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Pb Drying / Degassing PWB

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Pb Drying / Degassing PWB | 21 November, 2008

Hi ,

i desire to find the type of information :

When the Soldering must be performed after baking for the PWB to not have a POP problem link to the moisture ?

And the time for a flexible PWB ?

thanks best regards

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Pb Drying / Degassing PWB | 22 November, 2008

IPC-HDBK-001 w/Amendment 1 Handbook and Guide to Supplement J-STD-001 (Includes J-STD-001B to C Comparison). 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*C||Baking Time [Hour] 120||3.5 to 7 100||8 to 16 80||18 to 48 Table 7-2 Maximum Storage Times After Bakeout Relative Humidity %||Maximum Recommended Storage Time [Day] 36||20 40||11 50||7

IPC-FA-251 recommends 121*C for 30-60 minutes for 1 and 2 flex layer circuits. A rigid flex can take easily an hour per layer depending on the construction. The more layers the longer the time. 140*C is above the rating of acrylic adhesives and could possibly damage the bond.

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Pb Drying / Degassing PWB | 24 November, 2008

Thanks ,

Have you a table more complet than the Table 7-2 or a formule to calculate with a precise humidity value ?

because for me it's important to know if is necessary to solder the component on the PWB with a wave solder process 8 hours before to solder the components ?

thanks best regards

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Pb Drying / Degassing PWB | 24 November, 2008

No, but we find discussions about relative humidity fairly short sighted, because relative humidity is such an imperfect measure of the amount of moisure in the air. Dew point is much more appealing.

R.H. = (amount of moisture)/(maximum amount that could be contained) x 100% If the moisture in the air remains unchanged and the temperature rises, the maximum amount of moisture that the air could hold increases. The numerator is the same and the denominator is increased, so the resulting percentage will be lower. [NIWA Science, Weather Plots: Temperature and Relative Humidity]

If the barrel plating is at least 1 thou [2.5 um], you should have no problems wave soldering boards that have not been de-moisturized.

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