Factors causing the circuit board welding defects has the following three reasons:
1, weldability impact mass soldering circuit board holes
PCB hole solderability properly, it will produce Weld defects, parameters that affect circuit components, resulting multilayer components and the inner conductive line unstable, causing the entire circuit ineffective. Solderability is called a metal surface by molten solder wetting properties, i.e. where the metal surface to form a solder layer of relatively uniform continuous smooth film adhesion. The impact of the printed circuit board solderability main factors: (1) the solder composition and the nature of the solder. Chemical solder welding process important part, which consists of the chemical composition of the flux-containing material, commonly used low-melting metal is eutectic Sn-Pb or Sn-Pb-Ag. Impurities which have a certain division ratio control prevent oxide impurities flux generated is dissolved. Flux function by transferring heat to remove rust to help solder wetting are soldered circuit board surface. Generally use white rosin and isopropanol solvent. (2) soldering temperature and the metal plate surface cleanliness also affect the solderability. Excessively high temperature solder diffusion speed, high activity at this time, the solder will melt and the circuit board surface oxidizes rapidly, welding defects, contamination of the surface of the circuit board can also affect weldability resulting defects that including tin, tin ball, open, good gloss, and the like.
2, warpage generated weld defects
Warped circuit boards and components in the welding process, due to the stress generated by the deformation Weld, short circuit and other defects. Warpage is often due to the temperature of the upper and lower part of the board of imbalance. For large PCB, due to the weight of the board itself will fall warped. Common PBGA device from the printed circuit board about 0.5mm, if the circuit board with a large device, with the board after cooling back to normal shape, joints will be under stress for a long time, if the device is sufficient to cause elevation 0.1mm Weld open.
3, the circuit board design affect the quality of welding
In layout, the circuit board size is too large, although welding is easier to control, but the printing lines long, resistance is increased, decreased noise immunity, increased costs; too small, decrease the heat, welding difficult to control, prone to adjacent lines mutual interference, such as circuit boards electromagnetic interference and so on. Therefore, we must optimize PCB design: (1) shorten the connection between the high-frequency component to reduce EMI interference. (2) a large weight (such as more than 20g) components, should bracket, and then welded. (3) heating elements should be considered the heat problem, preventing a larger ΔT element surface defects and rework, sensitive components away from heat sources. Arrangement (4) components as parallel as possible, so not only beautiful but also easy to weld, appropriate for mass production. Circuit board design rectangular 4:3 best. Conductor width not mutated to avoid discontinuities wiring. When the circuit board is heated for a long time, copper prone to swelling and shedding, therefore, should avoid the use of large areas of copper foil.