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Jim Marsico <"marsico%Organization=Industrial & Mfg'g Engineering%Telephone=516-595-5879"@a1.allin1.ALLIN1.umc> |
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From [log in to unmask] Fri Apr 12 14: |
09:35 1996 |
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On Thu, 11 Apr 1996 11:29:01 -0700 Jim Marsico wrote:
> From: Jim Marsico <marsico%Organization=Industrial & Mfg'g
[log in to unmask]>
> Date: Thu, 11 Apr 1996 11:29:01 -0700
> Subject: DES: MATERIAL FINISHES
> To: [log in to unmask]
>
>
> Good day,
>
> We have a design that relies on intimate contact between the bottom
> side of an SMT PWB and an aluminum carrier. The board gets fastened down
with
> screws. The design requires a good ground between the board and carrier
for a
> period of 10 years in an outside environment. The carrier is irridite
coated,
> but the surface is abraded where the board is located. The original board
> finish was tin/lead.
>
> The question arose as to what effects oxidation will have on the grounding
> function. Which will oxidize faster, the tin/lead or the aluminum? Is
gold a
> better choice for the board finish? What alternatives are there for the
> carrier finish?
>
> Also, this same product includes components which have either nickel or
gold
> plated bodies. These components must also maintain intimate contact for
> electrical ground between the component body and a .003" thick copper shim.
> What would be the best finish for the shim? Keep in mind that the shim has
to
> be formed slightly, therefore the finish must be somewhat pliable.
>
> Thanks in advance...
>
> Jim Marsico
> [log in to unmask]
> (516) 595-5879
>
> ********************************
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> * SYSTEMS, INC. *
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>
Jim: 1.The "irridte" coating you refer to is a trade name for
chromate conversion coating.Class III is typically called out for electrical
connections.Since it is abraded away at the contact area this may be a mute
point.Is it abraded from the chasis possibly because you are using a heavier
coating(Class I)and it is acting as an insulator?
2.The aluminum will oxidize faster than the Sn/Pb,it forms an
oxide layer almost immediately when exposed to air.The Sn/Pb however also
form an oxide layer in a short time.
3.A gold pcb will not oxidize,however due to it's highly cathodic
(noble)state and aluminums highly anodic position in the galvanic series the
possibility of corrosion from a galvanic cell is very probable.This is
especially true in an outside environment that is subject to moisture and
humidity conditions.
4.You may want to condider a zincate/copper/tin finish on the
aluminum carrier and stay with the Sn/Pb on the board,these are very close in
the electromotive series.Another option could be to conformal coat the
aluminum/pcb assembly.
The real issue here is what do you require for"a good ground
between the board and carrier".In many cases the connection of the
board/screw/carrier as it is may be sufficient.How well sealed the enclosure
is,where it's located,type of environment etc. are all going to have an
effect on the reliability.
5.I would tin plate the shim.
Regards
Michael Barmuta
Staff Engineer
Fluke Corp.
Everett Wa.
(206)356-6076
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