Hi Phil,
Quite possibly the industry norm for this type of situation has now changed
with the advent of RoHS compliant solders and their particular
idiosyncrasies. You may be right on the leading edge of those that define a
new norm to address this fill situation. Top-filling the solder junction may
indeed be the preferred method for handling this. We've all gotten used to
many "can't do's" over the years with leaded solders. We may have to
re-visit some of them, even against our better judgment, based on the new
solder types.

My 2 cents worth,
Dale Ritzen
Quality Manager
Austin Manufacturing Services

-----Original Message-----
From: Phil Nutting [mailto:[log in to unmask]]
Sent: Tuesday, October 09, 2007 2:10 PM
To: [log in to unmask]
Subject: [TN] RoHS solder and wetting plus thermal reliefs


Hello Technetters,

Let me see if I can explain the problem.  We build some PTH boards with
4 oz. copper and connect some high power IGBT devices with thermal
reliefs on the pads.  In the past with tin/lead everything worked well.
More of our designs are being converted over to RoHS (SAC 305).  It
appears we are not getting as much flow through or wetting on these
devices.  So there is a two fold concern.  During a specific failure
mode the thermal reliefs act like a fuse causing arcing until enough
copper vaporizes to make a large enough gap that the voltage can no
longer jump. One solution is remove the thermal reliefs.  Part two is
the amount of current passing through the PTH.  We can't rely on the
barrel for full current carrying capacity so 100% barrel fill is desired
with a good fillet top and bottom.  Here is where we are having issues
with RoHS solder not fully wetting or filling the PTH.

I'm concerned that removing the thermal reliefs will generate other
problems.  I'm thinking we should use a different configuration relief.

If the RoHS solder does not fill the hole we would typically go back and
top solder the connection.  I know the industry considers this a no-no.

Any recommendations?

Thanks in advance.

Phil Nutting

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