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September 2009

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TechNet E-Mail Forum <[log in to unmask]>, Inge <[log in to unmask]>
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Mon, 7 Sep 2009 18:57:59 +0200
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Finally, I think I found a reasonable answer on the last issue, that was a 
copper rich and deteriorated solder mask over the copper traces:

6. Ingredients in the immersion process can be aggressive to the solder 
mask. Also, too rough a copper will cause insufficient homogenity and 
thickness of the solder mask. It's important that the solder mask matches 
the chemistry of the immersion process, as well as the requirement on copper 
planarity be fulfilled. So, the copper went through the (too) thin tin layer 
and found its way also through the destructed solder mask.

7. The conformal coating, that we apply, will seal the solder mask, and the 
board will work for period of xx months. If our  customer pay us, we will do 
tests to decide this time level.

With other words,  the board manufacturer has not been very accurate and our 
customer did not perform any Incoming Inspection on these boards.

Thank you for contributing.

Inge

----- Original Message ----- 
From: "Inge" <[log in to unmask]>
To: "TechNet E-Mail Forum" <[log in to unmask]>; "Inge" 
<[log in to unmask]>
Sent: Monday, September 07, 2009 5:52 PM
Subject: Re: [TN] Need clever comments


> OK, I think I've found more answers on the rest of my questions:
>
> 4. You never use nickel barrier for Immersion Tin, because the Immersion 
> Tin with nickel barrier won't readily form binding intermetallics to the 
> barrier,  while Tin in direct contact with Copper creates strong and 
> stable intermetallics.
> 5. What I've seen is probably the result of too thin ImSn. The below 
> article clarifies, that there is a dramatic thickness level, under which 
> the copper makes the bond pads unsolderable. If the ImSn is dense enough 
> and thicker than 1um, there ought to be no concerns about solderability, 
> granted that you don't pass fresh date.
>
> http://www.ecitechnology.com/articles/Surface_Evaluation_of_the_Immersion_Tin_Coatings_via_Sequential_Electrochemical_Reduction_Analysis_(SERA).pdf
>
> The remaining question is now only the copper on top of the solder mask.
>
> Sorry for rushing, but I have just one day to decide what to report.
>
> Inge
>
>
>
> ----- Original Message ----- 
> From: "Inge" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Monday, September 07, 2009 5:27 PM
> Subject: Re: [TN] Need clever comments
>
>
>> It seems as I have to reply to the questions myself:
>> 1. Immersion Tin has a typical cubical or prismatic organized 
>> superstructure. Like the boards I got.
>> 2. Tin whiskers are natural after ImSn processing.
>> 3. But after soldering components, there is no risk any more.
>>
>> http://www.atotech.com/fileadmin/pdf/papers/el/Immersion_tin_and_tin_whisker_growth.pdf
>>
>> So, I've reduced my concerns to the question of copper diffusion through 
>> both tin and solder mask and also what is the solderability of such 
>> (badlooking in my opinion) pads.
>>
>> Inge
>>
>>
>> ----- Original Message ----- 
>> From: "Inge" <[log in to unmask]>
>> To: <[log in to unmask]>
>> Sent: Monday, September 07, 2009 5:06 PM
>> Subject: Re: [TN] Need clever comments
>>
>>
>>> Observation 1: I don't make that coincidence, I ask you all about a 
>>> possible connection. Why? Because my experience is that one anomaly is 
>>> often followed by more. Like lack of vitamins can cause many diseases. 
>>> But my main worry is the anomalous solder pad look and also the presence 
>>> of copper on the solder mask surface.
>>>
>>> Observation 2: in fact, if you study the conductor lines under strong 
>>> light, there seems to be more copper colour than tin.
>>>
>>> Q2: Well, is pressfit positive or negative with nickel plating. 
>>> Ericsson, one of the biggest telecom companies, have forbidden pressfit 
>>> + nickel plating. And you say that Nortel have done it for years. Who is 
>>> poppycock?
>>>
>>> Inge
>>>
>>> ----- Original Message ----- 
>>> From: "Bev Christian" <[log in to unmask]>
>>> To: <[log in to unmask]>
>>> Sent: Monday, September 07, 2009 3:14 PM
>>> Subject: Re: [TN] Need clever comments
>>>
>>>
>>>> Inge,
>>>> Observation 1: How in the world can you make a connection between tin
>>>> whiskers and CAF?!!
>>>>
>>>> Observation 2: If copper is "penetrating" the solder mask, it makes me
>>>> wonder what it is doing to the tin overlayer!  I would be concerned 
>>>> about
>>>> tin/copper intermetallic growing to the surface and oxidizing.
>>>>
>>>> Q2 Popppycock!  Companies like Nortel and other put press fit 
>>>> connectors
>>>> into ENIG boards for years.  Of course it is always a good idea not to 
>>>> try
>>>> and put an elephant in a mouse hole.  Make sure the pin and hole are
>>>> dimensional matched, the pin is put in straight and the proper force, 
>>>> speed
>>>> and assembly apparatus are used to do it and that the end results are
>>>> cross-sectioned initially to make sure that it is right.
>>>>
>>>> Bev
>>>> RIM
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
>>>> Sent: Monday, September 07, 2009 7:22 AM
>>>> To: [log in to unmask]
>>>> 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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