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PCB Baking after Wash

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#57473

PCB Baking after Wash | 2 December, 2008

Need your opinion guys.. The old timers at the plant here feel that we should clean our board after Misprints withe the solvent cleaner over water wash. This is so they don't have to rebake the pcb for 24 hours before smt again. I don't believe this is true, or at least a minimal bake is needed. Please feel free to let me know your opinion.

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#57477

PCB Baking after Wash | 3 December, 2008

Hi System Steve,

What is a solvent cleaner over water wash? Solvent first and then rinse with water? And why do they feel the need to bake the boards? Do they think trapped moisture will cause board damage?

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#57481

PCB Baking after Wash | 3 December, 2008

boards are polyimide, and it is an ipa rinse with the solvent cleaner.

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#57495

PCB Baking after Wash | 4 December, 2008

Hi Steven,

The 24 hrs bake is an overkill according with various J-STDs. Assuming a bake temperature over the 100 Celsius degrees.

There are several factors to be considered before deciding the best methodology to wash misprinted board. However, there is no enough information in your previous message to formulate a recommendation.

However, you can contact me by email: maacolomer@yahoo.com or call me at 813-476-0755 to talk about the differents factors to be considered.

Thanks, M. Colomer, CSMTPE

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#57498

PCB Baking after Wash | 4 December, 2008

Dear Steven:

Often the recommendations for baking prior to assembly are derived from various J-stds. But J's currently concern baking moisture out of components, and fail to address the moisture issues of the PCB. Because of that, there is the IPC D-35 committee, of which I am a member. We're in our 4th year working to develop the IPC-1601, Storage and Handling standard, which intends to engage the whole of the electronic assembly (rather than selective componentry) in practices that will mitigate the effects of increasing thermal shock and latent moisture. The opportunity for the PCB to absorb moisture after wash-down is a given. How much moisture is a variable based on the PCB's type of exposure (spray vs. immersion) the kind of moisture (cleaners vs. water) time & temperature of the liquid and the type of substrate being exposed (hygroscopic properties are a variable). The consequence of latent moisture in the PCB can lead to delamination, a separation of the layers (or prepreg plies). These can appear and range from minute bubbles thru coin-sized blisters. The failure mechanism is simple: If the rate of trapped moisture expanding during a subsequent thermal pass exceeds the laminate's bonding agent strength, the bond fails and delamination begins. What the "old-timers" know is that a slow low[er] temp. pre-bake is good insurance against reflow or wave-shock delamination. eMail me if you're interested more on this subject, and I'll be glad to provide more from the PCB perspective.

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#57538

PCB Baking after Wash | 8 December, 2008

Hi Bob,

Do you have information about IPC D-35 for reference? What moisture sensitive level is the Sn-Pb/RoHS of PWB?

Does PWB supplier start to implement PWB drying packaging?

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#57543

PCB Baking after Wash | 9 December, 2008

SMT-ypw: Do you have information about IPC D-35 for reference? B3: The D-35 is a subcommittee responsible for developing industry guidelines on the proper handling, shipping and storage of printed boards and printed board assemblies. Moisture is a major concern and focus. To that end we are developing the IPC-1601 PRINTED BOARD HANDLING AND STORAGE GUIDELINES. As the 1601 is in Draft and not ready for Public release, I must refer you to IPC Staff Liason, John Perry, for specific reference materials: JohnPerry@ipc.org

SMT-ypw: What moisture sensitive level is the Sn-Pb/RoHS of PWB? B3: MSL's haven't been determined for PCBs yet. There is not a unified opinion within the Committee on this matter. Instead we may rely upon the substrate manufacturers' data on moisture dynamics for the various PCB base materials. Some of this information already exists, some does not, and there is not a standard method of refernce employeed by the substrate manufacturers. In the interim I recommend you pursue IPC-HDBK-001, Tables 7-1 and 7-2 for statements on relative humidity, storage and dehydration baking.

Hope that helps; you can eMail me for more information.

Bare Board Bob

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#57545

PCB Baking after Wash | 9 December, 2008

SMT-ypw: Does PWB supplier start to implement PWB drying packaging? B3: YES! The fabrication of the PCB involves a series of wet processing that requires counter-measures to remove moisure. While some of the PCB processes also include bake cycles that have dehydrating qualties (multilayer press, solder mask cure, letter screen cure, etc.) it is variously recommended that the PCBs receive a final bake specifically for dehydration purposes. So, yes, the PCB Fabricator starts the PCB drying process, but that does not remove responsibility from the Assembly operation to keep the PCAs in a low moisture state. - Bare Board Bob

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