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Bare PCB baking

Yngwie

#22429

Bare PCB baking | 19 November, 2002

Hi Experts out there, pls help me with the following matters.

How important is the bare PCB baking ? What is the temperature for baking @ what RH ? What would be the impact for the ENTEK finished board should the baking is really necessary ?

For the board that is to be reworked, what is the baking temperature ? Is the risk of getting the lifted pad can be eliminated or reduced if the board is sent to baking first ? IF so how does the moisture impact the hold-up strength of the pad onto the FR4 ?

Pls help. Thank you in advance.

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#22457

Bare PCB baking | 20 November, 2002

Q1: How important is the bare PCB baking ? A1: Baking boards is a non-value added activity.

Q2: What is the temperature for baking @ what RH ? A2: Search the fine SMTnet Arcives for bake recipes.

Q3: What would be the impact for the ENTEK finished board should the baking is really necessary ? A3: Solderability of Entek boards decreases during each additional thermal cycle.

Q4: For the board that is to be reworked, what is the baking temperature ? A4: See previous discussion on bake recipes.

Q5: Is the risk of getting the lifted pad can be eliminated or reduced if the board is sent to baking first ? A5: Lifted pads are generally caused by operator falling asleep while holding their solding iron on the pad. So, no baking is a waste of time and money and will not cure the sleeping operator problem.

Q6: IF so how does the moisture impact the hold-up strength of the pad onto the FR4 ? A6: If the board was fabricated properly, humidity has no impact on peel strength of pads.

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Yngwie

#22460

Bare PCB baking | 20 November, 2002

Hi Dave,

What do you mean by "if the board is manufactured properly". If the board is not manufactured properly, humidity will have an impact on the peel strength? Is that what you mean. How does the Humidity impacted the peel strength ? Thru' which angle ?

I don't believe that the peel off/or lifted pad is solely operator dependence. They must be something that could lead to the problem if I don't faced problem on all othehr products except for the one that I'm having th eproblem with. I just don't know what is that variable.

Anyway, is there a standard for reference should be there is a need for PCP baking ? I saw the thread from an archive on possibity of including this spec in the J-STD-033. Will that be materialised or just to make someone feel comfortable ?

Thanks for you input Dave.

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MA/NY DDave

#22475

Bare PCB baking | 21 November, 2002

Hi

I can't give you the answer for your question because I have too many questions and would have to know a lot more before I was comfortable giving any advice.

I have done mass Baking of piles of PCB's as a needed evil when piles of boards were exhibiting destructive outgassing during soldering.

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#22478

Bare PCB baking | 22 November, 2002

Q1: What do you mean by "if the board is manufactured properly"? A2: If the epoxy is noy cured prooperly, for instance, the copper will not be well attached to the glass. So, peel strength will be lower.

Q2: I don't believe that the peel off/or lifted pad is solely operator dependence. A2: It may not be. Why not do this correctly and give us a chance? Tell us about the materials, processes, and process parameters involved in: * Board fabrication * Assembly

Q3: Is there a standard for reference should be there is a need for PCP baking ? A3: There is no standard. It is not good practice to bake. The guidelines in the fine SMTnet Archives are all you get.

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#22483

Bare PCB baking | 22 November, 2002

The follwing is from the Technet archives, per IPC's Jack Crawford. I dug it up during some investigation of a supplier's issues with humidity in an Asian plant:

There is support for this in IPD-HDBK-001 w/Amendment 1 Handbook and Guide to Supplement J-STD-001 (Includes J-STD-001B to C Comparison). I've copied text below, and there are also pointers to other good reference documents. (Ok, I know--two HDBK responces in one hour, but it's a nice book with a tremendous amount of information to support soldering requirements; 165 pages and only $35 for IPC members.) Jack

During fabrication and storage, both components and PWBs will often absorb water. If left in the device, this water will vaporize at soldering temperatures and can lead to PWB delamination, soldering voids (especially in PTHs), and device cracking.

For PWBs, the bakeout removes water accumulated during the fabrication process and absorbed during storage. Recommended baking times and temperatures 2 are given in Table 7-1. Longer bakeout times and higher temperatures are not recommended, as they can degrade PWB and component solderability.

Water re-absorption begins immediately upon removal of the PWB from the oven and is linearly related to RH. For a storage environment of 20�C and 30% RH, a maximum interval of two to three days is recommended with the interval decreasing with increasing humidity .

Plastic encapsulated devices, especially ICs, also have a tendency to absorb water from the air, which is violently released during soldering. Typically, 1000 ppm of absorbed moisture is considered a maximum content beyond which device failure due to body cracking may result. Bakeouts similar to those used for PWBs have been successful in eliminating these defects. After baking, the parts again begin to absorb water. Recommended maximum storage times after bakeout as a function of RH at 25�C 4 are given in Table 7-2, based on the time to achieve 800 ppm of water (see 5.0.9).

Table 7-1 Baking Times and Temperatures Baking Temperature Baking Time 120�C 3.5 to 7 hours 100�C 8 to 16 hours 80�C 18 to 48 hours

Table 7-2 Maximum Storage Times After Bakeout Relative Humidity Maximum Recommended Storage Time 36% 20 days 40% 11 days 50% 7 days

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