Wednesday, November 02, 2005

Documents innside ESD packaging

As a producer of electronic printed board assemblies I work within manufacturing. I've recently been tasked with overseeing the effects of static during board production. As part of the storage of product I have found printed computer paper and photocopier paper placed directly on top of the boards in carriers without shielding. As far as I was aware with BS 61340-5-1:2001 this was found to create possible damage by static from the paper. However, I am locked into debate with the person concerned as I've always believed this to be unacceptable. Please could you advise.

I have not found any specific mention in 61340-5-1 or 61340-5-2 of inclusion of paper within ESD packaging. However I agree with you, I do not think that it is good practice. The characteristics of paper are extremely variable and this is introducing an unknown ESD risk into what is supposed to be a protected environment with controlled ESD risk. If you value your product, as I'm sure you do, why take the risk? A good way to put costs into perspective is to think about the potential cost of even one ESD failure in the customer's site. Usually this is sufficiently high to convince one that a small saving or doubtful practice in pursuit of convenience, is not worth while.

I have found computer paper in particular to often be at the insulating end of the paper spectrum and therefore possibly at higher risk of causing ESD problems. Usually in such cases it is much easier to remove the doubtful practice than it is to prove one way or another whether there is significant ESD risk.

If you can prove that the paper full fills the requirements for "intimate" packaging over the full range of environmental conditons (especially low humidity) then I concede you may have a technical argument for allowing the paper in with the boards. The requirements for "intimate" packaging are given in Table 2 of 61340-5-1 - the material should be low charging and at least static dissipative (< 1011 ohm surface resistance measured at 100V with a concentric ring electrode. I recommend testng at 12 % rh). My guess is that you do not want to perform this characterisation test for the paper, including periodic verification of any paper you may use for the purpose in the future. Special ESD paper may be available if you really want to use this practice. Note that the resistivity of paper typically increases by several orders of magnitude with reducing humidity and so measurements at higher humidity will not give worst case results.

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