I posted a question a few days ago about Press Fit Connectors. I've started
to understand the answer myself.
Press Fit connectors are a mechanical means of making contact with no soldering.
PTH connectors which go through a soldering process that requires CAD layout
to have thermal relief to copper planes. This means a connection to the via
which doesn't go all the way around the circumference but only makes contact
in two or four points. This is to reduce the heat sinking effect of the
copper plane on the via which would cool it during a thermal cycle
(soldering operation) and thereby create potential defects.
Press Fit connectors can be mechanically inserted without soldering
therefore the via can be connected to the plane along its full circumference
with no thermal relief (a solid contact). This solid contact in turn gives
lower inductance for power pins on a backplane.(Which is what the designers
want).
I get the impression that Press Fit is used not just by Compact PCI
protocall (in our case 6U card computers) but anyone heading at high bus
speeds in a backplane. Obviously the speeds are going up and this is to be
commended. But the trend of the assembly industry to use Press Fit
connectors to address this is seriously worrying. Why should the Design
fraternity dictate the assembly technique and not the end result required.
In other words if they want reduced inductance why don't they just specify
no thermal relief and let the assembly heads worry about the means of
connection and whether soldering will be a problem.
APPLYING A FEW TONS OF PRESSURE TO AN ASSEMBLY to attach a connector seems
to me to be primitive method. Yet this is the trend for manufacture in the
future for high speed applications. Why is this basic method being promoted
for the future? Do people out there have any means of automating this
process within a line? How can rework ever be possible considering the
assemblies are expensive board level computers worth thousands of pounds
(dollars). What is the typical yield for such a process? How easy is it to
destroy assemblies in a Press?
Does anyone out there use solid connection to vias on copper planes for the
wave soldering process?
Has anybody done any trials on this?
Pressumably the thermal aspect can be sorted by sufficient pre-heat. Do wave
solder machines need to use convection pre heat zones. Are such machines
available?
It would be nice to see the design people told that there should be a STOP
TO THIS BARBARIAN ASSEMBLY TECHNIQUE. It would be nice to see the connector
manufacturers make connectors which provide shielding and conform to Compact
PCI format and IEC1076 (or high speed bus formats) in Plated Through Hole OR
Surface Mount packages.
Pressumably SURFACE MOUNT WOULD HAVE NO THERMAL RELIEF DEMANDS as modern
Convection ovens have no problems dealing with solid contacts to vias. So
why not develop the Surface Mount version of the connector?
Please, please, please there must be someone out there with an opinion on this.
Thanks

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