A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. PCB's can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductor on different layers are connected with plated-through holes called vias. Advanced PCB's may contain components - capacitors, resistors or active devices - embedded in the substrate.
Most of the surface assembly manufacturing and process engineer's primary goal is to improve the early detection qualification rate of components. Therefore, engineers, of course, the main focus on the original manufacturing process, and less repair process. But now the PCB assembly, due to the solder joints, spare parts and materials are more, thus increases the chance of failure. Even the best manufacturing equipment, high density PCB repair also unavoidable.
Repair is probably the most difficult to according to the general consistency of industry knowledge, standards, and to define the process. In the repair, can find a lot of field failure. In order to ensure high quality repair, must consider the following basic questions:
The continuity of metal welding points and strength
The heat tolerance of PCB and components
Surface insulation resistance and corrosion
The subsequent process of repair, such as online test (ICT), cleaning and packaging
The basic conditions of repair
For fine pitch components, because of printing, SMT, is very tall to the requirement of coplanar sex, so the pin welding repair is very common.
Bad solder paste printing, incorrect component placement and soldering paste cold cave (cold slump) with hot cave (slump) hot, inadequate backflow, etc., can cause defects, such as open circuit, bridge, ball pins, virtual welding (cold be) or poor wetting. The most common and the most effective way to repair these defects is applying flux and drag along the wire soldering pen. If need be, can use electrode or with added solder flux core wire.
Fully understand the causes of the formation of defects, which not only helps to prevent the occurrence of defects, but also to determine the best way to repair and the subsequent repair problems such as to provide guidance. Open circuit, for example, are often caused by insufficient solder paste, so check the printing process, and moderate increase solder in the repair.
Due to the solder paste, printing, SMT or reflow soldering problem, discrete device may appear too much on the welding bead or solder ball.
Formation of weld bead and two of the most common cause of solder ball is quickly exhaust and thermal damage. Small toothed tools available to shave off the solder ball, however, it depends flux characteristics, the weld bead/solder ball size and scope.
Mount error or components on wetting degree of difference, may make the discrete component leakage is stuck, post partial or wrong. For a device with a pin, repair personnel often use flux, welding wire, and iron repair or supplementary discrete component.
If there is an electrical fault or packaging damaged, you must replace component. For most of the device, can use iron removal; But for array device, such as the PBGA, the situation is complicated. In the array device, general defects, such as open circuit, bridge and solder ball is likely to happen. In addition, it is important to pay special attention to the hole. Given the nature of the array device, it is necessary to use local, hot air or infrared reflow soldering equipment to dismantle them. Most need of repair by printing or point solder paste to supplement the solder paste.
The basic methods of repair
Methods include: basic repair soldering iron and hot air, focus IR. Hot air and focus IR is usually composed of bottom and hearth, the function of the base plate is to heat radiation to the PCB, and the function of the furnace is above the parts form local heating. For most local reflow soldering equipment, the use of special high temperature hot compressed air or nitrogen.
All the heating method, all should avoid excessive heating or bad compensation of different coefficient of thermal expansion (CTE), in order to prevent damage of welding plate cock, spare parts, internal metal forming and burned. On the contrary, if insufficient heat flux, will damage to processing process, form virtual solder joints.
Add solder methods including point or solder paste printing, applying flux, using solid wire (solid - core wire) or prefabricated welding (preform).
In most cases, do not need to supplement the solder paste. For example, in the case of the bridge, need to redistribute the existing solder paste can, without having to add solder paste.
Note: when you start printing the solder paste used, may not be suitable for repair. Can use different, but mutual compatibility of solder paste. Some manufacturers use low melting point alloy to avoid overheating. If repair when the alloy is harmless, flux heat fully (can be activated), and the left will not cause damage (no clean), it is a good method.
Even in the same workshop, with procedures, flux distributing method are also quite different. Common methods include coating, spray bottle (squirt bottle), spray (spray), sensing (felt pen), etc. In addition, flux type, also have active flux and low reactive nitrogen peak flux. Therefore, in the process of repair a mixture of various chemical substances. With the repair materials and methods, from cleaning, washing, or manufacturing processes, such as the saponated/solvent rinse may produce harmful chemical reactions.
Metal welding point
Repair method is the key to ensure stable metal welding point. According to the original amount of residue left after assembly and repair type, can determine the repair methods.
