Problems with Reflow Oven Profiles

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 With over 50 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 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
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, who normally try to get employment through ITM Consulting, but today, we're here to talk about your problems with surface mount technology related issues and it could be equipment, materials, processes, people, whoever, whatever.

Jim
We've got a reflow problem from J.R. in Abertine, Texas.

Phil
Okay, and let's see what old JR has to say. He says, "Our company is a low-volume, high-mix EMS." There are a lot of those out there. "I'm a good doobie. I profile every individual board."

"I use at least three TC's on every board and more on complex ones." So far so good. "I've heard you guys talking about attaching TC's with epoxy or a high-temp solder. Well, I use Kapton tape and I get good results."

Jim
You say the TC's stay attached. How do you know, JR? Do you ride the boards through the oven?

Do you have webcams in your reflow section? Kapton is a great material, that's not the problem. The problem is the adhesives. The adhesive that holds that Kapton layer on are temperature sensitive. They soften at high temperature and when it softens, contact between the thermal couple and what you're trying to measure can be reduced and it doesn't take much to give you an inaccurate reading.

Remember, the greatest risk is when the temperature's the highest. That's at peak reflow temperature, and that's probably the most critical point, particularly if you're doing lead-free and you're working with a really tight process window.

What's confusing is as you go through cooling, the adhesive re-hardens and it looks fine when it comes out. So the bottom line is, the Kapton tape works most of the time, but you can never be sure on any given profile on any given thermal couple.

So what should you do, Phil?

Phil
First of all, the best procedure we have found, and believe me, we've tried lots of things over the years. The first thing is once you pick your point, the joint where you're going to be attaching your thermal couple, you want to remove as much of the existing solder that's there first. Get it off there. And this is what's going to save you lots of headaches because otherwise if you leave the old solder it's going to undermine the connection.

And unless you want to be doing this over and over and over again. You want to do this right. Use some solder wick. Get as much of that old solder off and then you want to use a very small amount of solder, high temp solder or a conductive epoxy.

The reason we emphasize using a small amount is you don't want to have too big a mass at that interconnection that may give you wrong readings, inaccurate readings. You do that and you know what? You know all that Kapton tape you bought, use it for strain relief.

So again, this is the most time-tested method, either high-temp solder or the conductive epoxy. As Jim said, we've tried everything. Did I ever tell you about the time I tried using RTV? It was one of those things that seemed like a good idea at the time, but well, we won't go there.

Okay, we are Phil Zarrow and Jim Hall, the Assembly Brothers, and we bid you good day. And remember ...

Jim
Don't solder like my brother.

Phil
Don't solder like my brother.

Jim
And keep the kids away from the solder pot.