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.
Printed circuit board(PCB) provides a method to connect circuits and devices together. In addition to electrical connections, PCB provides a mechanical structure to hold the circuits and components together. Due to the inherent insulation properties of organic material, many PCB substrates are composed of fiber glass with organic resin. PCBs made from inorganic substrate like ceramic or metal can be counted as member of heat dissipation system to conduct heat away from the circuit. For consumer electronics and industrial application, it is common to require a circuit board to withstand 105 deg C to 130 deg C. Some application such as uninhabited airborne application may demand for higher temperature circuit. It is common to use inorganic substrate to combat such environment. For example, ceramic substrate can operate up to 200 deg. C without insulation breakdown and metallic substrate with aluminium and copper are being used in power module packaging so as to achieve high power density or output power per unit volume. As there are many kinds of PCB available, for training needs, we shall focus on PCBs that can be easily fabricated for prototype testing; the fabrication of single-sided and double-sided PCB. Traditional plated-through-hole fabrication techniques are discussed.
In the electronic industry, Single-sided PCB and double-sided PCB are the most popular interconnection device because of their low cost and easy to fabricate. In general, printed circuit board fabrication mainly consists of photolithography and photochemical techniques with chemical etching and surface finishing technologies. Some of the chemical processes in making single-sided PCB and double-sided plated-through-holes (PTH) PCB are similar. Minor adjustment is needed so as to adapt to different material or tolerance requirement. In a workshop environment when making PCB is mainly for the purpose of small scale prototype tests. Double-sided and single-sided processes are sufficient for most purposes. Making PTH requires a good drilling process and more complicate chemical processes and much longer time in fabrication. PCB Fabrication processes are designed with the production rate in mind. Process for mass production is very different from small batch production. In this section we shall discuss process available for training or sampling purposes for an electronics workshop.
The PCB is said to be "printed" because its conductive area is usually generated be means of a printing like process; artwork preparation, film development, photo-chemical etching, silk screen printing and surface finishing. Common thickness of rigid PCB is 1.6 mm, in some cases where space is critical, 0.8 mm thick laminate is a popular choice.
Depending on the number of layers of copper clad and process, 3 basic types of PCB are listed below in ascending order of interconnection wiring and component density.
1.Single sided PCB: conductors on only one surface of a dielectric base.
2.Double sided PCB: conductors on both sides of a dielectric base; usually interconnected the two layers with plated-through-holes (PTHs).
3.Multi-layer: conductors on 3 or more layers separated by dielectric material and interconnected by PTH or pads.
PCB Material
Printed Circuit Boards can be broadly categorized into 2 types; rigid and flexible. Flexible PCB is much more expensive and should only be used on areas where space is limited, for example, inside a camera or telescope. PCB or in particular rigid PCB are composite material which made from a reinforcement material immersed in a temperature resistant polymer. Conducting material like copper foil is bonded on top of the laminate. Foil thickness, regardless of the manufacturing process, is specified for printed circuit boards in ounce of copper foil per square foot. Some common materials for printed wiring boards are:-
1. Phenolic-resin-impregnated Paper, (e.g. XXXP, XXXPC, FR-2)
1. Epoxy-resin-impregnated Paper, (e.g. FR-3)
2. Epoxy-resin-impregnated cloth surface cellulose paper core, (e.g. CEM-1)
3. Epoxy-resin-impregnated cloth surface nonwoven fibre glass core, (e.g. CEM-3)
4. Acrylic-polyester-impregnated random glass mat, (e.g. FR-6)
5. Epoxy-impregnated fiberglass cloth, (e.g. G-10, FR-4) and
6. Other resin system like polyimides (Kapton) and polytetrafluoroethylene (PTFE).
Notes:-
1. Kapton is a registered trademark of E.I. du Pont.
1. The classification of material inside the bracket follows National Electrical & Manufacturers Association, USA, (NEMA) grading.
2. The number of X represents the amount of resin present in the board; the amount of resin increase with the number of X. P suffix indicate punchable at elevated temperature and PC suffix indicates that the board can be cold punched.
