Hi Mumtaz - just my opinion but I would not go to a low temperature alloy
for the issue you detailed as there should be other root causes that are
the problem. My guess is that you have a solderability or pad geometry or
some other contributing issue. One possible root cause would be that the
passive components being soldered do not have a termination barrier layer.
Lead-free solders are have aggressive dissolution rates and if your passive
components don't  have a termination barrier layer (typically copper or
nickel) then the soldering process may be leaching the termination finish
completely into the solder joint allowing for the issue you are seeing.  A
simple fix would be to use a better quality part and not have a complex
soldering process with two alloys.

Dave

On Tue, Jul 28, 2015 at 12:26 PM, Mumtaz Bora <[log in to unmask]> wrote:

> Hello Mike, Thank-you for your pointers regarding low melt solders. Much
> appreciated .
>
> The soldering is using reflow oven, not hand soldering. The concern is
> some passives are not soldering very well and falling off the board with
> the SAC alloys, so one recommendation is to switch to low melt solders.
>
>
> Thank-you
> Mumtaz
> 858-795-0112
> Peregrine  Semiconductor
>
>
> -----Original Message-----
> From: Mike Fenner [mailto:[log in to unmask]]
> Sent: Sunday, July 26, 2015 8:44 AM
> To: 'TechNet E-Mail Forum'; Mumtaz Bora
> Subject: RE: [TN] Soldering NiPdAu finish components with low melt solder
>
> Are you hand soldering or possibly have an oven with small number of zones?
> I assume the driver here is difficulty of getting the joints to flow
> because of the heat sinking effect of the ground plain, and possible
> concerns for heat damage through prolonged heating times.
> Low melting alloys such as SN/Bi [and its modified variants] may not
> improve matters. It's the tin that does the work in solder joint formation
> and in these low melting alloys it's relatively dilute so lower to work. In
> addition there is less energy due to the lower temperatures so the rate of
> joint formation is slow compared to eutectic and SAC types. If you then add
> NiPdAu to the mix that is decidedly not going to help. That stuff can be
> pretty close to wood in soldering terms.
> Flux performance can be similarly impaired. If you are hand soldering by
> all means have a go, but you might find it is better to adjust tool
> performance so that you get a rapid rise to soldering temperatures before
> the heat sinking flattens you and/or to have some background heating to
> reduce the burden on local heating.
> Don't forget that if choosing a lower MP alloy you are reducing the
> operating temperature range of your product and possibly reducing its life
> if it sees a lot of temperature cycling in service.
>
> --
> Regards
>
> Mike Fenner
> Bonding Services & Products
> M: +44 [0] 7810 526 317
> T: +44 [0] 1865 522 663
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Mumtaz Bora
> Sent: Thursday, July 23, 2015 3:46 PM
> To: [log in to unmask]
> Subject: [TN] Soldering NiPdAu finish components with low melt solder
>
> 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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