[sdiy] moving to SMD/SMT

[Jay Schwichtenberg]

John,

I was forced into SMT in my last job and I went kicking and screaming. Now
that I'm there it isn't as bad as I thought it would be. Even with my 50+
year old eyes and hands. It will take some practicing and patience to get
there though.

First thing is that I'll say most of this works for parts down 0603 size and
to .020 pitch. I wouldn't go any smaller than that hand soldering. I usually
use 0805, 1206 and .050 pitch parts. Then I'd start to think about the
toaster oven. Don't even think about BGA (Ball Grid Arrays) even with the
toaster oven.

I started out using a couple of Antex pencil irons. They worked OK but when
I upgraded to a Weller WES50. With that I use their smallest screwdriver tip
and stubby conical tips. It would be nice to have a curved one for SMT but
they don't make one as far as I know. Yep a MetCal would be nicer but this
was a whole lot cheaper. As far as solder I use .020 water sol. and no
clean. The solder is something a lot of people don't think of but it makes a
big difference. The smaller the gauge the better control you have on how
much solder you are putting on a joint. Even though it is expensive I would
recommend getting at least one spool of 0.015 water sol. or even 0.010 if
you can find it.

AS far as desoldering the solder sucker will suck a lot of parts up if you
are not careful. But in some cases that is what you want. Normally I use
solder wick and that works good. Some times I still hall out a pair of old
Antexs and use them like tweezers to remove 2 'leaded' parts. Also never use
a SMT part once you have removed it. They are very fragile and may have been
damaged in removal. Last thing you want to do is replace another SMT part.
Be careful in removing parts from the standpoint of lifting pads. Through
hole parts will have a via to help hold them on the board when you remove
them. SMDs don't.

That brings up some other tools that you will need. A good pair of tweezers
that can not be magnetized and that solder will not stick to. These will be
useful in placing parts. Once I get them placed and go to tack them down I
use a round wood tooth pick that I have cut the end off of. Be careful
putting too much force on some of these parts will break them.

There are a number of issues that you have to be aware of when laying out
your board. Initially I would not go for doing real dense boards. After you
get good soldering and have some experience then you can start getting more
dense in your layout. Biggest issues for me are two things. First is thermal
transfer unsoldering parts. If you have parts that share a trace that are
close together soldering one may unsolder the other. For me it works to
figure out what the optimal soldering sequence is before I start (or at
least in an area). I look for ways of maximizing the distance between solder
points so less heat gets from one part to another. I place the part as well
as I can then put my tooth pick on the part close to the area I'm going to
solder so I get a good physical contact between the part and pad. Then I
tack the part down either using the solder already on the board or a small
amount on the tip of the iron. I'll tack down a number of parts and then go
back and solder them down for real. The other issue is it is very easy to
place parts such that they interfere with soldering other parts. For example
those surface mount electrolytic stand pretty tall compared to other parts
and can block the space you need to get your iron into to solder resistors
near by. So you have to plan for that type of thing when doing layout.

Another issue is that some SMD are heat sensitive or very fragile. Some of
the poly caps I've use can melt very and I mean very easily. You can hand
solder them but you have to be extremely careful. I was reading the data
sheet for those Panasonic Thermometric Resistors we have been talking about
here. In the data sheet they say don't even use tweezers on them. If they
get there protective coating scratched they will loose their accuracy.

So it can be done but you need to start thinking differently and learn how
to solder differently. But like I said practice and patience can get you
though it.

Good luck.
Jay S.



> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of john mahoney
> Sent: Tuesday, October 12, 2004 3:57 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] moving to SMD/SMT
>
>
> Anybody here doing much with SMT? I'm looking to make the jump. Many
> ICs are only available in SOIC and TSSOP packages, as we all know. I
> like the idea of building small stuff, though, and those discrete SMDs
> are so small. (Widdle baby weesistors, awww, so cute! ;-)
>
> So, what kind of equipment do I need?
>
> I see the term "rework station" a lot, but I don't understand it. Is
> that the tool of choice? Does it do soldering *and* desoldering? My
> Soldapult would probably suck up an entire SMD! Maybe desoldering
> braid is better? (Or is the rework station really the way to go?)
>
> If an iron with a 1/32" or 1/64" tip is good enough, that'd be nice. I
> also read here http://protoboards.theshoppe.com/ that there's a
> soldering tip that looks like a miniature ladle. Do I need one of
> these, too?
>
> I've also read about the toaster oven reflow technique. Seems
> interesting, and I suppose that I'll try it, but it won't replace the
> need for a more focused soldering method.
>
> And since SMDs usually go on PC boards, is there anything special to
> keep in mind when making PCBs?
>
> Any other tips, warnings, or links?
> --
> john
>