3. XXXPC, FR2 and CEM-1 is normally used for single sided consumer electronics application.
4. CEM-3, G10 and FR4 are normally used for double sided computer/industrial electronics application.
1. CEM-3, G10 and FR4 are normally used for double sided computer/industrial electronics application.
Environmental Protection Issues
Most of the chemicals used in the PCB fabrication are inorganic acid or base. For batch type operation, spent concentrated solutions should be collected and disposed though chemical waste treatment plant. Rinse water should be treated before discharge into drainage. A table of chemical effluent can be prepared as it helps to provide a foundation for chemical disposal. In addition, material safety data sheet (MSDS) of individual chemicals should be gathered and analyses so as to formulate a safe operation procedure in chemical handling. For example, an air scrubber is needed to remove fume in a pcb processing plant. Chemical used in the air scrubber depends on the process. The goal is to neutralize the fume before releasing it into the atmosphere.
Single-sided PCB Fabrication Process
The first step of making single–sided PCB is to cut the raw PCB into proper dimension using tools. Raw PCB laminate is first cut into the required size by considering the machine available and the size of the process tanks. A guillotine can be used to cut the material into designated size at room temperature. For boards with epoxy resins, for example FR4, it can be cut at room temperature. For boards with phenolic resins, for example XXXPC board, the board must be heated prior to cutting to avoid cracks. The temperature depends on the shear characteristics of the material. Normally, a board temperature of approximately 40 deg. C is sufficient to avoid board cracking. For board punching, the temperature should be properly controlled since heating can affect the tolerance of the holes and the size of the board. This temperature is not critical and can be determined experimentally. For panel type of jobs involving daughter boards, a V-cut groove can be made on the PCB so that the PCB can be separated at a later stage. The V-cut groove can be made using a V-cut machine.
Etching
Unwanted copper on PCB can be removed by chemical etching. A subtractive process which is common in PCB fabrication. In general, photo resist is soluble in alkaline. Hence the etchant should be acidic. There are many different acidic etchants available in the market. They have different characteristics such as copper holding capacity, regeneration characteristics, close loop control characteristics, disposal and hazards. Common available etching baths are cupric chloride, ammonium persulfhate, chromic-sulfuric acid and ferric chloride.
Bath selection criterion are safety, speed, bath control, cost and maintenance. For example, in a workshop environment, the primary consideration should be given to a safe etchant that is low concentration, low odor, low fume, low hazardous, environmental friendly, easy control and minimal bath maintenance. The cost and speed of reaction, which are essential in a production environment, may be considered as secondary importance in a workshop environment. Ferric chloride solution is a relatively safe and adequate etchant for small workshop application. It is economical and easy to maintain. The composition of the etchant is mainly ferric chloride solution and hydrochloric acid (HCl). In addition to HCl, commercial available etchants contains chelating additive to reduce foaming, fuming, increase etching speed, improve etching uniformity and reduce iron hydroxide precipitation. Comparatively, ammonium persulfhate etchant is also a safe etchant, but the etching rate is slow and expensive, etchant can decompose spontaneousely and copper content dependent etch rate. In addition it causes black film on tin.
The etching rate of FeCl3 depends on its concentration and temperature. The concentration of ferric chloride is around 28% to 42% by weight. For optimum etch time, the concentration of FeCl3 range from 30% at room temperature to 35% at 70 deg. C. For our etching tank, we can prepare a 33% FeCl3 solution and set the temperature of the bath to around 35 deg. C . 5% HCl is added to the solution to increase its copper holding capacity. We do not set the bath at high temperature such as 50 deg. C. because higher temperature will improve etch time at an expense of higher risk and fume generation.
Ferric chloride decomposed into HCl when it is being dissolved in water as described by the following hydrolysis reaction:-
FeCl3 + 3H2O ® Fe(OH)3 + 3HCl
Adding 5% of industrial grade hydrochloric acid will hold back the precipitation of the insoluble Fe(OH)3. Copper etching starts when the PCB is immersed into the dark red-brown liquid. At the copper surface, ferric ion oxidizes copper to form cuprous chloride and the green ferrous chloride. Cuprous chloride is further oxidized to form cupric chloride as described by the following reaction:-
FeCl3 + Cu ® FeCl2 + CuCl
FeCl3 + CuCl ® FeCl2 + CuCl2
As cupric chloride builds up in the solution, a disproportionation reaction takes over and turn copper into cuprous chloride:-
CuCl2 + Cu ® 2CuCl
Air agitation introduced by bubble tank or spray processor helps in etching. This increase in etching rate is contributed by the reoxidation of Fe+2 to Fe+3 and Cu+1 to Cu+2. Black or dark green precipitates from the solution. The maximum temperature setting depends on the maximum temperture that the etching machine and the mask or dry film of the PCB. This reaction can be described by:-
4 CuCl2 + FeCl2 + O2 (from air) ® FeCl3 + 2CuCl · CuO
Etch factor is one of the figure of merit for etchants. Etch factor is the ratio of the depth of etch to lateral etch. The etch factor for ferric chloride etchant is around 2.4 at 35 ° Be¢ solution at 40 ° C. Degrees Baume' (° Be¢ ) is one way to express specific gravity (s.g.) of acidic solutions. It is given by:-
Degrees Baume' (° Be¢ ) = 144.3 x (1-1/s.g.)
