Simon,
your idea is very realistic, however, if too much copper is solved and begins to precipitate on the soldermask, WHY should it only be on top of the conductors? Electrostatic attraction?
Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of S.Huetter
Sent: fredag 11 september 2009 10:23
To: [log in to unmask]
Subject: Re: [TN] Need clever comments
Inge,
IMHO the cause for Copper residues on soldermask is the Im.Sn process.
Check out the process from Atotech:
http://www1.atotech.com/start.php3?cl_my_id=578839
I assume the board maker has not used the Atotech process as it is "the Whisker-Free process" but if you look at the Crystallizer process you'll see that the Copper content has to be controlled. The cause for the Copper residues is IMHO the overdue chemistry life time (max. MTO).
I can only compare it to our inhouse ENIG process as we outsource the Im.Sn-process.
If we would overrun the MTO we would get excessive plating on the base material and soldermask.
Regards
Simon
Inge wrote:
> Jack &Paul, FYI.
>
> Today I had a close look at the cross sections. What I found was this:
>
> 1. The Tin plating was done AFTER solder mask (Jack was right) 2. The
> solder mask was very uneven, thickness between 5um and 25 um.
> 3. Despite the corrupted surface, the solder mask is homogenous, no
> vertical cracks found.
> 4. The copper that I found earlier on top of the conductor, i.e. on
> the solder mask, that copper had no connection with the conductor
> copper. Which means that these contaminations had NOT migrated
> through the solder mask.
> 5. I can still not figure out from where the copper contaminations
> come.
>
> So, all that remains is the question about the copper contaminations
> tha embedded in the very surface of the solder mask. I have to adjust
> my report and resend it to our customer and the board maker.
>
> Thanks to your critisism, I can now redo the analys, starting from a
> more correct standpoint.
>
> Your are great!
>
> /Inge
>
>
>
>
> ----- Original Message -----
> From: "Inge" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Wednesday, September 09, 2009 8:09 PM
> Subject: Re: [TN] Need clever comments
>
>
>> Jack,
>>
>> SMOBC is the common industrial standard, as you pointed out, however,
>> there ARE some fabricators that apply the solder mask after
>> Tin/Lead-ing the copper traces. The later method has an obvioius
>> disadvantage, see below quoted from an article written by US
>> Environmental Agency:
>>
>> " This method predominates for several reasons. Copper is a surface
>> that lends itself to rigorous cleaning, which is essential for solder
>> mask adhesion. Tin-lead under solder mask will liquefy during
>> soldering and may cause the mask to blister and peel. The hot air
>> solder leveling process generally produces less waste water and
>> introduces less lead into the waste water stream than tin-lead
>> plating and reflow. Despite these advantages, well-known
>> disadvantages also exist. The shelf-life of hot air solder leveled
>> circuits is short and solder thicknesses on pads and hole barrels is
>> notoriously difficult to control. For these reasons, a small minority
>> of specifications continue to call for tin-lead plate and reflow or
>> other alternati air solder leveling, nomenclature screening, and
>> finally, gold edge plating if necessary. "
>>
>> I think that is what happened to our boards....." cause the mask to
>> blister and peel"...
>>
>> Another paper describes Tin under solder mask this way:
>>
>> " Facility F initially was concerned with the soldermask breakdown
>> where the Tin leaches underneath the soldermask....etc"
>>
>> Quoted from EPA (United States Environmental Protection Agency.
>>
>> When I started the investigation (had just some hours to spend before
>> reporting the result!), I was fully convinced that these boards were
>> SMOBC, but our customer said they used tinning before soldermask. I
>> have asked for a confirmation from the board fabricator, but got no
>> answer.
>>
>> Thanks for your comment, good critics.
>>
>> Inge
>>
>>
>> ----- Original Message -----
>> From: "Jack Olson" <[log in to unmask]>
>> To: <[log in to unmask]>
>> Sent: Wednesday, September 09, 2009 4:44 PM
>> Subject: [TN] Need clever comments
>>
>>
>>> I know I'm late to the game, but I can't resist asking this
>>> question:
>>>
>>>> From my experience, the tin is applied AFTER soldermask, so
>>> you have mask over bare copper, and tin over exposed copper.
>>>
>>> The tin in PHOTO2.JPG in the exposed area looks beautiful, so isn't
>>> the question (ignoring the whiskers for the moment) "How can bare
>>> copper erupt through the mask?"
>>>
>>> Unless I missed one of your previous posts, it seems to me that any
>>> speculation about copper poking through the tin finish is
>>> irrelevant. I'm only addressing Question 2 below, but you mentioned
>>> introducing a nickel barrier, and that will not be plated under the
>>> mask either, will it? only on exposed circuitry...
>>>
>>> just wondering,
>>> Jack
>>>
>>>
>>> -=-=-=-
>>>
>>> *Subject:* Need clever comments *From:* Hernefjord Ingemar <
>>> [log in to unmask]> *Reply-To:* TechNet E-Mail Forum <
>>> [log in to unmask]>, Hernefjord Ingemar
> <[log in to unmask]> *
>>> Date:* Mon, 7 Sep 2009 13:21:54 +0200 *Content-Type:* text/plain
>>>
>>>
>>> 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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