Possible causes: Based on your description, I suspect that the solder is
solidifying at different rates, possibly due to different thermal masses inherent in the circuit
layout, a thereby causing the tombstone phenomenon. Are the suspect termination pads attached
directly to or totally part of a large copper plane, without thermal reliefs? These pads would
solidify more slowly, allowing the termination to lift out of the liquid solder. Perhaps the
paste printing process is depositing different paste volumes on the pads? There may be thermal
shadowing from adjacent
components, but that's probably an unlikely scenario with convection
reflow.
I've experienced reflow tombstoning similar to yours with a specific type
of chip component. The parts are EMI filter-capacitors (also called three-terminal capacitors).
They're available in standard package sizes, however, have terminations on all four sides (for
example, check the
Murata Electronics page <http://www.iijnet.or.jp/murata/products/english/catalog/k01eemi.pdf>
using part type NFM40P for a photo and technical specifications). BTW, I'm
not affiliated with this company. Are you dealing with this type of device? Completed
assemblies are in-circuit tested (shorts & opens, values, orientation, etc.) which is certainly a
more consistent process
than visual inspection, once equipment and programs are in place.
Hope this helps solve the problem. Contact me directly if you want to
discuss this further.
Ellsworth D. Berkowitz, P.E.
Member of Technical Staff
Paradyne Corporation
phone:813-530-8121
****************************************************************
The opinions expressed herein are solely those of the writer and
do not necessarily reflect the views of Paradyne Corporation.
****************************************************************
LAMBERT.K.A- wrote:
> Hi all,
>
> We've been working at getting an automated line up and running. We're
> making one sided ceramic substrates. The substrates are both low
> temperature cofired ceramic and thick film copper. Both have a 63/37
> solder coating on the tracks. The substrates have passed
> solderability testing with ease. The pads have been designed
> appropriately to minimize tombstoning. Our components are solderable.
> We stencil print solder paste onto the substrates. A robot places the
> components. Reflow soldering happens in a Nitrogen convection oven.
> We are using RMA solder paste (still!)
>
> Now for the question. Why do we get unsoldered connections? Most are
> chip components where one end solders beautifully, the other end is
> sitting on a rounded mound of solder. The distribution across the
> substrate and from one substrate to the next appears to be random.
>
> To detect unsoldered connections, we are doing the old brute force
> method of having manufacturing personnel looking for them after
> reflow.
>
> What are other folks out there doing to:
> 1. Solve the unsoldered connection problem
> 2. Detect the unsoldered connections
>
> Thank you,
>
> Kathie Lambert
> Process Engineer
> Northrop Grumman
> Baltimore, MD
> [log in to unmask]
>
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