As far as possible to reduce the number of variable components used in the. For example, if the initial SMT process used in the medium/high residue, no clean or RMA solder paste, so in the repair of the bridge, virtual welding or the defects such as spherical pin, then no longer need to add flux. Residues of help weld ability, is usually enough to formation of solder joints. Thus, even a slight touch solder joints, soldering iron can complete repair work.
If there is enough heat to melt solder and flux activation, a reliable solder joints can be formed. Excessive heat can form a large number of tin copper intermetallic compound, the solder joint embrittlement. Using a soldering iron, this problem is very common. Operators in order to improve the efficiency, the solder iron temperature often switch to the highest. Repair engineer in the report pointed out that the actual temperature of the soldering iron is usually higher than required or recommended temperature 50 to 100 . In addition to the formation of intermetallic compounds, but also can reduce the activity of the flux, or lead to solder and flux cock burning and damage of parts/PCB.
Insufficient return not commonly found in using the iron work, but it is a problem of local hot air reflow soldering system. Insufficient heating can be made in addition to welding is not fully, can also produce thermal stress and corrosive residue. Generally there is no need to use nitrogen, for flux outer garment has to enclose the system, and most of the repair powder does not exist. But when added solder paste, nitrogen can provide good wettability and joint (coalescence).
Repair alloy used in the same as the alloy used in the initial assembly. However, in order to reduce heating demand as far as possible, and to avoid local circumfluence, qualified welds can use alloy at low temperature. With different alloy system may form a low melting point is the main problem that medium.
Flux of choice will influence the formation of the solder joints and long-term reliability. During the most use of the soldering iron repair operations, core type welding wire, crest, solder paste and flux to wetting. Flux liquid or cream to remove oxide requirements are lower than the solder paste.
Because the surface of the solder powder have been won't produce oxide, therefore, only need a small amount of activator, can clean the welding plate, pins and welding surface. Caused by inadequate flux function of virtual welding and such problems as poor wetting, may be formed by incorrect, overheating of soldering iron tsui, incomplete backflow or incompatible compounds.
The damage of the PCB and spare parts
In repair, damage to the PCB and components are common, the type of damage including plastic outer melting, burning resistance welding membrane, the shift of the spare parts or crack, tilted welding plate. By appropriate Settings solder iron temperature and training technical personnel of repair, can be the most defects to a minimum.
Due to the CTE is different, and need to heating at the bottom of the device, so repair array of spare parts is very difficult. To fully heat at the top and bottom to minimize displacement and crack. Ceramic parts repair quite difficult. An advantage of low melting point alloy is minimizing the due to different CTE and soldering pen overheating problem.
SIR and corrosion
The most easily neglected problems in repair, especially for no clean components, is defined to be used in a process chemicals. At the scene of the production, flux or wire is often casually piled up in some of the board. Many unknown compounds are mixed together, in the repair process may produce chemical reaction. Choose a repair flux is not difficult, but to ensure that the flux is the only, can be used to clean up the scene of the production materials. If the repair immediately after thorough cleaning, won't produce undesirable chemical reactions.
The activity of the flux should be enough, for repair. Most of the repair only, with little or no need activator. High activating flux, often can cause reliability problems. If the flux is not heated to 200 , this is especially obvious. Most no clean flux, especially used for solder paste, flux should be properly exposed to high temperatures, in order to make its residue harmless. Flux itself usually is conductive, and has slight corrosion. Entrapment within the element or away from the flux of pin was not influenced by the iron heating. Therefore, if the repair is used in the solder paste used in flux, you might expect SIR and reliability problems such as corrosion.
Repair flux chemicals don't have to be the same as the initial solder paste, but to be compatible with each other. Compatibility can be defined as similar as activator, resin and solvent. But the real test need SIR evaluation, including all can be sent to the chemical composition on the PCB.
Strict evaluation does not require heat flux for repair. Harmless solvent in repair flux can make no clean residue soft, it can make ion movement, thus reducing SIR value and slow the growth of dendrite.
If you use the help or solder paste, flux core wire must be completely heating, because of solder paste required local hot air reflow soldering process, and the flux in the wire will get all the heat of a soldering iron. If you need to supplement the same solder paste or flux, remember, a lot of solder paste flux of viscosity is not enough, so can't use in core type welding wire, and many template with the solder paste printing or flux is not fully point. Again, the key characteristic of the flux is compatibility. The activator, resin and solvents used don't have to be exactly the same, but similar. In the process, you can test by SIR, exposes many kinds of chemicals at the lowest possible temperature, to check for compatibility.
