"Lean" may be the rage, but fat solder joints are wholly reliable.

Aside from gobbling up too much solder, are fat solder joints in wave soldering more than a cosmetic problem? And is there anything that the process engineer must, or should, do?

Figure 1 shows a group of identical wave-soldered joints with different joint shapes: fat and standard. The reliability (mechanical and electrical) of a fat joint is actually the same as that of a commonly shaped joint. One should therefore never rework such joints to transform them into the common shape, as doing so will cause more harm than good.


The visual quality aspects set forth in several standards were defined at a time when almost all solder joints were made by hand with a soldering iron. In judging solder joints, one of the quality aspects is the solder fillet shape and wetting angle. This control aspect was used for hand-soldered joints to ensure that no poor solderable joints were “buried” with solder. In machine soldering, however, it is impossible to get (much) solder on a non- or poor solderable pad or lead. So, if we sometimes have much solder on joints that are machine soldered, we know there is no question about the solderability. The “fat” solder joint is caused by the solder drainage conditions and should therefore not be reworked, because it is a perfectly reliable joint.

When wave-soldering circuit boards without solder resist, one will never find fat joints. This is because of the liquid solder surface tension. Excess solder, which is drawn from the solder bath when the joint leaves the wave, will drain to the traces that are connected to the joint. However, in the case of a joint in which traces are covered with solder resist, excess solder will remain on that joint because there is no mechanism or opportunity for the solder to drain. A fat solder joint may thus be the result, but such a fat joint is nevertheless a perfectly acceptable joint if there is evidence of a component lead in the solder (e.g., by comparing adjacent joints or looking to the contour edges of connecting parts), because in wave soldering, solder will adhere only to a solder connection when the latter’s solderability is perfect.

When the solderability of a joint is not perfect, then such a joint will show non- or imperfect wetting and will never result in a fat joint. Thus, a fat joint in wave soldering is always a perfectly solderable joint. It is merely fat because more solder was left at the joint as it was separating from the last wave. Such a joint should not be touched up; it is perfectly functional, and the reason why these joints form is known.

Indeed, it is impossible, in a wave-soldering process on boards covered with solder resist, to avoid such joints, because the separation of solder from the board during its exit from the wave is more or less coincidental and very much dependent on the layout and solder drainage conditions.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. This column appears monthly.