Down the Drain

Figure 1 shows a closeup photo of a PCB assembly, it seems as though solder has flowed “down the drain” and away from the solder joint where it’s needed.

In fact it has, because the customer has inconveniently located a via right through the center of one of the two topside SMT pads for a surface mount component. When the assembled PCB is run through reflow, the molten solder drains away through the barrel of the via and out the other side of the PCB. There isn’t enough solder remaining post-reflow to create an acceptable solder joint per IPC-A-610. The joint is “starved”; this is unacceptable. What to do?

Figure 1. Insufficient solder, i.e., "starved" solder joint on an SMD pad.

Figure 1. Insufficient solder, i.e., “starved” solder joint on an SMD pad.

The via is there to stay, by virtue of the customer design. So, no matter how many times solder is added to the joint, every time the PCB is run through the reflow oven the solder is going to drain away because the PCB , including the via, is at reflow temperature.

Obviously, more than one run through the oven makes no sense. The only practical solution is to manually add solder to the individual solder joint, post-reflow, without running the entire PCB through another thermal cycle. It’s a touchup procedure that’s required to create a robust SMT solder joint that meets acceptability criteria. This is a manual PCB assembly soldering process that should be performed by a skilled hand-soldering or rework operator. Solder is added only to the joint, via cored wire solder or solid wire with flux, in order to build up the volume of solder at the solder joint to provide strength, connectivity, and an acceptable meniscus per IPC standards, covering the via drain-hole. The solder won’t flow through the via because only the surface joint area is heated.

Figure 2. The solution: Add solder to the joint manually via a touchup procedure.

Figure 2. The solution: Add solder to the joint manually via a touchup procedure.

It may seem tedious, but a skilled operator can touch up the joint in a few seconds, and if there is only one instance per assembly it won’t appreciably cause production delays.

Roy

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