PWAs & PWB bake out requirements

#51346

PWAs & PWB bake out requirements | 8 August, 2007

Hello All, I have been searching for the board assemblies bake out requirements acceptance criteria but unsuccessful. Does anyone knows if there are general guidelines for the percentage of the moisture absorption in the PWBs and PWAs?

Thanks, Tran

#51350

PWAs & PWB bake out requirements | 9 August, 2007

There are no requirements acceptance criteria for baking of board assemblies. What are your customer requirements?

The common reasons people choose to bake PCB are to prevent: * Delamination of multilayer boards * Measles, particularly on double sided boards * Outgassing solder joints, and solder voids / pinholes

The amount of pre-bake required depends on: * Thickness of the board * Construction of the board * Materials of the board * Amount of latent moisture * Acceptable level of moisture for the manufacturing process

Guidelines for baking are: IPC-HDBK-001 w/Amendment 1 Handbook and Guide to Supplement J-STD-001, Table 7-1 Baking Times and Temperatures

As an alternative to baking, consider having your fabricator package your boards in sealed vapor barrier bags (bulk quantities) immediately after final cleaning & post-lam bake. Keep the boards stored in the sealed bags until just prior to assembly.

#51353

PWAs & PWB bake out requirements | 9 August, 2007

DaveF, Our customer requirement is to comply with IPC-STD-001 but the J std doesn't specify the bake out temperature for populated boards and the moisture absortion rate. Right now we are baking the populated PWAs after aqueous wash at 80C at 18-48 hrs. I am trying to reduce the in process bake out time.

Thanks, Tran

#51354

PWAs & PWB bake out requirements | 10 August, 2007

Tran Since we all agree that J-001 has no baking requirement, why are you baking the boards? What is the purpose of this bake process?

#51360

PWAs & PWBs bake out requirements | 10 August, 2007

Davef, The J Standard-001-DS(Ref. 4.9.3)does required board assembly bake out and the purpose of the bake out is to demoisturize the PWB and to prevent board delamination during reflow like you have mentioned earlier. Thanks for your inputs. I have a new question for you Dave. Do you think if it's possible to reflow PWA that has QFP208 (fine pitch) on top and bottom side without using any glue? and if it is what are the meothods?

Tran

#51366

PWAs & PWBs bake out requirements | 11 August, 2007

Tran First, you never spoke of J-001DS in your initial post.

Second, J-STD-001DS requires no baking of assemblies, only boards [and for the anal, components].

J-STD-001DS, 4.9.3 Drying/Degassing. Prior to soldering, the assembly shall be treated to remove detrimental moisture and other volatiles using a documented process. This was changed from optional to a requirement because PCBs that are not demoisturized are susceptible to board damage and solder defects when exposed to soldering temperatures.

Finally, on your second side relow question: A second side QFP208 can be reflowed with out glue or fixuring. Admittedly its a QFP256, But if a 256 works, a 208 should be a can of corn. Look here: http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=29002

#51411

PWAs & PWBs bake out requirements | 14 August, 2007

My apology for not mentioned the J-STD-001DS. I didn't mean to trick you. Anyway thanks for your inputs. You are the man with the smt knowledge, I am sure that I will be asking you for some more answers again.

Thank you for the link. I will look for the double sided design formular that other have mentioned. Thanks, Tran

#51412

PWAs & PWBs bake out requirements | 14 August, 2007

Just talking off the top of my head, I'd guess that greater weights can be tolerated on the bottom side of two sided reflow when using lead free solder than leaded solder.

RDR

#51426

PWAs & PWB bake out requirements | 16 August, 2007

Here is what you need to do if you can,

Since you want to shporten the bake time you need to increase temp.

weigh a board prior to your current bake process (you need very hi resolution scale for this)

weigh the same board after current process and note what the amount of moisture was that you removed.

Increase temp to 125C and weigh another board before placing into the bake. Remove board on occasion and weigh until you have acheived the same result as before. there is your new process that should be significantly shorter.

I agree with Dave however that it would be better to improve storage conditions if you can. Maybe you could purchase a dry cab as well for PCB storage.

Your QFP will be fine if your conveyor is free from vibration.

Russ

#51460

PWAs & PWB bake out requirements | 17 August, 2007

My small reflow oven doesn't have the rails (only metal mesh). I personally don't think it was built for double side reflow assembly and there is vibration as well. We are a small low volume mfg house and our reflow oven doesn't have the capability to reflow QFPs. I think I will stick to the old hand soldering method.

Tran

#51461

PWAs & PWB bake out requirements | 18 August, 2007

Here's how to double reflow your QFP: * Reflow the first side with the QFP according to plan. * Paste second side and place components according to plan. * Lightly crinkle a small, QFP-size piece of aluminum foil. [Note: the degree of crinklization is dependant on the height of components on the first reflow side.] * Place the foil under the QFP on the mesh belt of your oven. * Reflow it.