>
> Would you indulge my curiosity ? How is "thermal relief" better than
direct
> connect on your board ?
>
> --
> David Cary
>

Jumping in
Soldering pth vias on planes improves the reliability of the connection and
to achieve this, thermal relief is recommended (IPC-D-279).
Cyclic thermal excursions cause a mismatch between the Cu in the plated hole
and the surrounding dielectric (eg.FR4) due to differing CTE's and can cause
seperation of the via land from the hole plating resulting in open or worse
still, intermittant connections. I have seen this happen and spending
several days in a walk in temperature cycling chamber debugging a batch of
boards isn't easy to forget.
The mismatch relies on the ductile properties of Cu to take up the slack but
over time, temperature cycles, vibration and any other environmental
hazards, this property is reduced (work hardening of metals). Solder is far
more ductile than Cu and thus offers greater reliability.
Wave soldering tends to fill untented vias anyway so they may as well be
filled properly.
The draw back of increased weight versus higher reliability is a no brainer
when it comes to our applications which are required to work under extremes
of vibration and temperature.

Best regards

Michael Beavis

Advanced Mining Technologies Pty Ltd
Research & Development dept
PO Box 376 Wyong NSW 2259 Australia
(20 Lucca Road, North Wyong, NSW 2259)
Tel: (+61) 2 4351 1778
Fax:  (+61) 2 4351 1764
Email:  [EMAIL PROTECTED]
Website:  www.advminingtech.com.au




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