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How Frequently Should We Recheck Profiles?



How Frequently Should  We Recheck Profiles?
Is it necessary to replicate thermal profiling during long-term mass production? Is it necessary to recheck accuracy of reflow oven profiles periodically?
Board Talk

Transcript


Phil
Welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting. We're the Assembly Brothers coming to you today from atop Mount Rialto at ITM Consulting's Failure Analysis Cave.

We're here to talk about electronic assembly; materials, equipment, components, practices, procedures, relationships, catastrophes and anything else that comes up.

So today's question is?

Jim
It's about reflow. It comes from H.A.

Is it necessary to replicate the thermal profiling process during long-term mass production?

As a reflow expert, I would say it is not necessary to replicate the thermal profiling process, but it is a good idea to recheck the accuracy. The recipe to create the profile on the board and how frequently?

Most modern ovens today are very reliable and they have a lot of self-checks. Many people have the practice, whenever they change an oven, after it's stabilized, they will run the profile board again.

Another really important thing is to save a permanent profile board with permanently attached thermo-couples so you can always validate it. That's a really good practice.

Or some people use a calibration device such as an oven rider. Run it through the oven to make sure that you are getting the profile on the board that recipe is supposed to generate.

In long-term mass production where you're not changing recipes, if you never turn the oven off, maybe not. If you shut down daily or weekends it might be a good idea to validate the profile using your test board or tool.

What are we looking for? The oven has temperature controls so it should be producing the same temperatures.

Things can happen. Errors like a thermo-couple can come loose. It's reading the temperature, but it's not reading the same temperature relative to the process gas flow.

More significant is variations in gas flow because, if something happened with the fan or the diffuser plate, flux plugs up some nozzles or holes in your diffuser plate so the air flow pattern changes, you don't get the same heating effect on the board.

They're very difficult to monitor within the oven control system. There also are a number of third party tools that you can use. Some of them consist of an array of additional thermo-couples that permanently mount inside your oven and continuously monitor temperatures and heating rates.

That is a belt and suspender system that gives you a continuous validation of your profile if they're installed correctly and validated as a control tool. Yes, I think you should check it.

How frequently will probably depend upon how much confidence you develop in your oven, but the general practice is, whenever you change a recipe, before you start the first board, you check the profile.

Phil
I would add that the operative word here is, is it necessary? Maybe yes, maybe no. As Jim said, ovens are really stable these days. We've come a long way. Jim and I can remember 20 years ago when IR was a key component in the heating methodology and a combination of IR convection ovens. You would tend to drift within the hour.

Things like drafts in the room could interrupt the flow because you didn't really have positive pressure inside the oven. They've come a long way and today's ovens are very, very stable, as Jim said.

So necessary, maybe, maybe not. On the other hand, best practice, absolutely.

Jim
Consider the risks.  Let's take this scenario. Something happens to the gas flow. A fan is affected or you get some nozzles plugged up and you're not getting the same level of heating and it results in inadequate reflow on the center balls of a BGA.

How quickly are you going to determine that that's happening through your inspection and test? And what is the risk? What is the cost to that board if that happens? So it's a matter of potential risk.

Phil

Whatever you do, don't solder like my brother.

Jim
And don't solder like my brother. 

Comments

If you're in a high mix shop, the safest thing to do is run the oven rider through the oven while you're adjusting your stencil and doing other setup operations for each unique part number. It's your last verification the operator punched in the correct profile. (Usually part number controlled) It's always more expensive to find out the wrong profile was used after you're running and 10-12 boards are destroyed.
Bradley Fern, Entrust Datacard
Another impact on gas flow is dirt building up in the exhaust stack that reduces the cross-sectional area of the pipe. Then if the pipe is cleaned or replaced after a long time, the CFM rate of the exhaust changes significantly and throws off all the profiles. Regular maintenance is important for stability.
John Hasler, Cognex Corp.
During the article, there was no real mention of "time between rechecks". I was hoping to receive info about a general time frame for rechecking profiles. A suggestion like "every 5th run of a PCA", or whenever you shutdown for the weekend, you may want to recheck your first day's profiles, to ensure proper operation. Info like that was what I expected to find with this article, not just that is was "best practice" to recheck occasionally.
Jeff Vaessen, Borg Indak, USA
It is good practise - and an essential part of QMS systems like AS9100 - to have a regular documented preventative maintenance and calibration schedule for all equipment used in a prod cution facility. This is not a question of 'maybe, maybe not' - it is a must do to retain quality certification.
Robert Sharman, Device Engineering Incorporated, USA

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