Assumes that the initial assembly and repair compatible, and can produce reliable solder joints, so the only problem left is the follow-up process.
The circuit boards, for example, repair, cleaning process are common. But too much heat will burn in the residue of repair, the residue is difficult to clean. In addition, in the DI water system with flux, no clean or RMA repair can make leaving residue on the PCB. Combined with zero cleaning and washing technology of manufacturing equipment, is likely to cause incomplete compatibility problems.
The online test has been very popular all over the world. No clean residue must make probe is easy to penetrate, left in the residue after repair must satisfy similar indicators. Online test after repair, the selection of flux, the heat and the amount of flux will influence the result of the test.
No clean residue on the conformal coating will also be recognised. Before you accept the process, the adhesion between coating and residue relay and the compatibility of the routine evaluation. However, flux and repair of repair method must be included in the test project, to ensure that qualified performance.
Should the formal research related to repair defects, the results to determine what kind of repair is the worst. In order to get satisfactory welds, must pay attention to repair process caused by the chemical effect.
The key to successful repair
Repair is a very reliable in electronics assembly process procedure. Besides must to train operators, set up or redesign repair process must also follow the following steps:
Repair use only suitable for all of the components of flux.
In the process of using active low flux as much as possible.
The solder iron temperature as low as possible.
Chemicals used in repair don't have to be the same as the initial assembly completely, otherwise may cause the failure of repair.
When using local hot air reflow soldering, should according to the suggestion of temperature curve for synchronous uniform heating and spare parts for printed circuit board.
For all chemicals on PCB thoroughly the reliability of the research.
Don't ignore repair subsequent working procedure.
The two key technology of the PCB repair
SMT help for successful repair of two of the most key technology, is also the two most likely to lead to ignore the problem:
To flow again before appropriate preheating of PCB. Reflow rapid cooling after solder joints.
Because the two fundamental process often ignored by repair technicians, in fact, sometimes the situation is even worse than before repair after repair. Although some "repair" defects can sometimes be process after inspectors found, but in most cases always look not to come out, but soon exposed in the circuit after test.
1, Preheat the premise - successful repair
Over a long period of time to be sure, PCB processing under high temperature (315-426 ) will bring a lot of potential problems. Thermal damage, such as welding plate and warping, lead substrate layer, raw white spot or blister, become angry. Board up and burned usually noticed by inspectors. However, precisely because not "burning board" is not to say "plate is not damaged. High temperature damage to PCB's "intangible" problem is more serious even than those listed. For decades, countless times test proved repeatedly PCB and its components can "through" rework after inspection and testing, the attenuation rate is higher than normal PCB. Within the substrate warp and "invisible" problem such as the attenuation of circuit components from different materials with different expansion coefficient. Clearly, these problems will not exposed itself, even has not been found at the beginning of the test circuit, but it is still lurking in the PCB components.
Although looks after the "repair" is very good, but as people used to say: "the operation is a success, but the patient died". Causes of the huge thermal stress, temperature (21 ) PCB components suddenly contact heat source for about 370 soldering iron, when I went to local heating welding tool or hot head, circuit board and its components of about 349 temperature changing, produce "popcorn" phenomenon.
"Popcorn" phenomenon refers to exist in an integrated circuit or SMD the moisture in the repair process in the device, a rapid heating, make the moisture expansion, a tiny burst or fracture phenomenon. Therefore, the semiconductor industry and circuit board manufacturing requirements before production personnel to flow again, try to shorten the preheating time, quickly rose to reflow temperature. In fact PCB assembly reflow process is included in the stage of preheating before flow again. Regardless of the PCB assembly is to use wave soldering, infrared vapor phase or convection reflow soldering, each method generally are conducted to preheat or heat preservation processing, temperature is in commonly 140-160 . Before the implementation of reflow soldering, using simple short preheating PCB can solve many problems at the time of repair. It has been for several years in the reflow soldering process history of success. Therefore, the benefits of PCB components before reflow preheated is multifaceted.
Due to plate preheating can decrease the temperature of reflow, wave soldering, so the IR/vapor phase soldering and convection reflow soldering can be welded at about 260 or so.
