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Re: OT: Soldering SMDs (was Re: [avr-chat] isp-usb)

From: Erik Christiansen
Subject: Re: OT: Soldering SMDs (was Re: [avr-chat] isp-usb)
Date: Thu, 16 Mar 2006 20:19:52 +1100
User-agent: Mutt/1.5.9i

On Thu, Mar 16, 2006 at 10:53:25AM +1000, Don Ingram wrote:
> Be very careful with this!!!!!!
> As the board heats it is constrained by glass mat in the X & Y axes
> but not in the Z axis. Epoxy expands considerably when heated above
> glass transition temperature and the Z axis is the only way it can go.
> If overheated or heated for too long the FR4 will swell and tear the
> barrels out of the via's & component holes. When the board cools they
> return to size and provide intermittent connectivity with the pads
> leaving a marvellous little box of intermittent tracks and pads.

Ooo - Err, that's worth avoiding!

A peek at www.speedingedge.com/PDF-Files/tutorial.pdf suggests FR4 will
increase in thickness by 5.1% at 300 °C, that eutectic solder melts at
185 °C, and FR4's Tg is 125 °C. Now, if copper has a Thermal Expansion
Coefficient of  17 x 10e-6/°C, then it'll expand by nearly 0.5% at 300
°C, leaving 4.6% expansion differential. Hmmm, maybe that's too much,
given that unannealed copper's elongation is only 2% at the yield point,
and that's already 93% of UTS, so rupture is probably staring us in the
face before 4.6% elongation. Ouch!

OK, if we're to keep the substrate under Tg, it seems better to direct
temperature controlled air onto the components. The greater thermal
conductivity of the leads and solder will hopefully result in the heat
going quickest to where it's most needed. No skillet, and no upturned
iron. :-(

> A test run with a low mass thermocouple affixed to a scrap PCB with
> high temp solder can give you some useful feedback. Similarly the temp
> sensitive plastic provided with the proto stencil kits can provide
> useful feedback. Toss the time/temp data into excel and have a look at
> the reflow profile that you see on your rig & compare it to a
> published profile from one of the IC manufacturers, this will allow
> you to tune your process to get a more consistent and reliable result.

Yes, that seems to be needed, to determine whether the pcb can be
warmed, to minimise heat sinking from the solder we're trying to melt.


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