For example, a solution with s.g. 1.2 equals to 24 ° Be¢ . The following table gives an approximation on etch factor for different etchants. Etch factor also depends on the type of equipment used for etching. For example, the etch factor of a PCB fabricated using a splash type or bubble type etching techniques are much higher than that using spraying technique.
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Drilling and Surface Protection
A good PCB drilling process produce consistent clean and burr-free holes. A drill bit is required to cut through both the copper foil and the dielectric lamination. Special drill bits made from tungsten carbide are available for drilling holes on a PCB. Most drilling machines will accept drill bits with a constant shank of 1/8th inch (0.125 inch). Carbide is a hard and brittle material. It must be handled with care. Dropping the bit on the ground or any external stress can break it easily. Comparing with ordinary HS drill bits, it will not worn out as easily and the life is longer. PCBs are normally drill in a stacks of 3 to 5 with a sheet of thin aluminum alloy (5 –30 mil) as entry material to prevent entry burr. For small operation, thin stainless steel sheet can also be used. In addition, backup material should be placed to support the PCB stack so that the drill bit can run through the holes. Backup material is used to prevent exit burr; a piece of phenolic laminate can be used. For large volume, in particularly for single sided board, PCB is almost never drilled. Instead, a punching tool is used to make all holes including circular holes, rectangular holes and other odd shapes because drilling and routing is expensive. For prototype purpose, a PCB router is needed to make non-circular holes on the PCB.
A high-speed drill should be used to drill holes in a PCB. A high-speed spindle with sharp drill bit can eliminate resin smear and produce burr free hole. High spindle speed can also allow high feed rate. While a speed of 3,000 to 100,000 rpm can be used for PCB drilling, common drill speed ranged from 14,000 to 80,000 rpm. For drill a single PCB, the drill should enter the PCB from the copper side to avoid the breaking of the copper foil at entry.
It is often necessary to protect copper from oxidation; for example, before and after soldering. There are many different surface finishing techniques available. For small quantity of PCB with short delay before soldering operation, it is sufficient and convenient to use oil-based spray type contact cleaner on the copper for short-term protection. For a better protection, RMA type no-clean flux in aerosol package like "SMFL" from Electrolube can be used. After soldering, conformal coating like Plasticote 70 from CRC Industries can be used. This conformal coating provides a thin layer of electrical isolation but also allows repair soldering without first stripping off the coating.
For volume production, solder resist should be used to permanently protect the PCB. Since Plasticote is a soft organic material, contaminants can still get through the soft coating in a period of time. Solder resist is much harder and can thus it is more difficult for contaminants to get through. Hence, in addition to the function of solder masking, solder resist can enhance the reliability of the circuit. In particular, it is essential to apply solder resist to PCB with dense and narrow traces because a small scratch at the copper surface can easily break many traces and sometimes it is almost unnoticeable and difficult to repair. Similar to photo resist, solder resist can be applied using dry film, roller, curtain spray or screen-printing techniques. Among these methods, screen-printing is most popular and cost effective. When tented holes are desired, thick dry films should be used since liquid solder resists do not tent holes as effectively that of dry film. Dry film used for solder resist is different from that used for etch resist. Dry film solder resist should be around 3-4 mil in thickness, flexible so as to minimize solder wicking due to air trapped between the trace, the solder resist and the laminate. For screen resist coating, it is difficult to control the thickness of the solder resist. A typical minimum thickness requirement for the solder resist is 0.6 mil at the edges of copper conductors. For dry film, a thickness of 1 mil is common. After the application of solder resist, it must be adequately cured by UV or IR according to the recommendation of the manufacturer of the solder resist. Undercuring can cause the solder resist to fail.