2, The advantages of preheating is various and comprehensive
First, before starting to flow preheating or "heat preservation processing components help activating flux, get rid of to stay on the surface of the weld metal oxide and surface membrane, and the flux of volatile matter. Accordingly, a flow again before activating flux of the cleaning will
enhance wetting effect. Preheating is to the whole component heated to a temperature below the melting point of solder and reflow soldering temperature. This can greatly reduce the substrate and its components of the risk of thermal shock. Rapid heating will increase production components inside temperature gradient and thermal shock. Components inside the large temperature gradient will form the thermal mechanical stress, caused by the low thermal expansion rate of material embrittlement, broken and damaged. SMT chip resistor and capacitor are particularly vulnerable to thermal shock damage.
In addition, if the component preheated, can reduce the reflow reflow temperature and shorten the time. If there is no preheating, the only way to further raise only reflow temperature, or to extend the time of reflow, no matter which way it's not reasonable, should be avoided.
3, To reduce the circuit board repair more reliable
Used as a benchmark of welding temperature, welding method, welding temperature is different also, such as: most of the wave soldering temperature about 240-260 , the temperature about 215 in vapor phase, reflow soldering temperature is about 230 . Properly speaking, rework temperature is not higher than that of reflow soldering temperature. Though temperature close to, but could never reach the same temperature. This is because that all need only to a local repair process components adopt heating, and then flow need to heat the entire PCB components, whether in wave soldering IR and vapor phase reflow soldering are so.
Also limit rework in lower reflow temperature is another factor in the industry standard requirements, namely the points around the components to repair the temperature must not exceed 170 . So, repair the reflow temperature should be component itself and to flow the PCB components size, due to the local repair is essentially a PCB, so repair process limits the maintenance temperature of PCB. Localized heating of repair range higher than the production process of temperature, to offset the heat circuit board components.
So, there is still no reason to explain the whole board repair temperature not higher than that of reflow soldering temperature in the process of production, and nearly the temperature of the semiconductor manufacturer's recommended target.
4. Before the repair or repair PCB assembly preheating in three methods:
Today, preheat PCB assembly method is divided into three categories: oven, hot plate and hot air tank. Before repair and reflow soldering remove components use the oven to preheat substrate, is effective. And preheat the oven to bake off some integrated circuit internal moisture and prevent the phenomenon of popcorn, baking is a favorable method. Popcorn phenomenon refers to the repair of the SMD components on the humidity is higher than the normal component of humidity in the micro cracks by rapid warming happens suddenly. PCB in the preheated oven baked for a long time, generally for eight hours.
A defect in the preheat oven is different from the hot plate and hot air tank, heat preheating and repair at the same time by a technician doesn't work. For drying oven and rapid cooling solder joints is impossible.
Hot plate is the most ineffective way to preheat the PCB. Because to repair the one side of the PCB assembly is not all, today is the world of hybrid technology, PCB components in the plane of the side all is smooth or is indeed rare. On both sides of the PCB in the substrate is typically installed components. The uneven surface by hot plate preheating is impossible.
Hot plate once the implementation is the second defect of solder reflow, hot plate heat can persist on PCBS. This is because, even after unplug the power supply, there will still be stored in the hot plate of residual heat continue to conduction to PCB hindered the cooling speed of solder joints. The block spot cooling causes unnecessary lead precipitation of the lead liquid pool formation, lower the solder joint strength and variation.
The advantage of air tank preheating is: hot air tank does not consider the appearance of the PCB components (structure) and bottom, hot air can quickly directly into all corners of the PCB components and fracture. The whole PCB components uniform heating, and shorten the heating time.
5. PCB components in the secondary cooling of solder joints
As mentioned earlier, the challenge to SMT PCBA (PCB assembly) repair lies in the repair process should imitate the production process.
Proved: the first, before reflow preheating PCB components is essential for successful production of PCBA, Second, immediately after flow rapidly cooling module is also very important. And the two simple process has been ignored by the people. However, in the hole technology as well as the sensitive element in the welding of miniature, preheating and secondary cooling is more important.
Common reflow equipment such as chain furnace, PCB components through flow area immediately after entering cooling area again. As the PCB components into the cooling zone, to achieve rapid cooling, ventilation is very important to the PCB components, general repair itself is the massing and production equipment.
PCB components after flow slow cooling can make the liquid solder don't need to be rich in the lead liquid pool will make solder joint strength is reduced. However, the use of rapid cooling can prevent the precipitation of lead, make grain structure more compact, solder joint is more strong.
In addition, faster cooling solder joint will reduce PCB components during reflow due to accidental movement or vibration and produce a series of quality problems. For production and repair, reduce possible misalignment and small SMD tombstone phenomenon is another advantage of secondary cooling PCBS.
