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Consensus for Baking Prior to Rework?



Consensus for Baking Prior to Rework?
What is the consensus for baking out moisture in circuit board assemblies prior to convection rework, localized mini-wave rework, or hand soldering rework? Phil Zarrow and Jim Hall, The Assembly Brothers, answer this question and share their own experiences.
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
Welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, pick and place, who by day go as ITM Consulting.  But here we are, Mount Rialto, Board Talk headquarters, here to answer your SMT Process questions.  

Jim
It's a rework question and comes from P.A.  

We often rework circuit board assemblies that may have disc drives, batteries, plastics, or other heat sensitive components. What is the industry consensus for baking out moisture in circuit board assemblies prior to convection rework, localized mini-wave rework, or hand soldering rework?

In each case we are scrapping the removed components. So we're not worried about baking time on those. In addition we are masking and shielding the adjacent areas to isolate the extreme temperatures of the rework site. Should we be baking the board per IPC J-STD-033? Or can we utilize IPC 1601, Handling and Storage Guidelines, which focus on the board material?  

He or she has really done their work. The simple answer is: we're concerned about the board so you want to use IPC 1601. As you said, in your rework process, you're shielding your other components. IPC J-STD-033 refers to components.  You've defined it very well. You're not worried about the components you're taking off.

You've got to think about the components you're putting on of course, and that should've been dry according to 033.  But what you're concerned about is delamination of the PCB.  Even though it's localized, if there is moisture globally in the board, you could damage it at or near the site where you are applying the (to use your term) extreme temperatures.  

So yes, and IPC 1601 gives you guidelines for exposure times and bake cycles and everything like that. Yes, you very carefully defined the issue and we agree it's the board you're concerned about in rework.  

Phil
Very good.  And nice work P.A. for doing your due diligence.  It's refreshing to hear.  

Jim
IPC 1601 is a good spec. The answer is that IPC 160 and J-STD-033 are parallel. They both talk about the absorption and moisture, potential damage during reflow (or in this case rework) and  J-STD-033 talks about components and 1601 talks about the PCB itself.  

Jim
Enough number dropping.  

Phil
Enough talking about boards, exactly. Speaking about bores - I mean boards, whatever you do when you are soldering your board.  

Jim
Don't solder like my brother.  

Phil
Don't solder like my brother.

Comments

I have seen a lot of PCB damage done to boards by assembly houses that don't pre-bake boards. A low temperature and a lot of time usually does the trick. 150F and about six hours should be sufficient, and should not harm any components on the board. You may need longer times if the boards have been out of their package a long while. This step should also be done before assembling if PCB's are not "fresh". PCB's can still absorb ambient moisture even if sealed in bags. IPC 1601 provides good guidance.
Bradley J. Fern, Entrust
Warming the PCB in a dry cabinet as another reader suggested is great. Just remember to give it enough time. If it's a complex PCB with internal copper planes, it will slow both moisture uptake and moisture release. If the board has been in ambient environment for some time, it will take a while (as long as 24 hours or more). If there is moisture under the copper, and you heat it, it will delaminate. Keep in mind the copper plane may be one or more layers deep from the surface pads.
Chris Hunrath, Insulectro
Warming the PCB in a dry cabinet as another reader suggested is great. Just remember to give it enough time. If it's a complex PCB with internal copper planes, it will slow both moisture uptake and moisture release. If the board has been in ambient environment for some time, it will take a while (as long as 24 hours or more). If there is moisture under the copper, and you heat it, it will delaminate. Keep in mind the copper plane may be one or more layers deep from the surface pads.
Chris Hunrath, Insulectro
You must also be aware of process sensitive components that may not be able to withstand the bake out conditions recommended by either standard. See J-STD-075 for more information.
Curtis Grosskopf, IBM
I cannot believe that people say to remove the components first, this is very time consuming. It is not necessary, you can dry them fast in a 60C 1%RH low Humidity cabinet, without causing any oxidization and wettability problems.
Jos Brehler, Totech EU BV, Netherlands
Your program did not address the issue of whether or not baking is required. It referred to the IPC spec and stopped there. In order to make your programs more valuable to the listeners you should be talking about case studies and the like.
Tim Bibens, Surmotech
Dry your boards, otherwise they'll develop local de-lams ("Measles") under clumsy point heating. Measles are non-repairable. Well-dried boards are much more robust to sequences and processes of rework and special soldering ops. Wet boards will cause all sorts of mischief. Bake-out under nitrogen if you have vulnerable pad finishes. Store in dry cabinets, in-process.
Tom Clifford. TJB Associates

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