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Finding the Cause of Cold Solder Joints



Finding the Cause of Cold Solder Joints
We have continual problem with our wave soldering system, especially cold solder joints. Could the cold solder joints be caused by using a selective soldering frame? Phil Zarrow and Jim Hall, The Assembly Brothers, discuss this question.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil
And welcome to Board Talk with Jim and Phil, the Assembly Brothers or if you are from a right-hand drive company, Phil and Jim. We are here to talk to you about surface mount assembly processes, conditions, situations, problems, everything. We are here to solve your problems, or at least give you a bunch of diatribe on it.

Jim, today we have a question from T.H. It is a selective soldering question. T.H. writes, we have continual problem with our wave soldering system, especially cold solder joints. Could the cold solder joints be caused by using a selective soldering frame? Okay, obviously here he is referring to the selective soldering aperture pallet. I assume that is what he means. These are a great way, for the most part, of implementing a wave solder machine for boards that have surface mount components previously reflowed on the bottom side. But they do have their caveats.

Jim
Wave soldering is tough anyway because it is a combined process. You have to apply flux. You have to apply pre-heat. Then you have to immerse into the wave and you have to get all of the bridges to fall off when you are all done. We all know the degrees of difficulty involved with this. But now when you have an aperture pallet or a selective soldering frame, whatever you are going to call it, you have to get that whole process up into a small aperture or a hole in the bottom of this frame or mass.

If you think about it, you have to get the flux to go up there into the aperture, then into the holes. Then you have to get the preheat up there to activate the flux all the way to the top of the board. Then you have to get the liquid solder to flow up there. I am guessing, from this description Phil, that they are getting inadequate preheat.

They have not adjusted their speed, their preheat temperatures and their configuration, their preheaters to get adequate heat up into the apertures in their pallet and then up into the lead and the barrel of the hole. They get the immersion. In this type of soldering actually, getting the liquid solder up there is the easy part because liquid flows. The flux and the preheat tend to be the problem.

I am guessing here that the preheat is just not adequate. That the frame, in that relatively small aperture, is not letting enough heat get up into the board and bringing the leads and barrel of the holes to wetting temperatures. So, the liquid solder gets pushed up in there but it doesn’t really stick. It is chilled quickly by the cold lead and or the barrel, giving you a cold solder joint. What he’s describing is a cold solder joint.

Phil
Maybe a certain degree of dewetting or non-wetting. It kind of boggles your mind. Wow, we are shooting a molten solder up into the hole, how can it be cold? It is hot and molten in the first place. Like Jim said, not getting adequate flux coverage among other things.

That could be everything from your flux delivery methodology, a lot of time is has to do with hole to pin diameter, and geometries that way. As Jim is saying, the frame itself the actual aperture fixture could be sinking heat in the wrong places and / or combined. Check all of this out. We have a friend, his whole profession is making aperture pallets for selective soldering, he agrees that when it comes to wave soldering, basically it is just another piece of duct tape trying to make wave soldering work with our modern day requirements. I guess that is what it is.

Jim
I am thinking technically, back to my theory the pre-heat. Perhaps if you could configure your machine to give you more top-side pre-heat. Putting more heat down into the board from the top might be a solution for your problem. Of course, the proper way to do it is to instrument one of the areas where you are seeing cold solder joints and maybe following our compatriot Bob Klenke’s thing. Put not just a thermal couple on the top and the bottom surface of the board but bury one in the hole. Make sure you are getting the heat all the way to the center of the hole, halfway through the thickness of the board. If not, then the molten solder is not going to stick and you are going to get wet or dewetted solder joint.

Phil
You have to do your due diligence. You have to check your pre-heat. This is never an opportunity to do things seat to pants. Well good, I hope we helped T.H. in his direction here. We look forward to whatever comments and additions.

Jim
I wonder if that stands for through-hole? Whatever you do, ladies and gentlemen, don’t solder like my brother.

Phil
And don’t solder like my brother, please.

Comments

A recommendation would be to make a profile of the different temperature zones, in this way I have been able to correct similar problems. Also check IPC-7530, Guidelines for temperature profiling.
Roger, Camtronics
Aperture pallets are large heat sinks. As usual, Phis and Jim are spot on with checking (and probably increasing) the pre- heat temperature.

Another thing to look at may be the pot temperature of the solder. In wave soldering, the amount of dross generated increases exponentially with the pot temperature. With the relatively small area of exposed molten solder, and a nitrogen inert environment at the nozzle, dross is less of an issue in most selective soldering machines.
Mitch Holtzer, Alpha Assembly Solutions

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