Inge,
I knew that you will ask this question :-)
Yes, it has to be the electrostatic/voltage potential.
You have a different potential over the Copper conductors causing the solved
copper to precipitate on the soldermask.
However, I am not a immersion Sn process engineer and have not seen this
particular defect before but that is what I assume. I recommend to do a
process audit if possible.
Simon
Hernefjord Ingemar wrote:
> 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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