Welding is actually a chemical process. Printed circuit board (PCB) is an electronic products, components and device support, which provides the electrical components and connections between devices. With the rapid development of electronic technology, PCB density increasing, more and more layered, and sometimes may all designs are correct (no damage such as circuit boards, printed circuit design perfect, etc.), but due to There was a problem on the welding process, resulting in welding defects, welding quality passing rate of decline thus affecting the circuit board, leading to unreliable quality of the machine. Therefore, we must analyze factors affecting the quality of the printed circuit board welding, weld defects analyzes the reasons for and make improvements for these reasons that the entire circuit board welding quality is improved.
2, causes welding defects
2.1 PCB design affects the quality of welding in the layout, PCB 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, cooling down welding difficult to control, prone to interference between adjacent lines, 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.
2.2 circuit board hole solderability affect the quality of welding circuit board hole solderability properly, it will produce Weld defects, parameters that affect circuit components, resulting multilayer components and inner wire guide through 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.
PCB solderability main factors:
(1) 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. Wherein the impurity content to have a certain ratio control points to prevent oxide impurities generated by flux 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.
2.3 warpage generated weld defects PCB and component warpage during welding, the deformation due to the stress generated by Weld, short circuit and other defects. Warpage is often due to the temperature of the upper and lower part of the PCB imbalance. For large PCB, since the plate will fall of its own weight warped. Common PBGA device from the printed circuit board about 0.5mm, if the board as the device under large, as the return to normal after the cooling circuit board shape will be a long time in the solder joint stress, if the device will raise 0.1mm Weld sufficient to cause an open circuit. At the same time warp in the PCB, component itself may also be warped, located in the center of the pad element is lifted away from PCB, produce air welding. When only the flux and no time to fill the gaps, which often happens paste. When using the paste, leaving the deformation due to the paste and solder balls connected together to form a short-circuit defect. Another reason is that a short-circuit during reflow element substrate delamination, characterized in that the defect is due to internal expansion in the device following the formation of a bubble in the X-ray detector, the short circuit can be seen often in the middle of the welding device .
In summary, by optimizing PCB design, the use of fine solder circuit board hole generating improved solderability and prevent defects prevent warping, you can make the entire circuit board welding quality is improved.