Poor hole weldability of circuit board will lead to virtual welding defects, which will affect the parameters of components in the circuit, lead to instability of multi-layer board components and inner layer wire conduction, and cause the whole circuit function failure.
The so-called weldability is the metal surface by molten solder wet property, that is, the solder on the metal surface to form a relatively uniform continuous smooth adhesion film.
The main factors affecting the solderability of printed circuit board are :(1) the composition of solder and the nature of solder. Solder is an important part of welding chemical treatment process. It is composed of chemical materials containing flux. Common eutectic metals with low melting point are sn-pb or sn-pb-ag. The function of the flux is to help wet the circuit surface of the welded plate by transferring heat and removing rust. White rosin and isopropanol solvent are commonly used.
(2) welding temperature and metal plate surface cleaning degree will also affect the weldability. Temperature is too high, the solder diffusion speed is accelerated, this time has a very high activity, will make the circuit board and solder melt surface oxidation, welding defects, circuit board surface pollution will also affect the weldability and defects, these defects include tin bead, tin ball, open circuit, gloss is not good, etc..
2. Welding defects caused by warping
The circuit board and components warping in the process of welding, virtual welding, short circuit and other defects due to stress deformation. Warping is usually caused by the temperature imbalance of the upper and lower parts of the circuit board. Large PCB also warps due to the weight of the board itself. The common PBGA device is about 0.5mm away from the printed circuit board. If the device on the circuit board is large, the solder joint will be under stress for a long time as the circuit board returns to its normal shape after cooling.
3, the circuit board design affects the quality of welding
In the layout, when the circuit board size is too large, it is easy to control the welding, but the printing line is long, the impedance increases, the anti-noise ability decreases, and the cost increases. When the temperature is too low, the heat dissipation decreases, the welding is not easy to control, and the adjacent lines are prone to interfere with each other, such as the electromagnetic interference of circuit board. Therefore, PCB board design must be optimized:
(1) shorten the connection between high-frequency components and reduce EMI interference.
(2) heavy (if over 20g) components should be fixed with brackets and then welded.
(3) heating elements should be considered heat dissipation problems, prevent components surface has larger Δ T defects and rework, thermal element should be far away from heat source.
(4) the arrangement of components is as parallel as possible, which is not only beautiful and easy to be welded, but also suitable for mass production. Circuit board design is 4∶3 rectangle best. Do not change conductor widths to avoid discontinuities in wiring. When the circuit board is heated for a long time, copper foil is easy to expand and fall off. Therefore, large area of copper foil should be avoided.
Related content: PCB circuit board why will appear to shake the copper phenomenon
PCB is an important electronic component, the support body of electronic components, and the carrier of electrical connection of electronic components. In the process of making PCB circuit boards, sometimes copper will be dumped. Here is the introduction for you.
1. Unreasonable PCB circuit design. Too thin circuit design with thick copper foil will lead to excessive etching and copper rejection.
2, excessive etching of copper foil, the electrolytic copper foil used in the market is generally single-sided galvanized (commonly known as ash foil) and single-sided copper plating (commonly known as red foil), common copper dumping is generally more than 70um of galvanized copper foil, red foil and less than 18um of ash foil has not appeared bulk copper dumping.
3. Local collision occurs in PCB process, and the copper wire is separated from the substrate by external mechanical force. This defect is characterized by poor positioning or directionality, obvious distortion of the dropped copper wire, or scratches/impact marks in the same direction. Peel the copper wire at the wrong place to see the copper foil hair, you can see that the copper foil hair color is normal, there will be no side corrosion, the copper foil peeling strength is normal.
4. Under normal circumstances, as long as the hot pressing section of the laminate exceeds 30min, the copper foil and the semi-solidified sheet are basically completely combined, so the pressing will not affect the adhesion between the copper foil and the substrate in the laminate. However, in the laminate stacking and stacking process, if PP pollution, or copper foil surface damage, will also lead to insufficient adhesion between copper foil and substrate after lamination, resulting in positioning (only for the large plate) or scattered copper wire fall off, but there will be no abnormal copper foil peeling strength near the delamination.