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Moisture absorbtion in circuit boards

Carol Stirling

#19829

Moisture absorbtion in circuit boards | 8 May, 2002

We have a concern with moisture absorbtion/humidity effects on PWBs specifically used for automated SMT (including BGA).

I'm sure there is a IPC spec on this and would appreciate the number - I've searched the IPC web-site -no results.

Could you tell me the recommended time boards can be stored in sealed bubble pack without impact on the manufacturing process please? Assume the boards have not been baked prior to sealing, nor stored in ambient temperature.

Also, what is the impact on solderability?

As usual, your help is very much appreciated.

Carol

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#19855

Moisture absorbtion in circuit boards | 9 May, 2002

There is no IPC specification, nor should there be. * There are just too many peculiarities in the design and fabrication of PWB. * The issue is not defining how to store boards. The issue setting expectation for reliable products and allowing you the freedom to manage your process to obtain those results.

Comments are: * Storage materials can affect solderability, but the type of solderability protection is the primary driver to the self-life of your boards. * We do not ask our board fabricator to seal our boards in bubble pack. [Bubble pack maybe good, but we just think of it to be too flimsy to much of anything, �cept maybe absorb a little impact, but then again if our stockfolk are dropping bricks of boards, they�re going to need to find other work.] We request heat sealed moisture barrier bags. * When we concern ourselves about moisture absorption by PWB, we�re usually thinking about out-gassing of moisture though plated through holes. So, this is more of a concern in wave soldering than reflow. * We are more concerned about air-born contamination affecting solderability that we are about moisture absorption.

Consider: * Buying boards in a prudent fashion that coincides with your demand, so that you do not store boards for more than six months. * Sealing your boards in moisture barrier bags. * Storing the boards in an adequate environment (20 to 30�C, 40 to 70% R.H.) * Resealing the bag to protect the remaining boards after using part of a brick of boards.

Help us understand your situation better by explaining your concerns about moisture absorption by your boards. For instance, talk to us about: * Type of boards. * Storage environment. * Percent moisture absorption of your boards. * Effect the moisture absorbed by your boards has on your process.

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Carol Stirling

#20002

Moisture absorbtion in circuit boards | 21 May, 2002

Hi Dave F,

Thanks for the reply, the question was Customer driven.

Could you clarify your recommendations for heat sealed moisture barrier bags please - does this include Nitrogen vac pack?

Our production runs are small, so there is an investigation into storing boards up to a year to save on manufacturing costs.

The board material is mainly FR6, with a potential to use FR406 or polymide on one product.

Storage at our facility is not a problem.We inspect, then seal with nitrogen & controlled temp/humidity. The concern is at the board house (not vac packed, not climate controlled).

Thanks again,

Carol

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#20003

Moisture absorbtion in circuit boards | 21 May, 2002

No N2. We could not fix a benefit for N2 in our minds. Just fold the excess bag material over to expell most air and then seal the bag. [Have you assessed the benefit of N2?] F Monette can tell you stories about little versus more pressure. If he doesn't join-in soon, consider contacting him.

I know this is not directly applicable, but thought I'd throw it in just for yucks. [And maybe get you pointed in the standards direction that you were looking for in your initial post.] Comparing J-STD-033 to EIA 583 reveals a couple of MAJOR differences. * J-STD-033 requires a bag with WVTR of 0.002 maximum. And also requires 10% MIH (Max Interior Humidity inside bag) * EIA 583 Class 1 requires a bag with WVTR of 0.02 max * EIA 583 Class 2 WVTR is only 0.08 max.

In 583, MIH is apparently left up to the user as information is provided for 20%, 30%, 40%, with a formula to use for calculation at other levels. And no guidance for determining safe MIH levels.

For LT storage, some companies interleave boards with: * Raw brown dessicating paper. * Antioxidant paper that also is a sulfur free paper.

Not all heat sealable bags are acceptable for electronic assembly. They should: * Leave no presence of mold release, silicone, S, Cl, or Br componds. * Release no materials that affect the board when heat sealed.

Just because we ask our fab to bag boards for us, there is no reason that you cannot decide to bag your boards after acceptance at receipt.

Finally, while not understanding [appreciating] your situation, for us the economics of volume buying of boards is fleeting. Buying a years worth of material just isn't worth the hassle. But then again, it's your customer that got you going on this project.

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#20068

Moisture absorbtion in circuit boards | 27 May, 2002

Carol, I beleive there is an IPC spec for Moisture control. We see all kinds of problems with boards coming from the vendors. We use to bake all bare fabs upon recieving them. This helped alot but waste time. We now rely on the board vendors to bake the boards before shipping them to us. We have no way to verify if they are doing it properly. We see moisture problems when we rework feild returns. It is our process to bake those board before rework, the board may be a problem but more over the components are the issue, class four and class five parts. Nitorgen is expensive , but if you do not have it avalible there are air dryers (desiceint) that can do a great job providing you a cost effective method of generating very dry air for your storage cabinets using shop air.

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Carol Stirling

#20072

Moisture absorbtion in circuit boards | 27 May, 2002

Thanks for the information Reworkman, We are leaning to the theory of better safe than sorry. I think I'd supply the bags and dessicant just to eliminate manufacturing issues.

What temps & times do you ask your boardhouse to bake to? Ours said some Customers request 2 hours @ 125 degrees C.

Carol

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Armynski

#20074

Moisture absorbtion in circuit boards | 27 May, 2002

Carol,

That time is also acceptable (2 Hrs @ 125 deg C) u can use this too... 6 Hrs @ 75 deg. C

regards,

Dreamy

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arcandspark

#20103

Moisture absorbtion in circuit boards | 28 May, 2002

Two hours at 125 C will do nothing. IPC's new spec is 48 hours at 125C, the old spec I use to use for years was 24 hours at 125C. I do not have all the spec in front of me but if you reduce the temp to around 60C it can take up to 48 days. We use to bake the bare fabs at 125C for 24 hours and lnever had any problems. This would be my recommendation.

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arcandspark

#20104

Moisture absorbtion in circuit boards | 28 May, 2002

Two hours at 125 C will do nothing. IPC's new spec is 48 hours at 125C, the old spec I use to use for years was 24 hours at 125C. I do not have all the spec in front of me but if you reduce the temp to around 60C it can take up to 48 days. We use to bake the bare fabs at 125C for 24 hours and lnever had any problems. This would be my recommendation.

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Dason C

#20162

Moisture absorbtion in circuit boards | 31 May, 2002

I think that the IPC spec only apply to the MSD. I had a paper from Lucent and forwarded by Francois Monette, please aware that the first 2 hours, the moisture doesn't bake out from the assembly/component and result show that it is increase the weight of the moisture on the assembly for the first 2 hours. General practice, the bare fab were baked at 105C for 24 hours since it is a FR4 or .. material (we don't bake OSP fab) and not the MSD with higher density of encapsulation material which is not easy to remove the moisture. However, I don't know who change the bake board temp to 125C internally. The ideal is standard the baking oven temp to avoid the operator error. Also, I am still under evaluate to bake the assembly or component at 55C and below 3%RH. If we only concern to meet the minimum safety level of the MSD and we don't need 68 days or 48 days.

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