Paul,
The copper exists evenly along every millimeter along all tracks of this
very large board, so (2) is unlikely to be the cause.
Yes, I would like to go deeper into this, waiting for our customer and the
board fabricator to respond.
Inge
----- Original Message -----
From: "Paul Edwards" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, September 09, 2009 6:58 PM
Subject: Re: [TN] Need clever comments
Inge,
Were you able to section any of the PCBs at the "growth area" and look at
them in a SEM to see the possible dendrititic growth...
I have seen similar structures on PCBs but found that they were either
caused by:
1) a Cu transfer from a bare Cu surface to the raised S/M areas on the
finished PCBs ( i.e. someone using a Cu-clad sheet instead of an interleave)
2) Cu particles settling on a packaging/or interleaving surface which became
pressed into the S/M surface (dirty work areas; Cu because of the density)
This was determined by checking the depth of penetration in the S/M and
shape structure of the Cu deposits.
Paul
Paul Edwards
Surface Art Engineering
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Wednesday, September 09, 2009 12:20 AM
To: [log in to unmask]
Subject: Re: [TN] Need clever comments
Wayne,
Normally, Copper is very stable, as all know, but there are circumstances
when even copper starts walking.
Quoted from Navy Mantech:
"Conformal coating is only effective at preventing metallic dendritic growth
if there are no ionic residues trapped under the coating. If residues are
present under the coating, small amounts of moisture will penetrate the
conformal coat, facilitate the migration of metallic ions, and affect PCB
performance. Thus, all residues must be removed prior to applying the
conformal coating. "
This is especially applicable for some acrylic coatings. We have had
catastrophic migration of Tin, Lead, Nickel, Iron, Silver etc between J-leg
packages, due to humidity that has gone through the coating and started the
migration.
/Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne - IIW
Sent: tisdag 8 september 2009 18:12
To: [log in to unmask]
Subject: Re: [TN] Need clever comments
Hi Inge!
Could you please provide a reference for your statement about copper having
problems associated with mobility even on boards which have been conformal
coated? (This was noted on your original post).
Thanks,
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Tuesday, September 08, 2009 11:34 AM
To: [log in to unmask]
Subject: Re: [TN] Need clever comments
Thanks, David.
On pressfit:
I'm pleased to hear that this can be applied on both ImSn and ENIG. I've
learned from the crimp connector technology, that the bearing idea is that
the crimping creates numerous GAS TIGHT contacts, which prevent from
oxidizing and corrosion. Likewise, if you perform gastight pressfit contacts
with the copper, with the nickel, with the Silver or whatever finish, these
joints should be as good as crimp ones, don't they?
On solder mask:
I've understood, that chemical situations can occur, resulting in a corrupt
solder mask, but also that this problem can be avoided by good process
control and that you must pay attention to the interaction(s) between the
immersion tin and the solder mask.
It's well described on page 22 in the below article (written many years ago,
but I guess it's basically the same today)
http://www.epa.gov/dfe/pubs/pwb/pdf/sf_guide.pdf
The solder leaching on the boards I got for analysis, is so extensive, that
you see red copper through the solder mask, instead of the typical ImSn
colour.
My report is sent to both the board maker and our customer.
Again, thanks for the help.
Inge
----- Original Message -----
From: "David D. Hillman" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, September 08, 2009 3:33 PM
Subject: Re: [TN] Need clever comments
> Hi Inge! Sorry for the these tardy comments (I took advantage of the
> Labor Day holiday here in the US and enjoyed a 4 day weekend of
> whitewater kayaking in Wisconsin) but hopefully they add to the
> information already
> provided:
>
> 1) Immersion Tin PWB Surface Finish - ImSn is a viable pwb surface
> finish and is used on a wide variety of IPC Class 1/2/3 products.
> However, ImSn does not have the greatest thermal excursion robustness
> and long term solderability shelf life - both issues due to the
> oxidation of the Sn and/or the oxidation of SnCu intermetallic phase
> (copper and tin diffusion interactions). There are a multitude of ImSn
> plating chemistries on the market and they are not all "equal" in
> terms of oxidation and tin whisker resistance. I highly recommend that
> before using an ImSn surface finish the oxidation and tin whisker
> issues be completely investigated. Correctly formulated and processed
> ImSn chemistries should not have tin whiskers and should have a 12 month
> solderability shelf life.
>
> 2) Pressfit Pin and PWB Surface Finishes - all of the pwb surface
> finishes
> - OSP, ImSn, ImAg and ENIG can be used with pressfit pin technology.
> As Bev described, very close attention must be paid to the plated thru
> hole dimensions/tolerances to insure that a successful pressfit pin
> process can be achieved. And yes, if no attention to the hole
> dimensions/tolerances is conducted, you get damaged holes and damage
> laminate. Rockwell Collins uses pressfit pins with the ImSn, ImAg and
> ENIG surface finishes successfully on a number of product designs.
>
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
>
> Hernefjord Ingemar <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 09/07/2009 06:21 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to Hernefjord
> Ingemar <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> [TN] Need clever comments
>
>
>
>
>
>
>
> Hi all, need some professional backup regarding MIL quality boards.
>
> Objects: FR-4 Class III double-sided multi-layer boards, populated
> with SOICS, BGAs,and a lot of passive components.
>
> Observation 1 : the non soldered board have lots of Tin whiskers on
> inside of the PTH barrel. My thought is this: if whiskers can grow
> long before the board is assembled, then ain't it likely that even CAF
> can be generated? See photo 1.
>
> Observation 2: Copper has somehow penetrated the solder mask. This
> can be found everywhere along the conductor traces. You need a very
> good light microscope and a SEM to see it. See photo 2.
>
> Board data: Copper with 0.8 micrometer Immersion Tin. No nickel barrier.
> Solder mask thickness not specified.
>
> Application: Typical MIL-883 environment
>
> Q1: What is your opinion about that thin Tin directly on copper? I
> dislike the concept. Copper is very mobile at high temperatures, and
> combined with humidity, there can be leakage currents and corrosion
> issues. Even if the boards are CCd, there is a risk with copper .
>
> Q2: I gave the advice to introduce a nickel barrier, but our customer
> claimed, that they can't because of pressfit connectors and pressfit
> test pins on the board. Furthermore, they had heard that one cannot
> have nickel platings when pressfitting, because the nickel will crack
> and oxidize and cause electrical disfunction. Is this your opinion
> too? Are there any relevant testing behind such statements?
>
> Thanks in advance
>
> Inge
>
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