Double-sided PCB Fabrication Process
PCB fabrication process of double-sided PCB is more complicate than that of single-sided PCB. One of the major differences is that double-sided PCB normally requires electroplating while single-sided PCB fabrication process does not. The design process using CAD is similar except that you have more layers for your circuit to route. In order to minimize cross talk and noise coupling, top and bottom traces should be routed at orthogonal to each other.
Additional fabrication processes of double-sided PCB involving electroless copper plating, copper plating and metallic etch resist plating. There are more parameters to be considered; for example, the size of PTH, top and bottom layer isolation and current density of PTH comes into considerations.
PTH is a metallized hole that provides a conductive path between layers of PCB. It is also known as via hole since it provides connection between locations. PTH can be made by using chemical deposit or direct metallalization. As the demands for more compact circuit increase, direct metallization will become more popular. In addition, laser ablation with fine line technology is needed for interconnection. Pin Grid Array Package (PGA) and Ball Grid Array package (BGA) with 50 Mil pitch make micro-vias and fine line technology indispensable. State of the art BGA technology requires a design rule of 6 Mil spacing, 6 Mil trace, 24 Mil diameter pad, 24 Mil diameter mask and 12 Mil diameter hole.
Drilling
The process flow chart for double-sided PTH process is given in the Appendix. The raw laminate is first cut into the required size for drilling. All holes, including PTH holes are drilled using a high-speed numerically controlled drilling station. For manual drilling, a optical drilling is necessary for accurate hole positioning so as to ensure a correct front-back registration. Noted that in a PTH process, the hole size should be slightly larger than that of the single-sided PCB to accommodate the increase in PTH wall thickness due to electroplating.
After drilling, the PCB should be deburred and cleaned so as to remove the dirt and oxidation on copper and debris in the hole. Remove all sharp copper edges so that it cannot interfere with the electroplating process. The PCB should be cleaned using alkaline detergent and slightly etch in a 10% sulfuric acid bath.
Electroless Copper Plating
Metallization is required on the wall of the drill holes to make them conductive. It can be done by chemical deposition or direct metallization. For double layer boards, chemical deposition is common. It is an electroless process because the deposition of copper is done in the absence of electrical current. The objective of the electroless copper plating process is to deposit a very thin layer of copper over the entire surface of the copper cladding and on bare hole walls. This layer should be thick enough to allow the electrical current during the subsequent electrolytic plating. Normally, a thickness of 0.001 Mil to 0.0025 Mil is acceptable for further electroplating at low start current. The thickness of the PTH will be increased to the required level after the electrolytic copper plating.
Several proprietary solutions are available for making electroless copper. Basically, a precious metal palladium is being used as an activator to provide a site for the adhesion of electroless copper. Electroless copper bath contains an alkaline chelated copper reducing solution that deposits thin layer of copper on activated surface. This process can be describe by the following equations:-
Pd+2 + Sn+2 ® Pd + Sn+4
CuSO4 + 2HCHO + 4NaOH ® Cu + 2HCO2Na + H2 + 2H2O + Na2SO4
The first equation described the process of exposing palladium from the protective colloid of stannic tin. Next equation describes the process of electroless copper through the use of palladium as catalyze.
The thickness of electroless copper needed can be observed from the following table which gives the thickness of copper foil on PCB. For reliable operation, PTH wall thickness should be around 0.7 Mil to 1 Mil.
Electroless copper plating process is consists of 4 baths; acid predip bath, activator bath, postactivator bath and electroless copper bath. Most of the process can be done at room temperature. We'll use PTH process chemicals from Enthone-OMI as an example. Circuitprep 3023 is used to prepare the acid predip bath. It is a low acid solution to replace a bath of high concentration hydrochloride acid. An immersion for 1 - 2 minutes in Circuitprep 3023 is sufficient to attain balance in the activator bath. The preparation of this bath is simple, 100% concentration is needed. It should be noted that no water should be allowed from this bath onward until the PTH process is completed.
The next bath is the Circuitprep 3316 palladium catalystic process. Major ingredients are palladium chloride and hydrochloric acid. This is a catalyst for electroless deposition on insulators. This process should provide uniform void-free metal coverage in holes and good electroless copper to copper clad adhesion.
