PCB Technical - PCB process BGA simultaneous dry welding and possible causes of short circuit

PCB process BGA simultaneous dry welding and possible causes of short circuit

When circuit board manufacturers produce BGA, they are always prone to short circuits and empty soldering, and they are all new materials.

Generally speaking, there are not many cases where there are empty welding and short circuit at the same time in BGA welding, but it is not impossible. The edge is upturned, forming a curve similar to a smiling face, and the circuit board is too long because of the too long TAL, and the temperature difference between the upper and lower furnaces of the reflow furnace is too large. The two-phase interaction forms the edge of the circuit board, causing the so-called crying. Face curve.

If this kind of crying and laughing curve is severely deformed, BGA short-circuit and empty welding will be formed at the same time, but it is usually easier to have such a problem when both occur at the same time. The picture below shows that the smiling face of the BGA and the crying face on the printed circuit board strongly squeeze the solder balls of the BGA, so that it is almost a short circuit. Generally speaking, you can consider reducing the slope of the temperature rise of the reflow furnace, or preheating and baking the BGA to eliminate its thermal stress, or requiring the BGA manufacturer to use a higher Tg... and other methods to overcome it.

Other possible reasons for the empty welding of BGA are:

The solder pads or BGA solder balls of the circuit board are oxidized. In addition, if the printed circuit board or BGA is improperly moisture-proof, there will be similar problems.

The solder paste has expired.

Insufficient solder paste printing.

The temperature curve setting is not good, and the furnace temperature should be measured at the empty welding place. In addition, the above-mentioned crying and smiling face problems are more likely to occur when the temperature rises too fast.

PCB design issues. For example, Via-in-pad (via-in-pad) will cause solder paste reduction, in fact, it may also cause solder ball voids and blow up the solder ball.

Pillow effect. This phenomenon often occurs when the above-mentioned BGA carrier board or printed circuit board is deformed during reflow soldering. When the solder paste is melted, the BGA solder ball does not touch the solder paste. When it is cooled, the BGA carrier board and the circuit board are deformed. Decrease, the solder ball falls back and touches the cured solder paste

The general methods of analyzing BGA air welding are nothing more than the following:

1. Use a microscope to inspect the outer BGA solder balls. Generally, you can only see the outermost row of solder balls. Even if you use fiber optics, you can only inspect the outermost three rows at most, and the more you see it, the less clear you can see.

2. X-Ray inspection. It is easy to check short circuit, check the dry welding to see the skill.

3. Red Dye Penetration (dyeing red test). This is a destructive test. It must be used as a last resort. You can see the places of fracture and empty welding, but you need to be careful and experienced.

4. Slicing. This method is also a destructive test, and it is more labor-intensive than the red staining test, which is a special enlargement of a certain area for inspection.