Hi Juliano,
I've been through this same situation in the near past where we had to establish baking requirements for an assembled CCA due to baking for repair causing part failures. The two standards don't quite address built assemblies specifically, but with a little bit of effort one can extrapolate a decent plan. Below is what we did and it seemed reasonable and thus far has been pretty good.
The IPC-1601 addresses an unpopulated board, so that is good before the fact, but not so good after.
The J-STD-033 pertains a bit more to components, but one can reasonably look at the assembly as component as well. So first we need to determine an MSL (moisture sensitivity level) for the assembly as a whole and then a maximum temperature it can be baked which will establish the time to bake.
MSL Level: The pwb we determine to be a MSL 3 in discussions with our fabricators due to # of layers and material. Then we reviewed the components for the highest MSL level on our particular assembly, some were 3. So for my example, I'm going to assume it's a MSL 3 for the whole assembly - first part done. If a part had an MSL level of 4, it would have to made the whole assy 4, but luckly we didn't.
Baking Temp: This one is a little tougher. Some components, such as, tantalums, LEDs, oscillators (especially as they can go off frequency) and many others don't respond well to high baking temperatures and I've seen issues with baking too high (e.g. 125°C). So we used the max storage temp and/or max operating as our maximum baking temperature because these parts will be going through an extend heat cycles (4hrs or more), more akin to storage/operation. We didn't use the max temp established by the reflow profile because that is really for very short duration(minutes), not several hours. For many parts their max storage/operational is 85°C (sometimes more) and we used this number to establish our max bake temp. So now we have MSL 3 at a max temp of 85°C.
Time to bake: Using our MSL 3 and max temp of 85°C, we referenced Table 4-1 of the J-STD-033. Here we also need to know the thickness, so we used the worst part as our guide and often this is pwb or a BGA. So assuming the worst in our assembly is the board at 1.5mm, we can use the 90°C (yes I know I said 85°C, but it's close) at MSL 3 to arrive at about 4 days = 96hours. So we have 85°C for 96 hours.
Floor life (e.g. rework time) at this point can be established by Table 5-1 in the J-STD-033, which for MSL 3 would give us 168hours. Most rework can be done in 168hrs. And floor life can be halted/extended by putting boards in dry storage or controlling the work environment.
Obviously, there are many knobs here. One is the MSL level of the board/assy, next is baking temperature and third is the thickness. In combination these should be evaluated for your design to determine the most appropriate bake time. One size won't fit all (it depends!).
Couple of notes here, the folks doing the repair may not care for the long bake time at a lower temper as will affect throughput. But we've had issues of part operational changes (e.g. lessons learned) due to too high of temps for too long and have found a lower but longer bake resulted in less change. And your repair folks need to plan for this additional time.
Hope that helps.
-Jeffrey
Jeffrey A. Jenkins
Sr. PCB Staff Designer, CID+/CIT
L-3 Communications, Linkabit Division
Work: 858-552-9832
Email: [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Datacom - Juliano Ribeiro
Sent: Friday, October 23, 2015 4:07 AM
To: [log in to unmask]
Subject: [TN] Baking to populated boards to rework
Hi folks
We would like to know how much time and temperature to baking before rework.
We have some boards that is necessary to replace the BGA, but the boards are in the warehouse around 6 months. We would like baking in those boards before replace the BGA, so what's the time and temperature?
Thank you
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Juliano Bettim Ribeiro
DATACOM
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