[sdiy] moving to SMD/SMT

[Tony Clark]

   Some additional bits inline with Jay's post...

> As far as solder I use .020 water sol. 

   This is a good size for pretty much everything.  I use this for 
thru-hole as well as SMT.  For fine pitch, I'd recommend a smaller 
diameter wire for sure.

> 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.

   Nice technique!  I end up just using a single iron to remove 
passives.  SMT parts tend to retain heat too well, so if you heat up one 
pad and quickly move the iron to the other pad, you can practically slide 
the part right off the pads.  I'd consider this an advanced technique, 
since obviously you do run a risk of damaging the pads.

> Also never use a SMT part once you have removed it.

   Very true.  I know it goes against the nature of many DIY'ers, but 
unless the chip is something really special (or very expensive!), I throw 
the parts away.  With IC's I usually use a fine tipped pair of clippers 
and clip the leads on one side.  Then use a fine bladed trimpot 
screwdriver to lift the one side of the IC up off the board.  You can 
then easily work the part back and forth until the remaining pins shear.  
Then use a soldering iron to brush off the legs from the pads.

> 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.

   Here you really begin to appreciate the quality of boards from 
different vendors!  I have had trouble in the past with prototype boards 
that were very fragile from a rework stand point.  It's definately a good 
idea to pay a bit more to get a high-quality board when doing SMT!

> 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.

   Sometimes a magnetic set is handy too.  :)  But for dealing with very 
small passives, definately non-magnetic!  Oh, and the trick to putting 
most SMD parts on a PCB is to simply apply solder to ONE pad first.  Then 
move the part into place with your tweezers and quickly secure it to the 
soldered pad.  Then you are free to solder the other pad(s) without too 
much worry.

> 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.

   This is true, but I haven't found this to be much of a problem except 
at the 0603 level, where thermal energy can transfer straight through the 
part and literally melt the solder on BOTH ends of a part!  It's harder 
to do that with 0805 and larger parts.

> For me it works to figure out what the optimal soldering sequence is 
> before I start (or at least in an area). 

   I usually just work with the smallest and work up.  As Jay said, those 
electrolytics can block out quite a bit of space, so I usually save those 
for last.
   One thing that really helps me (especially with my high-density 
projects) is to create a map of where parts go.  What I do is print out 
the pad arrangement of the PCB from my CAD program and then make a lot of 
copies of it.
   Then, using a colored marker, I mark the location of one type of part 
(like 1K resistors) on one sheet.  If the page isn't too busy, I'll then 
switch to a different color for a different valued part (say 10K).  I 
simply make a note in the page margin of the part value, then color code 
the text.  That way, when I'm faced with a very blank PCB, I can look at 
my colored pad printouts and use them like a map!

   Cheers,

   Tony

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