To sum up, the correct preheating and flow of the secondary cooling the benefits of the PCB assembly has a lot of, need to put the two simple program into the technical personnel of the repair work. In fact, when the preheating of PCB, the technician can also do other preparatory work, such as on the PCB board with solder paste and flux.
Of course, need to solve the problem of the PCB assembly process new repair, because it has not yet passed the test of the circuit, it is also a kind of real time saving. Obviously, not will cause in the repair PCB scrap and save the cost, an ounce of prevention is worth more than treatment.
Accordingly, can reduce because of substrate layer, freckled or bubbles, warping, faded and scorching and eliminate too much waste. The correct use of preheating and secondary cooling is two of the most simple PCB components, and the necessary repair process.
Only the three paint shield method of PCB circuit board repair components
Anti-corrosion coating of paint to the PCB circuit boards to form a solid layer of protective film tend to last line of the PCB assembly process, using the technology of products is increasing now. But after the coating protective film for final testing, may find components flawed, and at this point, requires three proofing paint coating can repair. Environment, characteristics of rework components remove this component determines the most appropriate method to save money and time.
Electronic component needs to use in high humidity environment corrosion protection, organic silicone protective film coated on the PCB is a cheap protection method, can provide all kinds of protection to the PCB: moistureproof and prevent pollution; Minimize dendrite growth, release stress; And the insulation effect. Electronic components are more durable, therefore, to improve the reliability of electronic equipment, reduces the warranty costs. In some cases, you can remove the expensive components on printed circuit boards, other parts of the discarded circuit boards. Is to stay the same, however, is equipped with components of circuit boards have considerable value, it is best to remove the protective film, replace defective components, don't put the circuit board. Some circuit board, notebook computer motherboards, plasma TV video driver board, aviation electronic circuit board on the plane - worth a few hundred dollars, or even thousands of dollars. In the case of repair aspect to consider there are two main types: one is the local position of PCB, remove the protective film, replacement of components; Another is the protective film all PCB are removed.
Local repair
To correctly to local repair PCB must be conducted by repair personnel a "surgery". Develop a comprehensive plan to remove the anti-corrosion paint protection film, make sure it won't to remove the protective film components below the base board, other electronic devices, or rework location near the structure damage. There are several ways to remove the protective film, these measures include the use of chemicals or solvents, micro grinding, mechanical method and through the protective film desoldering.
1, chemical solvents. This is a common way to get rid of anti-corrosion paint protection film, it's success depends on how to remove the protective film of the chemical properties and specific chemical properties of the solvent. In most cases, the solvent is not really the protective film dissolve, but to make it. Once protective film, can easily put it gently scrape, or wipe gently, such as silicone anti-corrosion paint. Solvents dissolve protective film, but must be very careful when operating, to ensure that use of chemicals will not damage the substrate and rework location adjacent parts.
This method is the key to the solvent must be kept where you need it. There is a kind of technology is to use as they were around to replace components the material form a dam, the solvent or protective film scavenger always stay in the designated place, but there are still some solvent out from the dam leakage. Another approach is to filling agent is added in the solvent, into a paste. Silica, fog silica filler or sodium bicarbonate is very appropriate. In some cases, may need to combine several removal technology to use. Remove protective film method and technology are introduced in this paper the IPC standard HDBK - 803 on the provision of protective film to rework and repair.
Components once to welding and removed, will remove all clearing agent, solvents or protective film to prevent they affect or dissolved coating of protective film again. In addition, still must clean solder area, when change into the new element, easy to weld. Put it after welding to the circuit boards, to wash away the flux residue with solvent.
2. Micro grinding. Use of micro grinding technology to anti-corrosion paint protection film, is the use of nozzle of high speed particles, grinding operation must be very skilled. Commonly used abrasive including grinding of walnut shell, glass or plastic beads, or sodium bicarbonate
powder. With these abrasive removed silicone protective film, grinding operation must be very careful, can't let these materials into the grinding position around the soft protective film. Want to use aluminum foil or plastic cover around the place. On the electrostatic charge of particles passing through the path would be harmful, therefore, many micro grinding system with antistatic ionizer, in the interior of the device has a ground.
3, mechanical method. Although mechanical methods to remove three paint shield is perhaps the most reluctant to use it, but it is the easiest way. PCB rework workbench has a variety of equipment for rework, including drilling machine, grinding machine, rotating brush, and can be purchased from the market similar equipment.
