I found ImSN to be more difficult to solder at selective than ImAg.
A number of items to look at that may be unrelated to the ImSn but are part
of the transition to PB free.

Check the flux, type, quantity, pattern.
Alcohol is better than VOC free (water).
Selective is limited in the preheat and air flow available to take the
water away.
The newest, higher grade machines have improved this.

Check the solder used.
SAC 305 costs more but solders a few degrees cooler than SN100 or other
alternatives.

What is temp of solder at tip of nozzle?
It is cooler than the solder in pot unless the nozzle is very  large.

I was watching a selective machine spend 6 seconds  making a dipped solder
joint last week.
The company has set up parameters where this works for them.
I would have blistered my PCB at this dwell time and been eaten alive for
the cycle time.

What are the grounds like around the solder joint?
No thermal relief is tough.
Heavy solid planes are tough.
That type of joint is difficult to put enough energy in to heat the
surrounding area and get the top side hot enough.
A larger nozzle to add more heat, running the nozzle over the surrounding
areas to act as a preheater are options. Rigs that act as preheaters over
the PCA can help during the solder process.

Going from Pb to Pb free is a challenge at selective.

At one site, I swear the surrounding chicken coops threw enough sulfur in
the air to mess with the ImSn finish.
Just because you could smell the chicken shit in the air doesn't mean it
will affect the product.

I prefer ImSN to HASL.
Been bitten by crummy solder flatness from a few suppliers.
The volume product worked ok with Pb free HASL.
It was undemanding for flatness.
The Quick turns were a disaster.

Your specific requirements must be examined.
Forced change from alcohol to VOC flux.
PCB designed for selective.
PCB designed with a higher Tg for the higher temperature solder.
Nozzle design compatible with requirement.








On Thu, Nov 10, 2016 at 2:32 PM, George Wenger <[log in to unmask]>
wrote:

> 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
>