Background:
After the production of PCB, the original components need to be assembled to be used further. At present, the most common assembly methods are wave soldering, reflow soldering and the combination of the two technologies. The quality of PCB has great influence on the assembly quality of the three processes.
Technological introduction:
Wave soldering is a bulk soldering process used in the manufacture of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the circuit board makes contact with this wave, the components become soldered to the board. Wave soldering is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued onto the surface of a printed circuit board (PCB) by placement equipment, before being run through the molten solder wave. Wave soldering is mainly used in soldering of through hole components.
There are many types of wave solder machines; however, the basic components and principles of these machines are the same. The basic equipment used during the process is a conveyor that moves the PCB through the different zones, a pan of solder used in the soldering process, a pump that produces the actual wave, the sprayer for the flux and the preheating pad. The solder is usually a mixture of metals. A typical leaded solder has the chemical makeup of 50% tin, 49.5% lead, and 0.5% antimony. The Restriction of Hazardous Substances Directive (ROHS) has led to the elimination of leaded solder in modern manufacturing however, and lead-free alternatives are used. Both tin-silver-copper and tin-copper-nickel alloys are commonly used, with one common alloy (SN100C) being 99.25% tin, 0.7% copper, 0.05% nickel and <0.01% germanium.
Influence of PCB Quality on Wave Soldering:
1) There is green oil in the element hole, resulting in poor tin plating inside the hole. PTH of components need to be inserted, ring green oil is not allowed in the hole; otherwise, when passing through the tin furnace, tin lead cannot be soaked along the hole wall smoothly, leading to insufficient tin in the hole. Therefore, the green oil in the hole of PCB element should not exceed 10% of the hole wall and the number of holes which has the green oil inside should not exceed 5%.
2) The thickness of plating layer is not enough, resulting in poor tin plating inside the hole.
The thickness of the coating layer on the hole wall of the element is not enough, such as copper thickness, tin thickness, gold thickness, etc. In general, the thickness of the hole wall should be above 18um.
3) The hole wall is too rough, leading to poor tin plating inside the hole or the pseudo soldering. If the roughness of the hole wall is too large, the plating layer will be uneven and if some areas of thin coating which can affect the effect of tin.
4) The hole is damp, resulting in pseudo soldering or bubbles. PCB being packaged when not being dried or not cooled after drying, and placing for a long time after unpacking, etc., will lead to damp inside the hole, resulting in pseudo soldering or bubbles.
5) The size of pad is too small, resulting in poor welding. Disconnections or gaps in the solder pads of the holes may result in poor soldering. Generally speaking, the size of the solder pads should be greater than 4mil.
6) The inside of the hole is dirty, resulting in poor welding. Insufficient cleaning of PCB, such as the gold plate has not been pickling, resulting in impurities and dirt residues on the hole and pad, affecting the tin effect.
7) The components cannot be inserted into the hole because the hole size is too small, resulting in the failure of welding.
8) The components cannot be inserted into the hole because the positioning hole is offset, which causes the welding not being carried out.
Reflow Soldering:
Technological introduction:
Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach one or several electrical components to their contact pads, after which the entire assembly is subjected to controlled heat, which melts the solder, permanently connecting the joint. Heating may be accomplished by passing the assembly through a reflow oven or under an infrared lamp or by soldering individual joints with a hot air pencil. Reflow soldering is the most common method of attaching surface mount components to a circuit board, although it can also be used for through-hole components by filling the holes with solder paste and inserting the component leads through the paste. Because wave soldering can be simpler and cheaper, reflow is not generally used on pure through-hole boards. When used on boards containing a mix of SMT and THT components, through-hole reflow allows the wave soldering step to be eliminated from the assembly process, potentially reducing assembly costs. The goal of the reflow process is to melt the solder and heat the adjoining surfaces, without overheating and damaging the electrical components. In the conventional reflow soldering process, there are usually four stages, called "zones", each having a distinct thermal profile: preheat, thermal soak (often shortened to just soak), reflow, and cooling.
Wave Soldering Process:
Preheat is the first stage of the reflow process. During this reflow phase, the entire board assembly climbs towards a target soak or dwell temperature. The main goal of the preheat phase is to get the entire assembly safely and consistently to a soak or pre-reflow temperature. Preheat is also an opportunity for volatile solvents in the solder paste to outgas. The second section, thermal soak, is typically a 60 to 120 second exposure for removal of solder paste volatiles and activation of the fluxes, where the flux components begin oxide reduction on component leads and pads. Too high a temperature can lead to solder spattering or balling as well as oxidation of the paste, the attachment pads and the component terminations. The third section, the reflow zone, is also referred to as the “time above reflow” or “time above liquidus” (TAL), and is the part of the process where the maximum temperature is reached. The last zone is a cooling zone to gradually cool the processed board and solidify the solder joints. Proper cooling inhibits excess intermetallic formation or thermal shock to the components. Typical temperatures in the cooling zone range from 30–100 °C (86–212 °F). A fast cooling rate is chosen to create a fine grain structure that is most mechanically sound. Unlike the maximum ramp-up rate, the ramp–down rate is often ignored. It may be that the ramp rate is less critical above certain temperatures, however, the maximum allowable slope for any component should apply whether the component is heating up or cooling down. A cooling rate of 4°C/s is commonly suggested. It is a parameter to consider when analyzing process results.
Influence of PCB Quality on Wave Soldering:
1) Insufficient plating thickness of pad leads to poor welding. Insufficient thickness of the coating on the surface of the solder pan fitted with the components can lead to poor welding. Fox example, the insufficient thickness of tin will lead to insufficient tin when melting at high temperature and poor soldering of the element with the pad.
2) The dirty surface of the pad causes bad welding. If the PCB is not cleaned properly, impurities will remain on the surface of the pad, resulting in poor welding.
3) The deviation of wet film causes the poor welding.
4) A defect in the pad may cause parts to fail to solder or fall off easily.
5) The image development of BGA welding pad is not clean and there are impurities, which may cause improper soldering or false soldering.
6) The plug hole at BGA is protruding, resulting in insufficient contact between BGA component and pad, and easy to break.
7) The distance between the positioning hole and the circuit diagram does not meet the requirements, resulting in the solder paste off-position or short circuit.
Mixed Assembly of Reflow soldering Technology and Wave Soldering Technology:
In the process of mixed assembly, a circuit board has to go through reflow welding and wave soldering. For example, if there are both pasted and inserted components on the circuit board, then the circuit board needs to go through reflow and wave soldering.
