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September 22, 2008

When is oven drying mandatory

I have in stock printed circuit boards that may have absorbed water over time, the boards are manufactured and vacuum packed with desiccant. We are seeing blisters after reflow. What is the recommended storage period before oven-dry process is mandatory?



A.V.

Experts Comments

Blisters after reflow could to be caused by wrong reflow profile in the reflow process. Check the temperature profile first, then the pcb material.

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EH Lim
Managing Director, Asia Pacific
ECD
EH Lim has been in the PCB Assy industry since 1985, starting at Thomson/Singapore for 5 years before moving to Electrovert Asia Pacifc. Lim was Sales Director for Vitronics Soltec prior to joining ECD in 2007 as Managing Director for Asia Pacific.

Just because your boards were stored in Moisture Barrier Bags (MBB) does not guarantee their safety for processing, and your symptoms certainly sound like vapor damage. First of all, not all MBBs are identical, with varying Moisture Vapor Transmission Rates as well as varying mechanical strength.

Also, the amount of desiccant required needs to be properly calculated according to IPC/JEDEC, EIA or Mil spec standards. Errors at packaging can negate the bags' effectiveness, and excessive package vacuum can stretch the bag material which makes it porous and more susceptible to leakage. The Humidity Indicator Card (HIC) is the only reliable gauge of the moisture content of packages in a MBB.

With the introduction of lead free, the risks associated with marginal or improper handling of MSDs has increased dramatically. Higher reflow temperatures drive up the saturated vapor pressure to double or even triple that present in tin lead reflow processes.

The higher temperature increases the water pressure inside the components, and as a result, the allowable moisture content and associated floor life have to be reduced. All moisture-sensitive components have to be requalified by their manufacturers for lead-free and they are typically downgraded by at least one and generally two levels of sensitivity.

This significantly affects how components must be are handled, processed and stored. Suddenly, manufacturers that are currently handling MSL 2 or MSL 3 components are now looking at processing MSL 5 or MSL 6.

The SMTA's MSD Council is among the those who have reported that even passive components (ceramic chip resistors and capacitors), normally classified as MSL 1 ( unlimited floor life) have shown field failures after lead free reflow due to presence of moisture in the component.

Another major impact of lead free reflow for assemblers is the handling and storage of PWBs, as the incidence of delamination has increased with the change in thermal processing. Many share the conclusion now that PWBs need to be treated as MSL classification 4, in other words processed within 72 hours of open shop floor time.

The newest generation of high precision, ultra low humidity desiccant cabinets offer both storage and a drying solution for all MSDs. They utilize Zeolite A known to be the most effective desiccant available. Referred to also as "molecular sieves" these are desiccants with some differing properties to those of silica gel typically used in MBBs. (http://www.superdry.info/)

They are synthetically produced, highly porous crystalline metalalumino silicates and possess many internal cavities that are linked by window openings of precise diameters. The patented technologies of companies such as Totech Super Dry that incorporate Zeolite A have recently been expanded to offer 0.5% RH and thermal controls offering optimum storage as well as accelerated drying per IPC JEDEC standards.

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Richard Heimsch
Director
Protean Marketing
Now a director at Protean Marketing, Mr. Heimsch has worked in the electronics industry 25+ years in a wide variety of international sales, marketing and operations roles. Rich spearheads Protean's international business development, specializing in Brand Management and Strategic Communications.
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