Top Side Reflow Causing Solder Balls

Top Side Reflow Causing Solder Balls
When selective soldering the secondary side, we are producing solder balls on the primary side. What can be done to prevent this from happening?
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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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.


Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, coming to you from high atop Mount Rialto. What kind of question do we have today?

It comes from C.W. When selective soldering the secondary side, we are producing solder balls on the primary side. We are also reflowing surface mount components and it's on the primary side. What can be done to prevent this from happening?

Stop - my button has been pushed.

You've got to define what we mean by primary side and secondary side. I'm assuming that this board has been double-sided reflow, and that the primary side was soldered first, and the secondary side was soldered second. And now we are selective soldering this board from the bottom side where the primary side is now on top.  

Actually, Phil, it really doesn't matter in what order they were reflowed. The assumption I have to make is that the primary side is the side that is facing up while we're doing the selective soldering from the bottom. You see how easy it is to get confused about the physical situation we're talking about when we don't have standard terminology?  

But everybody's heard me rant about this. I think, Phil, that the pertinent question here is reflowing surface mount components on the primary side. And I'm assuming that that means re-reflowing. These are parts that have been reflowed once and now are reflowing again during the selective soldering process.  

Kind of like refried beans, except in this case, this is not a good thing. This just screams of a process out of control. Why are those top side components reflowing?

It's funny that C.W. asked about the solder balls first and oh, by the way, topside's re-reflowing. Oh, man. This is a mess. 

Well, obviously you're getting too much heat to the top side.   

Whatever combination of pre-heating and immersion in your molten solder, nozzle, or wave or whatever form of selective soldering you're using, you're getting too much heat into the top side of the board. It's just like in wave soldering, you can't reflow the parts on the top side.  

Actually, what's the worst thing that you can do is to partially reflow. And if you're getting reflow on some parts, you're probably getting partial reflow, and it's usually the partial reflow that will damage you.  

But the point is you're getting too much heat. You've got to go back to profiling your selective soldering process, putting thermocouples on the top and bottom side and adjusting those heating parameters so that you don't take the top side up above reflow temperatures.  

And I think that I agree with my brother - can you believe that, ladies and gentlemen - that the solder balls are just a secondary occurrence because of this overheating and reflow on the top side.  

What are the advantages of selective soldering? And I say this in the broadest sense of selective soldering. When I think of selective soldering I immediately think of using a selective soldering machine. But C.W., maybe you're using fixtures, which is termed as selective soldering.

One of the advantages with those pallets or selecting a machine is the idea is that you are protecting the rest of the area. You're isolating where you're applying your thermal process in this case where you really need it. 

Phil, listen to the question, brother.  He's producing, while soldering selective soldering the secondary side. So, if he is using a palletized wave soldering process, that pallet is covering the parts on the secondary side. The reflow is occurring on the primary side, the top side.  

The combination of preheating and wave exposure, or preheating and nozzle exposure in the selective soldering, is getting too much heat to the top of the board, and you're going to have to adjust those parameters and profile the board properly.  

What do you think this board looks like?  

Does he have a surface mount component so close to a through hole that he's soldering on the bottom side? It's transferring that much heat to reflow them.

Remember, Phil, top-side preheating in selective is something that's only become available lately. It may be on certain machines, it isn't understood or controlled well enough. People are improving and releasing new things about selective soldering machines every day. I can't pretend to keep up and be knowledgeable about all of it.  

Yeah, but I'm going add to that. The old standard still applies. C.W., hook up your profiling device, recorder, find out what the heck you're preheating that top side to, because it sounds like it's way too much if that's what's adding to it. You need to sit back and take a look at that process.  

If you need help, C.W., reach out to your selective manufacturer.

I think we beat this to death, Phil. So, ladies and gentlemen, wonderful Board Talk listeners, thanks again, and don't solder like my brother.  

Don't solder like my brother, selectively or otherwise.


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