Peter is having problems with immersion TIN not immersion S
Lee,
Peter is having problems with immersion TIN not immersion SILVER
Regards,
George
George M. Wenger
Andrew Solutions
Senior Principal FMA / Reliability Engineer
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [Office] (732) 309-8964 [Cell]
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee parker
Sent: Tuesday, September 22, 2009 3:03 PM
To: [log in to unmask]
Subject: Re: [TN] Imm Sn soldering issue.
Peter you mentioned that the defects are normally on the second side. I
was also wondering if the defects are usually associated with the same
features. If so you may have problems other than the immersion silver
surface. Issues I have uncovered are soldermask residues and soldering
fixtures (if you are using fixtures) that prevent contact of the board
and the solder. I suggest that the next time this occurs you process a
bare board with no no soldermask or fixturing and see if you get the
same result. Quite often I do not.
Best regards
Lee
J. L. Parker Ph. D.
JLP Consultants LLC
804 779 3389
----- Original Message -----
From: Peter Barton<mailto:[log in to unmask]>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Tuesday, September 22, 2009 12:33 PM
Subject: [TN] Imm Sn soldering issue.
Dear Technetters,
Hopefully there is someone out there who can help me understand a very
specific issue we have using PCB's plated with Imm Sn. namely,
non-wetting of solder to some lands on the second side of population
after reflow. We have an understanding of this surface finish with
respect to the assembly processes. We are careful how we handle the
unpopulated PCB's, the PCB's are shipped directly to the line and are
only removed from the manufacturer's packaging at the point of use. They
are not pre-baked and are subjected to 2 relatively benign reflow
cycles.
We only ever see the issue on the second side. It occurs on various
component types. When it does occur the solder deposited at the printing
stage is reflowed and is attached to the component termination,
favouring this as more wettable than the PCB land. The problem does not
always occur. The assembly is populated in 2 up panels and we have
examples where one panel has soldered joints across the piece, whilst
the other PCB in the same panel exhibits non wetting in some areas.
The assembly has been profiled with a good distribution of
thermocouples on a sample assembly to confirm a low delta T and the
profile itself is right at the centre of the paste manufacturer's
recommendations. We like to have a good process window.
I am aware that the 2 possible reasons for this condition are (a)
thermal degradation of the finish and (b) possible plating quality
issues.
We have dismissed excessive thermal input as these have only been
subjected to 2 non-RoHS thermal cycles and the PCB's are fresh from the
manufacturer. Peak temperatures are only around the 227 deg. C mark. The
Sn thickness specification is 0.8 - 1.2 uM. The applied thickness has
been measured using XRF as a process control and averages 1.0 uM. I am
aware that XRF is only a guide as it cannot segregate out 'available'
pure Sn from Sn/Cu intermetallics.
We have had an independent lab analysis conducted and example lands
where this condition is seen are shown to have very little or no
remaining Sn at the surface, the majority being Sn/Cu intermetallics.
Other unpopulated plated areas have also been subjected to SEM and EDX
for comparison and exhibit what is described as 'islands' of Sn in a sea
of Cu/Sn intermetallics.
We have not had 'virgin' PCB's analysed yet for comparison as it is
costly and we are not sure if we would pick the correct example to look
at - it could be a good one.
For the PCB experts out there out there I have a couple of questions:
1. If the application of Sn is a displacement reaction with the
underlying Cu is there any way that there can be less Sn on some surface
areas than others?
2. Are there other reasons that there could be variation in Sn
deposits or could lead to the 'islands' of Sn as described by our lab
that is seen after thermal processing?
3. Are we missing anything else?
Any questions please feel free to ask - this is causing me the biggest
headache going.
Peter Barton
Senior Process Engineer
ACW Technology Ltd
Dinas Isaf West
Tonypandy
Mid Glamorgan
CF40 1XX
Tel: 01443 425275 (direct)
Fax 02380 484882
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