PCB rework started with remove protective film around to rework compenent solder. Here, the protective film is often the most thick, may have more than 20 mm thick. On target solder joints with thin blade or knife gently cut a V groove is enough.
SMT components installed only on one side of the PCB (left), remove the protective film is easier. Rework instrumentation components (right), you must first remove the protective film on the two sides PCB. And then the solvent or clearing agent injection blow out the protective film of V groove, should pay attention to not let them flow to other places. Once the V groove protective film up, the other can use tweezers to lift up, and then to solder to welding. For rework parts of the edge, to dissolve the partial modification.
The instrumentation components, when welding the parts rework, the protective film on the solder with thin blade scraped off directly, when use vacuum to welding tool heating solder joints, soldering can flow easily.
Through the protective film to welding. Sometimes, go to welding technology to burn off protective film or make the conformal coating degradation. This welding method is first open a drain hole in the protective film, allow molten solder to discharge. In order to prevent resistance welding pad off or damage to the film, be careful to prevent overheating to welding parts. Ventilation should be enough, when I went to welding smoke spread out, the smoke is protective film thermal decomposition.
The comparison of SMT components and instrumentation components
SMT components are generally mounted on one side of the PCB, only when the rework the protective film side PCB SMT components are removed. For instrumentation technology, on the other hand, need to put the boards on both sides of protective film is removed, and then to redo the element. Remove all the protective film on the PCB.
Both sides need to remove the circuit boards of all protective film, the whole circuit board must be immersed in the solvent or solvent/cover removal agent. Due to liquid protection coating of various places in the circuit boards is not consistent - this is due to the capillary effect, due to the vertical components around the protective film thicker, protective film thinner place will first separation and circuit board.
To remove the protective film all, circuit board may soak for a few hours, depending on the circuit board and use of protective coating. Heat the solvents or protective film removal agent, can reduce the soaking time, but there are flammable or their steam soaking liquid light.
The rework site coating again
Because of many organic silicone protective film this own adhesive ability is not very good, so in repair and rework, we strongly recommend using single-component moisture-curing silicone protective coating. Choose a primer or slightly rough surface, can improve the protective coating on the surface of adhesive ability.
In the new protection coating before coating to rework parts of the solder joint, be sure to thoroughly clean for coated surface. With the brush or syringe to coating rework surface, a new protective film must go over the edge of the original protective film is at least 3 mm..
In a word, in choosing a protective coating of chemical materials, you must take into account the replacement element when the element out and rework. Preferably, the rework as a part of the circuit board design.
However, more often, in the design of circuit board after considering protective film, and there is no design standard. Sometimes, discarding the defective circuit board might be more benefit, but in expensive circuit boards and components must be saved or repair, we particularly recommend the use of one-component moisture-curing silicone protective coating.
A successful turnaround strategy, should consider to remove the protective film, welding and replace defective components, use new materials to coating rework parts and a series of problems. Although our purpose is to try to reduce and eliminate rework costs, however, remove and repair the protective film may need to take all kinds of working procedure, I know it is very important. Every rework each have their own requirements, there is no a fixed, applicable to all the rework process.
Repair of double-sided PCB lead-free grid array device
In recent years, the circuit and components are becoming more and more small and fine, at the same time also more and more complex, to more and more functions. At the same time, fierce competition for the manufacture of each link caused great pressure, and productivity and profitability as the key. In this background, the repair (rework) has not dispensable or occasionally for work, and become the unavoidable task. At the same time, it is also one of the most risky work, is one of the most demanding link in the electronics industry, especially in the grid array of occasions.
Indeed, with high quality and efficiency of repair is the success of BGA device assembly, the only important factors so as to exert its profitability.
With the import of lead-free welding technology, the quality of the repair more as the key, this is because the need higher temperature lead-free technology, may directly or indirectly caused the sensitive plate temperature and component of irreversible damage.
Whether used with lead or lead-free technology, on a regular or grid array device, repair the main steps of (except welding, open, clean and reinstall) are basically the same. Repair the personage inside the line of these are very clear, and equipped with perfect document says, don't need to say more here. Therefore, this article will focus on heating ways (that's biggest problem) for lead free repair, and how to better for large size double grid array pieces of work on the PCB.
