Hi George - Rockwell Collins/Celestica did some copper dissolution work
with the SAC305 alloy as part of the NASA DoD project. For a temperature of
260C, the dissolution rate of a QFP pad was 0.043 mils/second. With that
rate, I can dissolve a 2.5 mil thick copper pad in about 60 seconds. We
published the data at the 2013 SMTA ICSR in Toronto.

Dave Hillman
Rockwell Collins
[log in to unmask]

On Mon, Nov 21, 2016 at 10:24 AM, George M. Wenger <[log in to unmask]>
wrote:

> Juliano,
>
> You suggestion would be to cross section one of the pads before anything
> is soldered to the pad to see how much copper there is below the immersion
> tin.
>
> The first step in the immersion tin plating process is to microetch the
> copper (which removes some copper) and then during the immersion tin
> plating more copper is removed and replaced with tin. The thicker the
> immersion tin is applied the more copper is removed. So I think you need to
> determine how much copper there is left so that when you go through the
> soldering process you should be able to determine if there will be any
> copper left after soldering.
>
> I haven't seen any specific dissolution rate info for Pb-Free solders but
> I know the dissolution rate for copper into SnPb solder at 435F given in an
> old Wally Bader paper is 4 u"/sec. so if your solder is molten for a minute
> and a half you'll dissolve 360 microinches of copper.
>
> George
>
> ----- Original Message -----
>
> From: "Richard D. Stadem" <[log in to unmask]>
> To: [log in to unmask]
> Sent: Monday, November 21, 2016 8:47:54 AM
> Subject: Re: [TN] Immersion Tin - problems
>
> Thanks for your input Juliano.
> If others have also seen this, it would be interesting to see just how
> widespread this issue is. IPC should have guidelines on what to do to
> prevent it (ENIG finish, 1/2 oz copper minimum, etc).
> As I posted earlier, I have seen it happen many times at many client
> companies, and I am simply trying to get the word out. Many have no idea
> what may have happened, and be unfairly blaming the PWB fabricator.
> It is a process/design issue.
> dean
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Francesco Di Maio
> Sent: Friday, November 18, 2016 8:12 AM
> To: [log in to unmask]
> Subject: Re: [TN] Immersion Tin - problems
>
> Hello Juliano,
> I think you are facing a problem of copper dissolution as Mike wrote.
> This problem become more evidentemente with lead free solder alloy, also
> it may be presente with Sn/Pb alloy.
> In our experience the use of Sn100 alloy may help to mitigate the effect
> of the annullar ring partial or complete dissolution if compared to
> SAC305 or SAC405 alloy.
> We noted this problem during solder pot (solder fountain) rework used to
> remove and resolder pth connector.
> The selective soldering with long contact time, higher solder temperature,
> higher solder flow rate in my opinioni may cause the problem you noted.
> Try to reduce these variables and problem should be mitigate.
> Preheating the assembly before or during the selective wave soldering, may
> help.
>
> Regards
> Francesco Di Maio
> SEM Comunicazione & Gestlabs
>
>
> -------- Messaggio Inoltrato --------
> Oggetto: Re: [TN] RES: [TN] RES: [TN] Immersion Tin - problems
> Data: Thu, 17 Nov 2016 16:25:03 +0000
> Mittente: Mike Fenner <[log in to unmask]>
> Rispondi-a: TechNet E-Mail Forum <[log in to unmask]>, Mike Fenner
> <[log in to unmask]>
> A: [log in to unmask]
>
>
>
> There could be a number of reasons. I offer the following to add to your
> list of things to investigate.
>
> The immersion tin will dissolve very rapidly into the Solder which will
> then start to dissolve the Cu pad underneath to form a solder joint. If
> your pads/tracks are small/thin then it is possible the Pb-free solder
> could dissolve the pad entirely. Pb-free solders can be more aggressive in
> this respect than Sn/Pb due to the higher tin content. The Sn is more
> concentrated/less dilute compared to Sn/Pb. Processing temps are also
> higher. The problem can be increase by a turbulent or fast wave.
> Reducing the exposure time may help.
> I suggest you might want to consider your Pb-free finish for unrelated
> reasons. Immersion tin is a variable process and has a relatively short
> storage life. It's hard to think of any advantages it has, save possibly
> for cost.
>
> ---
>
>
> Regards
>
>
> Mike
> www.chrisfennerfund.org
> www.facebook.com/chrisfennerfund/
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Datacom - Juliano
> Ribeiro
> Sent: Thursday, November 17, 2016 11:46 AM
> To: [log in to unmask]
> Subject: [TN] RES: [TN] RES: [TN] Immersion Tin - problems
>
> Many pads
>
> _____________________________
> Juliano Bettim Ribeiro
> DATACOM
> +55 (51) 8446-2135
> +55 (51) 3933-3000
> Ramal: 3484
>
>
> -----Mensagem original-----
> De: Stadem, Richard D. [mailto:[log in to unmask]] Enviada em:
> sexta-feira, 11 de novembro de 2016 11:40
> Para: TechNet E-Mail Forum; Datacom - Juliano Ribeiro
> Assunto: RE: [TN] RES: [TN] Immersion Tin - problems
>
> One, or many?
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Datacom - Juliano
> Ribeiro
> Sent: Friday, November 11, 2016 5:21 AM
> To: [log in to unmask]
> Subject: [TN] RES: [TN] Immersion Tin - problems
>
> Damaged = "Pad disappeared" the track has broken
>
> _____________________________
> Juliano Bettim Ribeiro
> DATACOM
> +55 (51) 8446-2135
> +55 (51) 3933-3000
> Ramal: 3484
>
>
>
> -----Mensagem original-----
> De: TechNet [mailto:[log in to unmask]] Em nome de George Wenger Enviada em:
> quinta-feira, 10 de novembro de 2016 17:32
> Para: [log in to unmask]
> Assunto: Re: [TN] Immersion Tin - problems
>
> What do you mean by "damaged"? Do you mean solder doesn't wet the pad or
> is there physical damage?
>
> Sent from my iPhone
>
> > On Nov 10, 2016, at 1:16 PM, Vladimir <[log in to unmask]>
> wrote:
> >
> > Hi Juliano,
> >
> > The board finish most probably has nothing to do with the problem. It
> > must
> be your process.
> >
> > Regards,
> >
> > Vladimir
> > SENTEC
> >
> > Sent from my BlackBerry 10 smartphone on the Rogers network.
> > Original Message
> > From: Datacom - Juliano Ribeiro
> > Sent: Thursday, November 10, 2016 13:13
> > To: [log in to unmask]
> > Reply To: TechNet E-Mail Forum
> > Subject: [TN] Immersion Tin - problems
> >
> > Hello to all,
> >
> >
> >
> > We recently are changing to Lead Free process to specific products and
> > we need to change the finishes boards from (HAL Tin Lead) to Immersion
> Tin.
> >
> > But during the Selective Soldering process the component pad is damaged.
> >
> > Are there limitation about Immersion Tin about Max. Temperature, Max.
> > Reflow/Wave process, Max Soldering Contact Time, etc.???
> >
> > In this case the HAL Lead Free is better?
> >
> >
> >
> >
> >
> > Thank you
> >
> > Juliano Ribeiro
>