Homebrew PCBs

Hi list,

For the first time today I used solder paste. Much nicer way of soldering
fine pitch SMD - using the soldering iron to do it was getting old rather
fast. My method was to lay down a bead of paste on each row of pads, place
the chip, then use a hot air gun. It was nice not to have to spend 15
minutes carefully lining up a big LQFP then bodging it when trying to tack
the first pin and starting again. I had read that the component will try
and centre itself thanks to surface tension, and this turned out to be a
good tip.

My main problem at the moment is putting down to much paste rather than
too little... but I had far less solder to wick up after soldering than
doing it by hand and the result is much neater.

The paste I got is Edsyn CR44 - it seemed reasonably easy to apply. I
bought a syringe of the stuff.

What's the best way of making the solder paste keep for as long as
possible? I had heard that it had to be kept refrigerated, and that it
would only be sent same day frieght and all this sort of thing but as far
as I could tell, Farnell didn't treat the order any differently than any
other component (it certainly didn't go with any priority - took 2 days to
get to me!)

I have a customer that uses our temperature monitoring equipment to make sure their solder paste never gets colder that 37 degrees or warmer than 45 degrees fahrenheit during storage.

Stuart

  ----- Original Message ----- 
  From: Dylan Smith 
  To: Homebrew_PCBs@yahoogroups.com 
  Sent: Wednesday, April 23, 2008 2:15 PM
  Subject: [Homebrew_PCBs] Solder paste advice


  Hi list,

  For the first time today I used solder paste. Much nicer way of soldering
  fine pitch SMD - using the soldering iron to do it was getting old rather
  fast. My method was to lay down a bead of paste on each row of pads, place
  the chip, then use a hot air gun. It was nice not to have to spend 15
  minutes carefully lining up a big LQFP then bodging it when trying to tack
  the first pin and starting again. I had read that the component will try
  and centre itself thanks to surface tension, and this turned out to be a
  good tip.

  My main problem at the moment is putting down to much paste rather than
  too little... but I had far less solder to wick up after soldering than
  doing it by hand and the result is much neater.

  The paste I got is Edsyn CR44 - it seemed reasonably easy to apply. I
  bought a syringe of the stuff.

  What's the best way of making the solder paste keep for as long as
  possible? I had heard that it had to be kept refrigerated, and that it
  would only be sent same day frieght and all this sort of thing but as far
  as I could tell, Farnell didn't treat the order any differently than any
  other component (it certainly didn't go with any priority - took 2 days to
  get to me!)



   

[Non-text portions of this message have been removed]

No Farnell probably couldn't care less how long the paste keeps after
you opened it, i bet they just stock it like all their other stuff.

The Edsyn CR44 seems to keep far better than other types. I've
continued to use the same dispensing tips after many days of storage
in the fridge (tip separately from capped syringe, just in case i
don't need it for an extended period of time).
But i've also left the used dispensing tips on the bench and they were
just fine next day. With other paste they were dry and clogged after a
few hours.

Anyway, i'm fairly sure this paste could keep well at room
temperature, but i still put it in the fridge (in a zip-lock  bag,
together with the tip).

Of course there's a drawback too. The CR44 is just about the worst
paste i have ever used in regards to reflow behavoir. The stuff
softens like butter as soon as it gets hot, and builds huge puddles on
the PCB. Any other paste i ever used (apart from plumbing paste ;-) )
was far better in that regard. A SMD paste should "keep shape" under
heat until reflow. This would also keep the components in place better
(with CR44 i have more that misalign).

Maybe one day i will find a paste that has both, decent shelf life
(fridge shelf is OK with me), and decent reflow properties. But until
then i prefer to stick with a paste that still comes out of the
syringe when i need it, even if the reflow properties are rotten.

Too bad i didn't think to ask, had a rep. from a soldering tool
company visiting this week.

ST

On 4/23/08, Dylan Smith <dyls@...> wrote:
> Hi list,
>
>  For the first time today I used solder paste. Much nicer way of soldering
>  fine pitch SMD - using the soldering iron to do it was getting old rather
>  fast. My method was to lay down a bead of paste on each row of pads, place
>  the chip, then use a hot air gun. It was nice not to have to spend 15
>  minutes carefully lining up a big LQFP then bodging it when trying to tack
>  the first pin and starting again. I had read that the component will try
>  and centre itself thanks to surface tension, and this turned out to be a
>  good tip.
>
>  My main problem at the moment is putting down to much paste rather than
>  too little... but I had far less solder to wick up after soldering than
>  doing it by hand and the result is much neater.
>
>  The paste I got is Edsyn CR44 - it seemed reasonably easy to apply. I
>  bought a syringe of the stuff.
>
>  What's the best way of making the solder paste keep for as long as
>  possible? I had heard that it had to be kept refrigerated, and that it
>  would only be sent same day frieght and all this sort of thing but as far
>  as I could tell, Farnell didn't treat the order any differently than any
>  other component (it certainly didn't go with any priority - took 2 days to
>  get to me!)
>
>
>  ------------------------------------
>
>  Be sure to visit the group home and check for new Links, Files, and Photos:
>  http://groups.yahoo.com/group/Homebrew_PCBsYahoo! Groups Links
>
>
>
>

Speaking of puddling...

I have a syringe of SYNtech paste, specifically because it doesn't
need to be refrigerated.  I've got a bit of old wire insulation to cap
the end (the insulation is itself plugged, so no air gets in).  It's
been sitting on my bench for over a year and half now, in sporadic
use, and still works like when I got it.  Puddles when warmed, though,
so unless you've got a precise mask (i.e. no excess paste) there's not
a lot of point in being too careful.

The grind is pretty good, too - solder particles are too small to be
individually discernable.

On Wed, 23 Apr 2008, DJ Delorie wrote:

Hmmm...

> been sitting on my bench for over a year and half now, in sporadic
> use, and still works like when I got it.  Puddles when warmed, though,
> so unless you've got a precise mask (i.e. no excess paste) there's not
> a lot of point in being too careful.

Perhaps that's the tradeoff - the less puddly solder pastes have short
shelf lifes and need refrigeration (and are probably designed more for the
factory), and the solvents in the stuff they sell in syringes are assumed
to be used in prototyping, so small amounts and sporadically - so are
forumlated for long life at the expense of other properties?

I did notice the Edsyn stuff puddling a bit where there was a lot of
paste, but then again, doing hand assembly without a stencil I'm drawing a
line with the stuff anyway. But I'm a true believer in solder paste now,
it was so much faster and easier than drag soldering (especially the big
square chips).

Dylan Smith <dyls@...> writes:
> But I'm a true believer in solder paste now, it was so much faster
> and easier than drag soldering (especially the big square chips).

Yeah, once you get a syringe and some reflow device (I use a
hotplate), you use it for everything.  I do all the top-side SMTs with
paste now, even resistors and capacitors.  Just dab paste all over the
place, place the parts, and heat.  Done!  I've found I can use paste
on 0603's even with the iron - dab paste, place part, touch with iron
to melt paste.  It avoids the surface tension issues that arise from
wire solder and small parts.

Can anyone comment on:

RM10 - SN62 AGS83

This is the only solder paste that my regular supplier holds.  Quoth the 
catalogue:

"RM10 solder pastes have been forumlated as general purpose RMA type 
products showing a good combination of features for most SMD assembly 
processes."

My main criterion is that it should be suitable for occasional use.  If 
it's not, I'll have to look to other vendors.

Cheers

M


-- 
Matthew Smith
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