IMHO, nitrogen will allow you to lower the Tp 3-5 degree and change reflow
profile to shorter.  it does have an impact on the bridging, and warpage. 
the solder ball claps (reflow) of corner and center under nitrogen showed
little difference, but not in air... your HOP or bridge will tell you the
device might be tilt to one side or warped.  nitrogen will help, but not
cure all.  my 1.5 cents.
            jk
> Bob,
> Did you see this? Lots of good process information.
> http://www.ti.com/lit/an/swpa182c/swpa182c.pdf
> Hope it helps.
> dean
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Wettermann
> Sent: Monday, January 23, 2017 11:29 AM
> To: [log in to unmask]
> Subject: [TN] Nitrogen Reflow and Warpage Issues on POPs
>
> Dear Technetters:
>
>
>
> We have a Class 3 customer with very complex, high density placements
> (think cell phone for the military). This board is laden w/0201s, 0.4mm
> pitch BGAs, shields, mirrored BGAs and the TI Omap POP (0.4/0.5mm pitch).
>
>
> The problems in this board assembly for some time have been isolated to
> this OMAP part.  We have lots of shorts and opens (randomly-spaced) as
> confirmed by electrical testing, dye and pry and cross sectioning. The
> Omap has been shown to warp 5-7 mils depending on the reflow profile.( It
> is a no clean SAC305 process). We have profiled the reflow process by
> embedding TCs in the corners of the balls of the device (ball interface
> with the board where all of the problems are) , the die,  neighboring
> components etc. We dialed in the profile as low of a temp as possible.
>
>
> We are running several DOEs to come up with a placement/rework process.
>
> As part of the investigation the client is fixated on nitrogen reflow
> being part of the solution to these warped BGAs. Nitrogen reflow IMHO is
> the solution for joint aesthetics, wetting issues and enhanced reliability
> of the interconnection.
>
>
> ? Does anyone have experience with hot air reflow using nitrogen where its
> addition helped to mitigate warpage issues?
>
>
> Thanks!
>
>
> --
> Bob Wettermann
> BEST Inc
>