SAC alloys under the best of circumstances do not readily wet out to begin with. That characteristic alone should not be the sole basis for switching to a low-melt alloy, especially when soldering to NiPdAu-finished components. A low-temp Bi-based solder joint may display excellent wetting, but it does not mean it is better than a SAC alloy with limited wetting but a much stronger IMF.
Just a caution note: I am assuming you received a customer waiver to do so. Nearly all assembly drawings specify which solder alloy to use; one cannot simply switch due to design issues. You will need to make sure you track which assemblies use any alternate alloys not specified on the BOM or drawing. The low-temp alloy attribute will need to be on the As-Built record for traceability, just in case there IS a reliability issue down the road. It doesn't matter if it is a Class 1 assembly or a Class 3 because if you have 10,000 Class 1 assemblies failing in commercial automotive entertainment centers it can cost you as much or more as 10 defibrillators or 1 flight control computer.
Nothing is ever free; what you gain in being able to solder at a lower temperature you are going to give up in SJ reliability, be it increased ductility, or brittleness at the opposite extreme, or whiskering, SJ fracturing, or whatever. You need to determine if the particular worst-case application/environment the CCA sees will cause issues with the low temp solder joints that are not seen with the specified alloy. One must realize that soldering NiPdAu-finished components with Sn42.0Bi57.6Ag0.4 is not an everyday occurrence in our industry. It should be well-qualified before jumping into production. Don't assume customer buy-in frees you from any liability; they are depending on you for your expertise in such matters.
Dean
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mumtaz Bora
Sent: Tuesday, July 28, 2015 12:56 PM
To: [log in to unmask]
Subject: Re: [TN] Soldering NiPdAu finish components with low melt solder
Hi Joyce, Board has heavy copper planes and a lot of topside passives on the board are not wetting very well with SAC alloys, so the switch to low melt Sn /Bi solder to facilitate the soldering of passives.
Thank-you
Mumtaz
858-795-0112
Peregrine Semiconductor
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Friday, July 24, 2015 8:01 AM
To: [log in to unmask]
Subject: Re: [TN] Soldering NiPdAu finish components with low melt solder
The question is why they need to use low melting temp solder - something else on the board is not allow to be heat up? if so, the finish of the parts is not a good selection (should get caught during BOM design review). Just curious.
jk
> Hi Mumtaz - two things we have seen that are important when soldering
> NiPdAu component finishes are (1) soldering temperature/profile. Pd
> dissolves much slower into Sn than other elements so running the
> soldering temperatures 10 degrees hotter than typical and increasing
> the soldering time 3-5 seconds longer than typical can be beneficial;
> (2) flux formulation. Some fluxes work better than others when Pd is
> included in the soldering situation. Hope this helps.
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
> On Thu, Jul 23, 2015 at 10:46 AM, Mumtaz Bora <[log in to unmask]> wrote:
>
>> Dear All, Good Morning, Looking for some knowledge base on soldering
>> NiPdAu finished components with low melt solder such as
>> Sn42.0Bi57.6Ag0.4
>> to avoid wetting issues on a heavy copper plane board. Any insight
>> and advise will be appreciated.
>>
>> Thank-you
>> Mumtaz
>> 858-795-0112
>> Peregrine Semiconductor
>>
>>
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