PCB Circuit Board Welding to Produce Defects is What is the Reason?
PCB used in modern electronics is very wide, with the rapid development of electronic technology, PCB density is also higher and higher, the welding process requirements are also more and more. Therefore, it is necessary to analyze and determine what factors affect the quality of PCB welding, to find out the causes of welding defects, and then improve the overall quality of the PCB board. So, what are the factors affecting the PCB board welding?
Circuit boards and components in the welding process to produce warpage, due to stress deformation and produce defects such as virtual welding, short circuit. Warpage is often caused by the temperature imbalance between the upper and lower parts of the circuit board. Warping can also occur on large pcb's due to the weight of the board itself falling. Ordinary PBGA device distance from the printed circuit board about 0.5mm, if the circuit board device is larger, with the circuit board to return to normal shape after cooling, the solder joints will be under stress for a long time, if the device is raised 0.1mm will be enough to lead to false soldering open circuit.
B. Circuit Board Design
In the layout, the circuit board size is too large, although the welding is easier to control, but the printed line is long, the impedance increases, the anti-noise capability decreases, the cost increases; too small, the heat dissipation decreases, the welding is not easy to control, easy to appear adjacent lines interfere with each other, such as the circuit board of electromagnetic interference and so on. Therefore, the PCB board design must be optimized:
a, Shorten the connection between high frequency components, reduce EMI interference.
b, The weight of large (such as more than 20g) components, should be fixed with brackets, and then soldered.
c, Heat generating components should be considered to dissipate heat, to prevent the surface of the component has a large ΔT defects and rework, heat-sensitive components should be far from the heat source.
d, The arrangement of components as parallel as possible, so that not only beautiful and easy to solder, suitable for mass production. The circuit board is designed as a 4:3 rectangle most preferred. Do not change the width of the wire to avoid discontinuity of the wiring. When the board is subjected to heat for a long time, the copper foil is prone to expansion and fall off, therefore, large areas of copper foil should be avoided.
C. Solderability of Circuit Board Holes
Circuit board holes weldability is not good, will produce defective soldering, affecting the parameters of the components in the circuit, resulting in multilayer board components and the inner layer of the line conduction is not stable, resulting in the failure of the entire circuit function.
Factors affecting the solderability of printed circuit boards are mainly:
a, the composition of the solder and the nature of the solder. Solder is an important part of the welding chemical treatment process, which contains flux chemical materials, commonly used low melting point co-melting metal for Sn-Pb or Sn-Pb-Ag. Which impurity content to have a certain sub-ratio control in order to prevent impurities produced by the oxides are flux dissolved. The function of the flux is to help the solder to wet the surface of the soldered board circuit by transferring heat and removing rust. Generally use white rosin and isopropyl alcohol solvent.
b, the soldering temperature and the degree of cleanliness of the surface of the metal plate will also affect the solderability. Temperature is too high, then the solder diffusion speed up, this time with high activity, will make the circuit board and solder melting surface rapid oxidation, resulting in soldering defects, the circuit board surface contamination will also affect the weldability and thus produce defects, these defects include tin beads, tin balls, open circuit, bad gloss, and so on.
In short, in order to ensure the quality of PCB boards, in the process of making PCB boards, we must choose excellent solder, improve the solderability of PCB boards, as well as the prevention of warping, to prevent the production of defects.
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