PCB Tech - Talking about the Three Causes of Welding Defects in HDI Circuit Boards

There are three reasons for the welding defects of HDI circuit boards:

1. Welding defects caused by warpage PCB and components warp during the welding process, and defects such as virtual welding and short circuit due to stress deformation. Warpage is often caused by the temperature imbalance of the upper and lower parts of the PCB. For large PCBs, warping will also occur due to the drop of the board's own weight. Ordinary PBGA devices are about 0.5mm away from the PCB. If the components on the PCB are large, the solder joints will be under stress for a long time as the circuit board cools down and return to normal shape. If the device is raised by 0.1mm, it is enough to cause a virtual solder open circuit. .

2. The solderability of PCB holes affects the quality of soldering. The poor solderability of HDI circuit board holes will result in false soldering defects, which will affect the parameters of the components in the circuit, resulting in unstable conduction of multi-layer PCB components and inner wires., Causing the function of the entire circuit to fail. The so-called solderability is the property that the metal surface is wetted by molten solder, that is, a relatively uniform continuous smooth adhesion film is formed on the metal surface where the solder is located.

The main factors affecting PCB solderability are:

(1) The composition of the solder and the nature of the solder. Solder is an important part of the welding chemical treatment process. It is composed of chemical materials containing flux. Commonly used low melting point eutectic metals are Sn-Pb or Sn-Pb-Ag. The impurity content must be controlled by a certain proportion, In order to prevent the oxides generated by impurities from being dissolved by the flux. The function of the flux is to help the solder wetting the surface of the circuit to be soldered by transferring heat and removing rust. White rosin and isopropanol solvents are generally used.

(2) The soldering temperature and the cleanliness of the metal plate surface also affect the solderability. If the temperature is too high, the solder diffusion speed will increase. At this time, it will have a high activity, which will cause the HDI circuit board and the molten surface of the solder to oxidize rapidly, resulting in soldering defects. Contamination on the surface of the circuit board will also affect the solderability and cause defects. Defects include solder balls, solder balls, open circuits, and poor gloss.

3. PCB design affects the quality of welding. In the layout, when the PCB size is too large, although the welding is easier to control, the printed lines are long, the impedance increases, the anti-noise ability is reduced, and the cost increases;, It is easy for adjacent lines to interfere with each other, such as electromagnetic interference of circuit boards.

Therefore, the PCB board design must be optimized:

(1) Shorten the wiring between high-frequency components and reduce EMI interference.

(2) Components with heavy weight (such as more than 20g) should be fixed with brackets and then welded.

(3) Heat dissipation issues should be considered for heating elements, to prevent defects and rework caused by large ΔT on the surface of the element, and heat sensitive elements should be far away from the heat source.

(4) The arrangement of components should be as parallel as possible, so that it is not only beautiful but also easy to solder, it is suitable for mass production. The best PCB design is a 4:3 rectangle. Do not change the wire width to avoid wiring discontinuities. When the PCB is heated for a long time, the copper foil is easy to expand and fall off. Therefore, avoid using large-area copper foil.