Incomplete Joints on a PCBA Circuit Board

1.If the airflow in the solder reflow oven gets inhibited, it can cause some parts to not get heated enough for a good solder joint to be formed,which is usually called incomplete joints. This can be caused by larger parts blocking smaller components in a similar way to what happens with the shadowing of parts during wave soldering. Fortunately, there are steps that you can take while designing the circuit board that will prevent these problems from occurring.

In Figure 1, the lead-to-hole ratio is excessive, which has made soldering difficult. There is also evidence of resin smear on the edge of the pad. It may be possible, even on this design, to improve the soldering performance by decreasing the conveyor angle from 6 to 4°. This reduces the drainage performance of the wave but can lead to the incidence of shorting. Reducing wave temperature has also been seen to overcome the problem.

As a guide, the hole-to-lead ratio is normally the lead diameter plus 0.010", which is the normal guideline for automatic insertion.

Figure 1: The lead-to-hole ratio here was excessive.

Incomplete solder fillets are caused by poor hole-to-lead ratio, steep conveyor angles, excessive wave temperature and contamination on the edge of the pads.

The example shown in Figure 2 is a result of burring on the copper pads. During either drilling or punching, the copper on the surface of the board was deflected in some areas, making soldering difficult. The same thing can occur if resin is smeared onto the edge of the pads.

Figure 2: Burring on the copper pads caused this defect.