We are doing 100% vertical fill on 5 mm thick 32 layer boards with multiple
ground planes, Pb-Free solder. It requires preheat.  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Nutting
Sent: Tuesday, October 09, 2007 3:10 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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