PCB Technical - PCB manufacturer, the principle of processing the via hole in the pad

PCB manufacturer, the principle of processing the via hole in the pad

Vias in pads are a very headache for electronics manufacturing plants, especially when the vias are placed on BGA (Ball Grind Array) pads, but the design unit is often not based on its design. Overcome the reasons and require the assembly factory to follow suit.

In fact, with the shrinking of electronic products, the high density of circuit boards is getting higher and higher, and the number of layers is increasing. Therefore, many CAD layout enginggers place through holes on the solder pads, especially the ball pitch. Small BGA pads (pads) don’t have much space for the vias, but placing vias on the pads saves space on the circuit board, but it’s a disaster for SMT and manufacturing engineers., Because it is likely to cause the following problems:

If the via is placed on the solder pad of the BGA, it is likely to form head-in-pillow or air bubbles inside the solder ball.

Because the solder paste is printed on the via hole, the air will be enclosed in it. When the circuit board goes through the high temperature of reflow, the air in the via hole will expand and escape due to the heat. Void/bubble will be formed in the BGA solder ball, and in severe cases, it will even cause head-in-pillow failure.

When the air accumulated in the via hole flows through the reflow oven (reflow oven), the air expands due to heat, which may cause out-gassing. This usually occurs in the reflow profile with poor preheating. When the temperature rises too fast, the air expands rapidly, and the gas cannot escape and eventually burst out of the solder ball.

The solder paste will flow into the via hole due to the pertaining to capillar, resulting in insufficient tin or lack of solder, etc.; or even flow to the opposite side of the board, causing a short circuit.

However, as product designs become smaller and smaller, layout engineers have reached the point where they must compare the territory of the circuit board, and sometimes there should be room for compromise. Therefore, there are some alternative methods to deal with the through holes on the solder pads. The following icons represent five types of through holes from A to E and their impact on the SMT process:

A) The vias are not processed at all. This should not be accepted by manufacturing engineers, because tin will flow through this via hole after being heated, resulting in insufficient soldering, empty soldering and other undesirable phenomena, and the amount of tin is completely uncontrollable, and it may affect the parts on the other side of the board.

C) Blind hole. It can barely be used, but there is still a big risk. The amount of tin can be controlled, but when the solder paste covers the semi-buried hole, the air will be blocked in the semi-buried hole. When the circuit board is heated by a reflow After the temperature is warm, the air will explode the solder paste due to expansion, or form an escape channel. Short-term use may be no problem, but after long-term use, it may crack from the escape channel, resulting in poor contact.

B) and D) are the best via design. There are no holes on the solder paste pad to affect the amount of solder paste, and no additional bubbles are formed.

E) It can be used, but the price is more expensive. A copper plating process can be added after the circuit board process to fill up the semi-buried holes. The filled holes will be slightly sunken, so they must be controlled within a certain size, especially for boards with 0.5mm picth BGA . Note: The board of this process generally increases the price by about 10%.