The next bath is the Circuitprep 4044 postactivator process. Working in conjunction with CP3316, CP4044 accelerates the initial deposition of copper onto an activated surface. Mechanical agitation is required to ensure the removal of air bubbles in the holes and a complete wetting of the PCB. The active ingredient is fluoboric acid. It remove the protective stannous from the CP3316 so that electroless copper can adhere to the activated surface easily.
Enplate CU-406 concentrates are used for electroless copper disposition. Heater is normally required in electroless copper bath. The solution itself is highly unstable and we made use of its thermal property to restrict the displacement of copper from the solution. Thus, depending on the formula, temperature can be used to control the process so that it won't precipitate all the copper when the solution is idling. In fact, a cooling coil can be installed in the bath to cool it down to around 12 ° C to save it for a longer period in time.
Copper Plating
Electrolytic copper plating is needed to build up the thickness of copper. If the wall thickness is too thin, PTH can be easily cracked which may cause intermittently open circuit. A common practice in the industry for large PTH is to fill it with solder so as to enhance its reliability against mechanical stresses. There are two common methods for copper plating double-sided PCB after electroless copper; pattern plating and panel platting. They are named after the methods they are using in plating. Flowchart in figure 11 illustrates the difference in the process flow of these two methods.
In panel plating, the entire PCB including all PTH is plated with copper immediately after electroless copper and before image transfer. In pattern plating, only holes and circuitry are plated with copper and the metal etch resists. Hence, the copper plating is being done after the image transfer.
Pattern plating method can generates plating problems due to variation in current density during copper plating. This may cause the burning of thin tracks. In addition, the width of trace increases with the thickness during plating. This can cause overhanging problem. These problems are absent in panel plating because the entire PCB is plated before image transfer. Etch factor can be minimize with good process control and spray etching. In general, panel plating renders a better quality board with less effort in control when comparing with the pattern plating. The major disadvantage for panel plating is that it requires excessive copper etching which induce high operating cost and environmental cost.
Metal Resist
Metal etch resist has many advantages over organic photo-resist. It can protect the PTH and the copper trace of PCB from the attack of etchant and contaminates before and after assembly. When solder or tin is used as metal resist, they can help in enhancing the solderability of the PCB. Common metal etch resists are solder, tin, tin-nickel, nickel and gold. The most suitable etchant for tin and solder metal resist is alkaline ammonia. Ferric chloride cannot be used to avoid solder plate attack. Gold with an undercoating of nickel or tin-nickel plating provides excellent protection against all the common copper etchant. Although solder and gold are common finishing for PCB, tin is selected for workshop use because it is less hazardous than solder plating. Comparing with 60/40 solder, pure tin is more expensive.
Etching Solutions
Alkaline etchant with ammonium hydroxide complex is used as etchant for double sided PCB. It is compatible with tin or solder metal resist. Both batch type and continuous closed system can use ammoniacal etchant. High speed nozzle is needed to produce straight side wall.
In a bleed and feed closed loop system, a constant etch rate can be maintained by measuring the specific gravity (s.g.) of the solution. As the PCB are etched, copper dissolved and the s.g. increases. When the density sensor reach a upper set point, a pump is activated to remove the spent etchant and replace with fresh ammoniacal solution or the replenisher.
Batch type system have a low copper capacity and the etch rate dropped off rapidly as copper content of the etchant increases. It is necessary to add oxidizing agent to speed up the rate and increase copper capacity. Batch type system is more suitable for low volume production.
The compositions of alkaline ammonia solutions are proprietary. Alkaline etching solutions dissolve copper by oxidation, solubilizing and complexing. Ammonium hydroxide and ammonium salts combine with copper ions to form cupric ammonium complex ions Cu(NH3)4+2 which hold the dissolved copper.
Surface Finishing
For production board, the tin should be stripped off and the board is applied with a solder mask. For prototype purposes and hand soldering, we can leave the tin coating on the board as it can provide adequate protection. Depending the application or the environment of service; for example, if the board needs the wave soldering or dip soldering process, a epoxy type or UV cured type solder mask must be used. For final protection, hot air leveling (HAL) is common for double sided board. For surface mount application, organic coating is used for a tighter tolerance.
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