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October 2005

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Subject:
From:
"Barmuta, Mike" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Barmuta, Mike
Date:
Fri, 14 Oct 2005 11:15:00 -0700
Content-Type:
text/plain
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text/plain (191 lines)
Sona: Back in the "old" days we did a lot of tin plating as the final finish
on Teflon MLB's, typically for high frequency signal generators. You
definitely didn't want to mess these up or it could get real expensive rear
quick.

It's a very tricky process but if all factors are controlled properly it
works. Unfortunately most people doing tin plating these days are only using
it for etch resist and are not aware of the multiple issues needed to
produce an acceptable electroplated tin final finish.
So let me give you some pointers on how we did it.

1. Bath Type: Lea Ronal(Rohm and Haas) Bright Acid Tin Sulfate Electrolytic
Plate. This is a fine grain structure finish not requiring any type of
reflow processing. Due to low porosity and tight grain structure of the
deposit it is not as susceptible to staining as a matte/semi-mate tin.

2. Underplate: To help prevent slivering you need an electrolytic nickel
underplate between the copper and tin

3. Rinsing: To minimize staining after tin plate you need multiple rinses
cold followed by hot. Any acid drag over in final rinsing will result in
staining.

4. Resist Strip: The resist stripper chemistry has to be designed for
compatibility with bright tin, if not you will get staining.

5. Etch: You need to control your etcher(ammoniated), pH, chemistry, temp,
speed, etc, to minimize undercut which will produce slivers. Again rinsing
is critical here too.

6. Soldermask Prep: Do not scrub the panels.

7. Soldermask Bake: Bake at 140C for 60-80 mins AT temperature. If you have
discoloration you probably have done a poor job of rinsing somewhere above
or have poor water quality. Poor air circulation in the oven can also be a
cause.

8. Tin Plate Bath Control: Control of the multiple component organic
brightners, levelers and grain refiners is critical. Also the stannic,
stannous, sulfuric acid constituents, cathode agitation, amount/velocity of
filtration, trace metal impurities, organic build-up, etc, need to be
monitored and properly controlled.


If done right the end result will be an acceptable product but screw up just
one factor and you will have garbage. You need to ask yourself how much you
are willing to commit to offer this type of final finish and is it worth it.

                                                        Regards

Michael Barmuta

Staff Engineer

Fluke Corp

Everett WA

425-446-6076




-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Fineline Circuits,
Inc
Sent: Friday, October 14, 2005 9:39 AM
To: [log in to unmask]
Subject: Re: [TN] Electrolytic Tin Plating


Colin:
2-3 years back we ran an experiment with a tin vendor where we did
electrolytic tin plate, etch, and mask.Tin was supposed to be the final
finish per vendor.Vendor told us that he had customers that did this.
However the tin got tarnished and dark grey after cure and we just did not
like the look off the final finish. We did not pursue it further.
Sona

----- Original Message -----
From: "Colin McVean" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, October 14, 2005 10:33 AM
Subject: Re: [TN] Electrolytic Tin Plating


Are you suggesting that you use the tin plate as your final finish? I'm
no chemist, but I think that could spell trouble, with the open
structure of the electroplated tin giving rise to whiskers.
We use Electroplated tin, but only for the etch resist application - we
strip it off, solder mask, then final finish, either Immersion Silver,
Immersion Ni/Au, or lead free HASL dependant on customer requirements.

Colin McVean M.Inst.C.T.
Production Manager
Artetch Circuits Limited
Main: 01903 725365
DD:    01903 712926
Email: [log in to unmask]
www.artetch.co.uk
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Fineline Circuits,
Inc
Sent: 14 October 2005 15:20
To: [log in to unmask]
Subject: [TN] Electrolytic Tin Plating

Hi Technetters:
Now that Leadfree is here or close to, we would like to change the
electrolytic tinlead plating tank to electrolytic tin tank.
I would like to know if the other fab houses have done this and if so
which
vendors tin chemistry has a wider window of operation. Are they seeing
any
tin whiskers/sliver problems etc..especially if the boards are stored
for
few months.
Also it is my understanding that you can mask over this plated tin since
it
does not flow. In this case you can totally avoid stripping the tin and
tin
levelling.
We currently do offer other finishes including electroless tin ,
immersion
nickel/gold, silver....
If there are other consideration in making this change I would
appreciate
your input.
Franklin, like you I would like to see more fab houses participating but
I
would like to remain with technet.
Thanks

Sona Sitapara
Fineline Circuits Inc.

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