What Causes Wave Solder Bridging?

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers. What do we have today?

Jim
A very generic problem, but it's very detailed. We are having continual lead bridging problems with our wave soldering system. How would you rank the following most likely and least likely causes and areas to investigate?

Here's the list: flux quantity; flux type; solder pot contamination; solder pot temperature; component angle through the wave; wave solder ramp angle; and other.

The first thing that jumps out is solder pot temperature. If your pot is too cool, the viscosity of the solder at the end of the wave is too high and the solder bridges don't fall off.

Remember, when your leads are in the pot, you have 100 percent bridges. Now, you're counting on the weight of the solder to cause the bridges to fall off. If the solder is too cool, the viscosity is too high, that doesn't happen.

Then the one that isn't on the list is preheat profile.

The flux has got to be preheated properly. You have to make sure it's fully activated, but not preheated so much that the activators are burned up and you get re-oxidation, because you're counting on your flux to hang around in the wave so that it helps the bridges to flow off.

Phil
There are a lot of other factors like pad spacing, the length of the leads, the design.

Jim
If you've got bad pad spacing that's outside of the IPC guidelines, you're going to have bridges. If you're trying to wave solder some of these new connectors with 40-mil pitch or 25-mil pitch, you're also going to have bridging problems.

You may want to go to a selective soldering.

Phil
These are all important parameters that have to be controlled, whether they're affecting bridging or not. So, they should all be controlled including solder pot contamination. It may not affect the bridging but you better have that under control.

Jim
All of these things could contribute to bridging, so it's a good list. I think we've given you some input on what we feel is important, but check them all.

Phil
This is Phil Zarrow and Jim Hall ...

Jim
Saying don't solder like my brother.

Phil
And don't solder like my brother, and watch for him under those bridges, too.

The flow off the face of the PC Board must be equal or greater than the horizontal speed factor.
PC board and design along with the correct flux selection. Law of Physics does not change.

Regarding IPC spec they are referring to. I suspect that it is IPC-D-275. Section 5 of that document goes into mind-numbing detail on everything you would ever want to know regarding pad and conductor spacing.

Using a rotational pallet, can allow the user to find the optimal angle to reduce or eliminate bridging prior to ordering production pallets. Also, optimizing the wave dynamic to achieve optimal dwell and peel off on at the exit of the wave will be critical to success.

Great intro to bridges. You reference an IPC Guideline for pad spacings- what IPC doc is this please?

Once you've verified the solder parameters (i.e. solder bath temp, contamination, etc) are in compliance, then the most likely source of solder bridging is lead length. Normal lead lengths seem to be about .030" to .060" protrusion. That also happens to be the worst length for shorts. We aim for flush to .020" protrusion and get no shorts at that length. Surprisingly, lead lengths at the long end can have less shorts than the .040 to .060 leads.

If the layout of your product allows, have a pallet designed with the product image rotated. The goal is to orient to provide the greatest amount of separation between connector leads as they travel across the wave.

I've used ~27 degrees on most dual row headers in our designs, but this could vary for your product. Also, order your pallets square, if possible. This provides flexibility to turn the product when responding to problems.