Higher temperatures
Lead melting point at 183 , solder reflow soldering temperature range is 210-220 ; And the most general lead-free SAC solder alloy melting point at 217 , reflow soldering temperature is between 235-250 . The temperature tolerance limit of temperature range is very close to yuan device. The bare chip IPC up to 260 , for example, it has too high for certain components manufacturer. Process window is obviously more narrow than the past. At the same time, to avoid warping, such as circuit boards, components of molten, open circuit, short circuit, bridge, circuit boards layered, blowholes, even yuan device damage defects, such as must be precisely set and control the process temperature curve (such as reflux temperature, heating rate, holding time and temperature), heat exchanger size and location. Now welding need higher temperature, and no lead solder must be heated to at least 230 to ensure the reliability of the welding points. Due to the process temperature has a tolerance of + / - 5 , so the temperature must be at least 235 by iNEMI standard (or reached 245 ), just can get reliable solder joints.
BGA encapsulation body damage
At that temperature for repair welding, is one of the biggest difficulties such as how do BGA encapsulation is repair body remain at a relatively low temperature. For large size of the BGA, high temperature can make the Angle of encapsulating body twisted down (this is a major cause of the bridging defects), the components are difficult to correct welding is good. The cause of this situation should might be to use inferior BGA substrate materials. Made some components manufacturer has not yet been transforming their original with seal for lead-free temperature loading process, only is simply using lead-free solder balls; Or is it in the real lead-free packaging when switching, provide cheap transitional product.
Temperature difference
Repair is another factor to consider when the board on the temperature difference between different components (i.e., T). Good repair process with strict process control, to keep the temperature difference between the solder ball is repair nearly 5 but not more than 10 . Similarly, in order to avoid the plate warping and may lead to reliability problems, some manufacturers designed door rules the whole board temperature less than 7 .
Heating rate
Time is money, and the higher productivity, higher profit margins. This whether to repair or any other link is true, therefore, reflux treatment as soon as possible. This will force repair personnel to improve the heating rate of lead-free solder, partly because the target process temperature increased. Cooling rate will be faster, this is to ensure that in the melting temperature and peak temperature of the time as short as possible. Process control is still the key here.
Heater and air temperature
When the grid array device heating, convection (through a nozzle) heater heater is more practical than heat transfer way, because of its quick effect, and can make the whole components and encapsulation body heats up.
Another important factor is the heater is relative to the size and location of PCB. If the quantity of heat only focused on the repair area, the temperature of the substrate can bear will greatly exceed the limit, and the rest of PCB is cold. Resulted from warping, this will cause PCB board connection under stress, at the same time, the very high local temperature can make the solder resist film sparkling on the PCB.
To avoid these problems, the heat have lead PCB standard method is not only the BGA heating to repair, but with the largest up to 220 hot air heating, on the whole PCB to the reflow temperature. So, components surface temperature can be reached 190 to 200 , and solder will reach 220 .
Unfortunately, for some reason, some companies pay in the coma now, this method cannot be used for lead-free PCB, for improving hot blast temperature will bring disastrous consequences, especially for the back of the plate. This is because the back and is the surface has a sensitive components and connectors. These components can damage deformation and plastic part of the chapter, additional work, or their pin may also part/full circle; The resulting solder elongated, prolapse of the components and connectors, or deformation, even from the plate on the back of the loss. Because there is a potential defects and the risk of damage, when after completion of repair should be treated like a positive components, check on the back of the components for burning bad, fading and deformation.
Some repair personnel in order to avoid the risk from the back to lead-free boards pre heat to a far lower than the temperature of the melting point (217 ), and then directly from the surface heating, until the formation of backflow. Although this is better than no preheating, but in order to make the solder ball underneath all melted, BGA or exposed to exceed its can tolerate extreme temperatures, resulting in the risk of heat damage.
Have the best technology of BGA device of double panel temperature curve
Considering these problems, on the large size of double panel array sealing device for repair, need to carefully design can be sustained controlled process the temperature curve, makes enough reflux device can accommodate and support the PCB does not produce deformation;
And there is enough power rapidly preheating the whole piece of circuit board; And in the correct time and temperature range only to return the part be repair.
For specific circuit board, of course, the best temperature curve will take on its size, quality, component types and density. For arrays of a BGA encapsulation device, for example, the large size of double-sided PCB, there must be a pre heat; Using a large array of heater from below the entire component heated to a temperature below the melting point of solder, ideal temperature is 190 . After preheating to the temperature, will board the large heater shut down below, reuse is focused on the BGA back to fix the upper and lower sides of the hot nozzle will be added to the reflow temperature, temperature and keep a very short period of time. With preheating section, low hot air temperature can be adopted to achieve the process temperature. This can greatly reduce the BGA, encapsulation, and around the risk of resistance welding film thermal damage.
