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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

Humidity Control

Victor salazar


Humidity Control | 22 November, 2000

We have some humidity issues due to monsoon season in southern Arizona, I am having issues with water soluble solder paste, and moisture sensative IC's. We are trying to maintain a 32% relative humidity for EESD purposes. What is the standard, and what are some common methods of humidity control

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Chris May


Re: Humidity Control | 24 November, 2000


I am wondering why you are keeping the RH as low as 32% ??

The lower you go, the higher your ESD problems will be. The voltages generated at about 40% are roughly halved when you reach 55%

I believe that an acceptable level is about 55%-65%.

As far as control goes, well, how much money have you got ? how many areas and volumes are you talking about ?

For small areas (labs etc;) you would be able to buy portable plug in appliances, but for a "shop floor" you probably need a big expensive HVAC system.

If you have large areas, chop them up into smaller ones if practicable.

Probably not much help, but over here in cold damp England, we don't get the same issues that you do.

Good Luck,


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Dason C


Re: Humidity Control | 27 November, 2000

Chris, you are not completely right. Our industrial is following the IPC standard as our guideline.

Per IPC J-STD-033 has specify the floor life at para. 8 (normal condition at 30C/60%RH). If Victor can control his environment at 32% then the component floor life can extend and protected.

Further, if we can't complete to use up the component within the period, baking process must required. It will cause the oxidation and intermetallic growth.

Victor, can you advise what is your RH level at your area and how you can control the humidity at 32%. Currently, we are using a elextronic dry box and place at every line.

Rgds. Dason

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Re: Humidity Control and Moisture Sensitivity | 5 December, 2000

Victor, here are a few more elements of information to answer your question.

Both the temperature and relative humidity on the manufacturing floor have a significant impact relative to the rate of diffusion of moisture inside a plastic package and the resulting maximum floor life. This limit will either decrease or increase if the ambient conditions are above or below the 30C/60%RH.

The IPC/JEDEC standard J-STD-033 (available for free at includes a table to take this into account. (Re. Table 5. Derating due to factory environmental conditions). Unfortunately moisture diffusion is a complex physical phenomenon and the de-rating table reflects this situation. The de-rating factor depends on the actual classification of the component (level 1 to 6) and the package body thickness. The impact of relative humidity is very significant. For example, a thin Level 3 package (TSOP, TQFP), which is safe for 7 days at 30C/60%RH, will actually exceed the safe limit after only 1 day at 70%RH.

It is a very good practice to use dry box on your manufacturing lines as Dason suggested. However, you must be aware that putting previously exposed parts in dry storage does not mean that they are completely safe. Initially, the previously absorbed moisture will continue to diffuse towards the center of the package where is is more likely to create damage during reflow. A recent paper by Shook and Goodelle from Lucent cited an example that a PLCC device classified Level 5 (normally 48 hours floor life) will actually exceed the critical moisture level after having only been exposed 16 hours, followed by 70 hours of dry storage. This is why the IPC/JEDEC standard does not allow any provision to stop or re-set the clock of exposure time once the parts have been exposed more than one hour (except for a complete bake or for a very specific condition of room-temperature desiccation as defined by the rule for short-duration exposure).

I have done a lot of research lately on the subject of Moisture Sensitivity and would be glad to answer any of your questions on this subject.

Regards, Francois.

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