[sdiy] SMD experiences

[Tim Parkhurst]

On Fri, Sep 9, 2011 at 1:08 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
>
>
> What's a "thermal relief pad" and why would I only need one for through-hole?
>

Hi Tom,

A thermal relief isn’t really meant to keep solder out of vias. A
thermal relief is designed to allow a via (or any other plated through
holes) and a copper plane to expand / contract at different rates
without cracking.  For the uninitiated, a via is basically a tiny
plated through hole that is used to connect one layer to another on an
SMT board. A thermal relief is made by connecting a via to the copper
plane with four small strips at each quadrant of the via (rather than
a continuous solid connection, which could become a stress point under
thermal expansion). If a via is near a solder pad, it should be
separated from the pad by a short, thin trace (with solder mask over
the trace). Vias in pads should be avoided whenever possible, as they
can actually wick too much solder in and create an unreliable joint.
However, if you absolutely have to have a via in a pad, it should be
located in a corner of the pad to minimize the amount of solder that
gets pulled in.


Tim (did my time installing pick and place machines) Servo
-- 
"Sire, the church of God is an anvil that has worn out many hammers."
- H.L. Hastings