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TechNet E-Mail Forum <[log in to unmask]>, Inge <[log in to unmask]>
Date:
Thu, 10 Sep 2009 18:24:48 +0200
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I will post a handful SEM images to Steve, so you get an idea what it's all 
about.
It may take a while, because he is probably on the market place to buy 
curtains and  wallpaper
for his new appartment. Or jogging. Or having a drink on the balcony.
/Inge


----- Original Message ----- 
From: "Inge" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, September 10, 2009 5:10 PM
Subject: Re: [TN] Need clever comments


> 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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