Somebody asked me off-list for further details of the kind of packaging
materials I mentioned. The stuff I was buying was made in Switzerland
and was actually the bags used for packaging ready-made Rösti,
unprinted. I did a quick search and found
http://www.keepsafe.ca/barrier.shtml
but I'm sure there are many sources, as well as configurations. Note
that this site confirms my assertion that ordinary plastic bags are
porous and afford little protection.
Brian
Brian Ellis wrote:
> Sorry, cannot agree with you. 1) polyethylene bags provide zero
> long-term protection. Small gaseous molecules, including water vapour,
> pass through the stuff like a sieve 2) vacuum packing is even worse,
> because the pressure inside is much lower than the osmotic pressure of
> the water vapour outside. 3) In addition, plasticisers in the PE will
> sublime out of the bag and can condense onto the PCB, both during and
> after the sealing process. If you must bag the circuits, with or without
> vacuum, use tripack bags with PE on the outside, aluminium foil in the
> middle and polypropylene on the inside. The PE gives the bag the
> robustness it needs; the aluminium makes it hermetically impermeable and
> the PP provides a non-plasticised, non-contaminating, heat-sealable
> interior. This is why tripack bagging is used for some foodstuffs
> designed for long-term storage at room temperature - straight plastic
> bagging will not work, nor will it for PCBs.
>
> Rather than vacuum, it is better to use dry nitrogen which provides a
> slight overpressure, so that minute pores in the sealing will not draw
> in air.
>
> Desiccant bags should not be placed in contact with the PCB. Any
> desiccant powder on the PCB (and it can be ground off the silica gel
> crystals and implanted into the solder mask if vacuum sealed) may cause
> electrical failures in humid service.
>
> Furthermore, I suggest your remarks re OSP may be due to using
> inadequate packaging materials. An OSP-finished circuit packed
> individually or in bulk in tripack bags with dry nitrogen will preserve
> its solderability for at least 2 or 3 years.
>
> Been there, done that!
>
> Brian
>
> Jeffrey Bush wrote:
>
>> PCB finishes will dictate storage conditions. HASL and ENIG finishes
>> are generally preserved well beyond a year when stored in Poly bags in
>> an environment 65-75 F and 40-60 RH. IT/IS finishes should be stored in
>> VAC packaging available with most PCB suppliers - you may want to add a
>> RH indicator to let you now if any seal failures from handling. OSP is
>> basically a finish that should be avoided, but stored in VAC packaging,
>> with a indicator and desiccant should preserve this finish for at least
>> 6 months although the history of OSP as a preservative is not good -
>> mainly was used in captive shops where time to assembly was managed
>> easily, as well as rework which seems to be a prerequisite with OSP.
>>
>>
>>
>>
>>
>> Jeffrey Bush
>> Director, Quality Assurance and Technical Support
>>
>> 76 Technology Drive - POB 1890
>> Brattleboro, Vermont 05302-1890
>> Tel. 802.257.4571.21 Fax. 802.257.0011
>> [log in to unmask]
>> http://www.vtcircuits.com
>>
>>
>>
>> -----Original Message-----
>> From: Brian Ellis [mailto:[log in to unmask]]
>> Sent: Wednesday, March 24, 2004 2:38 AM
>> To: [log in to unmask]
>> Subject: Re: [TN] pcb storage
>>
>>
>> Dry, cool conditions, in sealed hermetic containers (preferably metal),
>> but NOT in polyethylene bags.
>>
>> Brian
>>
>> Rey Degollado wrote:
>>
>>
>>> Hello guys,
>>>
>>> does anybody has an idea on what suitable storage condition for pc
>>> boards (humidity / temp.)? we just want to know how to store solder,
>>> gold and osp finished boards.
>>>
>>>
>>> Thanks,
>>>
>>>
>>> Rey Degollado
>>>
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