In addition, keeping the temperature of PCB other parts in the lower 190 to 200 , the temperature of the repair parts can be quickly and easily return to a safe temperature, avoid the risk of local deformation circuit board. And, because the rest of the PCB keep temperature below the melting point, the risk of damage to the device will be on the back down low, reflow soldering is only in view of the desired part. The use of this method can also can keep the temperature within the scope of the whole circuit board in a very small range.
All in all, experience, patience and a lot of trial and error has been offline for double panel repair process outlined a contour, and using special software to optimize the specific product process provides a valuable basis for temperature curve.
Accuracy and speed is the key to success, efficient repair, use only the simple hardware "raise the temperature" are usually not sufficient.
Function of repair equipment must strong enough, can the lead-free process required under high temperature and fine it easily to work; At the same time, the closed-loop feedback mechanism and special control software to high array device quality, speed up the repair and reduce the cost of repair is to more and more important.
"Constructive" use PCB repair equipment, create additional value
As part of the PCB assembly process, a lot of people think PCB repair is a tedious process excess, also do not create additional value. Now, however, there have been some repair equipment suppliers and PCB assembly manufacturers began to advocate the concept of a "constructive" maintenance, makes maintenance work of "negative" is no longer just in order to reduce scrap, but improvement for product quality and process of ascension for the whole play a constructive role in a positive force.
"Constructive" use of repair equipment has not been paid attention to by the PCB manufacturer. But he believes that as more and more repair equipment equipped with optical and other testing means, PCB manufacturers will gradually change negative understanding of repair equipment, repair equipment more actively used to create some additional value.
In addition to the regular production to provide data for product failure analysis, advanced repair equipment can also be contributing to important new production line of the audit. The new production line inspection involves a large number of process parameters adjustment and test, very time consuming. Customers in the production line will be confronted with a large number of defective boards, commissioning when using the real purpose of repair equipment is not in order to improve the qualified rate, but for the production line process problems exposed in and out in time. In the new production line commissioning, craftsmen to make a lot of DOE test, which involved some quantitative test. Using Mr Sally a repair equipment, the technologist can measure the height of the solder paste printing and dip Angle, tablet devices, data volume, so as to judge the solder paste printing machine and equipment parameters of the placement machine are set appropriately. Using the measured data and image information, technical repair computer aided system can provide reasonable Suggestions to solve the problem.
A new generation of system will be based on factory network and a central database, at the appointed time, including repair workstation should different SMT production test equipment can be easily Shared production, design, process, testing and maintenance of data, such as repair workstation data were also more likely to contribute to the improvement of production process.
We have already seen on repair equipment equipped with certain optical detection system can make it beyond "maintenance" of the traditional role. SMT equipment industry have been discussed, in fact, m, ore cost-effective and more creative ideas, make some repair equipment and testing equipment could eventually be in harmony are an organic whole. On, e of the developing direction is to put the automatic X-ray and repair workstation integrated as a whole or the automatic optical inspection (AOI) and automatic repair workstation int, egrated as a whole. The advantage is that once found problems board detection subsystem, rework subsystem automatically to repair i, t instead of a mark on the board, and then deliver to artificial maintenance. Use of such a system will make the equipment vendors and customers both to get a new experience: is a PCB manufacturing defect will be found automatically, automatic correcting, simultaneously the source of defects were automatically quarantine in time, and does not involve human intervention. This is a trend of the future, but before that day arrives, repair equipment must contribute to the improvement of production process, and that only in the whole manufacturing process under the premise of fully realize network connection and closed-loop control can be fully realized.
About PCBway
Since 2003 PCBWAY has been the leading PCB quick turn manufacturer specializing in both Prototype and Production quantities, Initially produced single-sided and double-sided printed circuit boards for the consumer electronics market. PCBWAY is ranked among the top 4 board fabricators in asia and is well-known for its expedited turn time capabilities and its reliable best on-time shipping record.
Today, we have over 450 operators with high modern facilities to manufacture multi-layer PCB up to 12 layers. Backing up with a group of professional engineers, and well established quality system. PCBWay has grown to become a major PCB manufacturer in Asia to serve in diverse customers base such as electronics appliance, communication, educational electronics, power supplies, Automationsetc.
Our mission is to become one of leading PCB manufacturer that provide in high quality product with total customer satisfaction.
For more information about PCB, or to learn more about the online quote and ordering process, please visit //www.PCBway.com.
