Sorry about the late response. I don't have direct experience with tin residues, but I've seen similar defects from LPI residues in the hole. The residues were in the form of a ring of almost transparent LPI soldermask in holes of varying diameter. The rings were more prevalent in smaller diameter holes, but could occur in .187" dia holes. The residue was very thin - usually around .002 to .005 inch wide with even less thickness. The ring is almost impossible to see - even under a microscope, but it would act as a solder dam that prevented solder wicking from through the hole for both hand soldering and wave soldering.
A brief soak in an immersion tin bath will coat both bare copper and immersion gold with a light colored finish. A soldermask ring will appear to be a different color - possibly dark or the color of the substrate. The appearance of a thin ring is dependent on several factors and the exact appearance is difficult to predict. Excessive immersion time could undercut the ring and remove the evidence.
Don Vischulis
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]]On Behalf Of John Burke
>Sent: Thursday, March 22, 2007 10:50 AM
>To: [log in to unmask]
>Subject: Re: [TN] Solderability problems on through holes only.
>
>
>If the tin stripping has issues it will "poison" the nickel deposition
>leading to poor or non existent nickel skip plating effects. Generally this
>will not impede the gold deposition process which will deposit straight onto
>the copper. This will generally not be visually apparent (particularly down
>a hole).
>
>This is normally a condition though seen on very small via holes due to
>fluid exchange in the hole, although with an incomplete tin strip is
>possible on a through hole. This is also an issue if the holes are blocked
>by mask residue during the stripping process. Since not all holes are
>impacted I would be looking at this as a first cut.
>
>
>
>
>
>John Burke
>
>(408) 515 4992
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
>Sent: Wednesday, March 21, 2007 2:25 PM
>To: [log in to unmask]
>Subject: Re: [TN] Solderability problems on through holes only.
>
>Yes, you are probably correct on that. Do you know if there is a
>fabrication or plating issue that would cause nickel skips in the
>annular rings? If there were no nickel at all, I would imagine there
>would be oxidized copper under the gold dewetting into the solder. The
>thing that puzzles me is why they are having difficulty soldering to
>through-hole vias/pads, but apparently the SMT pads are soldering up ok.
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
>Sent: Wednesday, March 21, 2007 4:15 PM
>To: [log in to unmask]
>Subject: Re: [TN] Solderability problems on through holes only.
>
>On this one I'll have to disagree and side with Rudy.
>Richard,
>
>On this one I'll have to disagree and side with Rudy. I'm not a
>metallurgist but it is my understanding that solid state diffusion of
>copper through nickel is extremely slow. Even if it did migrate to the
>surface of the nickel what problem would it cause? Certainly not a
>solderability problem. It is my understanding that most fluxes were
>formulated to reduce copper oxide not nickel oxide. The flux chemistry
>should certainly work better on the copper oxide on the surface than if
>there was nickel oxide. From my old hybrid integrated circuit days I
>was lead to believe that the people doing wire bonding wanted thick
>nickel and thick gold. Their reasoning was that the nickel is a
>leveling agent and provided a "hard" surface as will as prevented a
>diffusion barrier. There is certainly lots of published work indicating
>that thick nick was advantageous for wire bonding but I have not seen
>any that has shown you need thick nickel to get good soldering. There
>is certainly lots of published work indicating that you need a good
>quality gold coating to protect the underlying nickel from oxidizing.
>
>
>Regards,
>George
>George M. Wenger
>Andrew Corporation Wireless Network Solutions Senior Principal FMA /
>Reliability Engineer 40 Technology Drive, Warren, NJ 07059
>(908) 546-4531 [Office] (732) 309-8964 [Cell]
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
>Sent: Wednesday, March 21, 2007 4:40 PM
>To: [log in to unmask]
>Subject: Re: [TN] Solderability problems on through holes only.
>
>The nickel barrier may be too thin, allowing copper to migrate through.
>This would lead to poor solderability.
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of R Sedlak
>Sent: Wednesday, March 21, 2007 3:29 PM
>To: [log in to unmask]
>Subject: Re: [TN] Solderability problems on through holes only.
>
>I would suspect quality of gold plate. If the gold plate was light,
>and therefore porous, the Nickel under the gold might oxidize during the
>SMT, and become unsolderable?
>
>Rudy Sedlak
>RD Chemical Company
>
>
>Peter Wong <[log in to unmask]> wrote: Hi Technetters,
>
>
>
>We are having a solderability problem confined to only through hole
>locations where we need some help or advice.
>
>
>
>We are soldering a no-clean SAC305 alloy to an ENIG finished board. All
>SMT solder joints (both sides) are fine. During hand soldering in our
>dedicated lead free area, we are having problems with solder wetting to
>some of the annular rings and barrels of through hole locations, not all
>holes. The assembly is an 0.062" thick 4 layer 1"x1" board so it is not
>thermally challenging.
>
>
>
>We've built thousands of these in the past without this particular
>problem.
>
>
>
>The time lag between smt and through hole assembly is 1-2 days.
>
>
>
>We've tried using a more aggressive flux such as OA flux with no
>improvement.
>
>The only way that we can get solder to wet is to mechanically scrape the
>annular rings with a pick.
>
>
>
>Has anyone seen this before? Why would this only occur on some of the
>through hole locations and not on smt pads?
>
>
>
>We have isolated this to a specific lot code of boards and are waiting
>for some advice from our board vendor as well.
>
>
>
>Thanks
>
>Peter
>
>
>
>
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