On 12:30 PM 21/02/2001 +1100, Michael Beavis said:

> >
> > 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

Good points - this is especially relevant for anyone using teflon based 
PCBs as teflon undergoes a phase change at about 19deg C (room temp) where 
its Z-axis CTE changes very quickly - even quite mild temperature cycling 
of marginal quality teflon boards can readily show barrel cracking as 
Michael said - I too have debugged intermittent failures due to this.

However a problem with making relief connections on Teflon is that usually 
a teflon PCB is designed fro high freq work and so the increased inductance 
of the relief is undesirable :-(

Ian



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