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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Fri, 11 Nov 2016 15:59:26 +0000
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Yes, George Wenger is absolutely correct that it could very well be a thin foil issue and a microsection is always a good idea.  I have seen many, many dissolved pads caused by too long of a dwell time in the selective solder, or too high of a solder temperature, or both in the client companies that I work for. 1/4 oz. copper dissolves quickly if left in flowing solder above 500 deg. F for more than a few seconds, especially if that 1/4 oz copper foil is a little bit on the thin side and the solder being used is lead-free.
If a connector is select-soldered, and then for whatever reason has to be removed and replaced as part of a rework operation, quite often there is so little copper left after the select solder process such that when you remove/replace the connector later, what little copper is left disappears during the rework.
I have seen this happen at many companies, more so in the commercial CEMs using SN100 in the selective solder pot, but also at some hi-rel companies using Sn63.
You see, when the old wave solder machines were used for soldering connectors ALL of the rows of pins were pretty much soldered at the same time with a single exposure to the molten wave that maybe lasted 3 to 5 seconds. The heat transfer from the large wave of solder in a wave solder machine was very sufficient for good topside solder wetting results.
Now consider what happens in a selective solder machine by comparison; a much smaller jet of molten solder is going up and down each row of connector pins. A longer dwell time is needed because this very small solder fountain does not transmit as much heat as the large fountain of molten solder in the old wave solder machines, in spite of the preheaters used. Often the process engineer will attempt to lengthen the dwell time in the jet of molten solder and increase the temperature and solder flow rate to improve the topside wetting.
Also consider; the selective solder machine cannot help but reflow the previously soldered rows of connector leads while soldering subsequent rows, so each row sees multiple dwell times during the soldering of a single connector. This reduces the copper on the PTH annular rings on the bottom (solder source) side of the PWB.

That is why 1/2 oz copper should be used for CCAs with large connectors that will be soldered using a selective solder process, and/or the finish for such a PWB is preferably ENIG/ENEPIG because the nickel barrier protects the copper during the selective soldering process. The copper never sees the molten solder; the IMF is formed only with the nickel, and nickel's much slower dissolution rate into either Sn100 or Sn63 prevents complete dissolution of the barrier. So take my word for it, and never mind what others have said about nickel not making a difference; I can state case by case of where this has happened, and changing to an ENIG finish fixed the issue every single time. I know because I was the one who dealt with it, not once, not twice, but many times. While I am normally not a fan of ENIG (at least not from a BGA standpoint), it is the best finish possible for preventing copper pad or annular ring dissolution.

-----Original Message-----
From: George Wenger [mailto:[log in to unmask]] 
Sent: Friday, November 11, 2016 8:34 AM
To: TechNet E-Mail Forum; Stadem, Richard D.
Subject: Re: [TN] RES: [TN] Immersion Tin - problems

Going back to a previous comment Richard made I too would never use immersion tin as a PCB surface finish. My preference still is immersion silver. Now to Julianos comment about a pad disappearing during selective wave soldering. I can't believe a pad disappearing has any thing to do about immersion tin surface finish or selective wave soldering parameters as much as it base to do with poor PCB fabrication. I would cross section a pad before soldering to see what the copper thickness is

Sent from my iPhone

> On Nov 11, 2016, at 8:40 AM, Stadem, Richard D. <[log in to unmask]> wrote:
> 
> 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

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