[chbot] SMD experiment #1

[Chris Hellyar]

Hurro,

An update on my quest for stampalikes; Last episode I'd decided to
switch to SMD...

Well, I picked up a couple of scrap boards from Huntsbury which had
0805, SO8 and SOT23 devices on them..

I bought a cheap thermocouple thermometer off ebay, and waited..

The thermometer turned up this morning, and as I was in town I went into
Bricoes and picked up a 1400W toaster oven for the grand some total of
$69.99 on special.

I was originally going to get a 2nd hand one, then I found out how cheap
they are new, and also someone pointed out that my workshop would
probably smell like burnt cheese forever.

I already have a pile of 0805 resistors, and I removed a SO8 IC and some
SOT 23's from a dead Hard disk PCB.

>From here on in the pictures are worth a thousand words:

http://cpix.co.nz/myio/reflow

look at them in order, 0 being first

0 - I used a syringe to put the solder paste on, (got that from Sicom).

1 - placed the components, badly as it happens, should have used better
light and a magnifier, but this was

2 - stuck it in the oven, with the thermocouple right on top of the PCB.
Note that I did this in the kitchen, hence low light...  I don't
recommend this unless you have a very good relationship with your
significant other!  Kate was out, and one of the kids is a bit sick so I
didn't think it was a good idea to hide out in the shed in case he got
up and couldn't find a parent.  (My excuse, sticking to it!)

3 - Raise the board up to 150c and hold it for about 2 minutes..  You
can see the solder paste has started to flow a bit, this is not because
it's melting, but because the flux is, which allows it to settle quite a
bit.

4 - Reflow time, take the board up to 210c for 30 seconds, then cool in
a semi-controlled way (Crack the door open a little).  The solder melts
at ~185c or so.

5 - Ta-Da!

I put too much paste on for the SOT23's by far, and misaligned the SO8
IC, but the 0805's don't look too bad.  A bit of experimentation and I'm
sure I can make them look quite tidy.

Next step is an automatic controller, the onboard thermostat in the oven
is junk, and using the thermometer and turning it on/off is only
slightly better.  I've got some more bits coming to let me drive it from
a PC, which will allow me to program in the 'profile' and log the
temperature actual against target etc..

I also need to look into flux solvents or no-clean solder paste, the
loctite one I've used has fairly horrendous residue.  I used to use
isopropyl on the through hold stuff, might try some on this.  Any one
got a favourite for cleaning SMD stuff, or a source of syringe packed
no-clean or water based solder paste?

Onwards... :-).

Cheers, Chris H.