Advice for switching to SMT?

[CrazyBillyBob]

Well after years of working with through board components I'm looking to start using SMT devices. I've got allot of basic questions, but any additional advice is welcome.
Basic questions:
What type of additional equipment is needed to work with SMT's?
When designing PCB's what are some general rules for SMT's? (pad sizes, tolerances, etc)
Is it crazy to etch your own PCB with SMT's?
Do you have to use solder paste? If so where can you order it (In qty's less then 500G).

If there's a good article on starting to use SMT's point me in there directions.

Thanks for the Help!

[MrAl]

Hi there,


I hope others will respond to this thread too as i'd like to hear more
about these issues too.

I've used some SM parts and made a couple SM boards, but the
smallest i've done was the 0.05 inch spaced pins and small resistors.
These are not too hard to do i guess, with a little practice, and you
can use regular thin solder.
I havent made any boards that carry the SSOP footprint, but i know
of people who have. They do report it's hard to do but somehow it
works i guess. The SSOP SM parts i have done in the past were all
on a board that was already made.

As far as solder paste goes, i'd like to hear more about this too.
I have thought about purchasing some to try but would like to
hear more about it from people who have used it firsthand.
The drawback here is i think the tube only lasts about 6 months
or so?

[Engineer1138]

I have pretty much switched to all SMT except where I have lots of through-hole parts I need to use up.

Start out with 1206 resistors & caps and SOIC ICs if possible. They are easy to solder by hand. I bought some 0805 LEDs by accident and they were harder to use: can't even breathe hard around them!

You don't need solder paste unless you're going for batch methods. Just use fine solder and a small tip.

Moving to paste - I bought a tube from Circuit Specialists for $15 and it will probably do a few dozen boards. A little goes a long way. I haven't used it with ICs yet, but for LEDs, resistors & caps, I put down flux and paste, place the parts and then put the stuffed boards on an aluminum sheet. Five minutes on an electric stovetop and they're done! They also look better this way: parts are all nicely aligned because the surface tension of the melting solder pulls them in line with the pads.

SMT is not hard at all. Just take your time and learn as you go. Pretty much all your soldering skills will transfer.

[Bob Scott]

I agree with Engineer1138 on the 1206 part size. 1206 and 1210 are possible to work with. Anything smaller is too small, but the manufacturers seem to be making parts smaller all the time.

I'm not very experienced at this. For resistors and caps, I found it easiest to pretin only one pad first. Then mount the part while the solder is molten, and finally solder the second pad. It's the only way I can mount the part so that it isn't crooked...otherwise in order to straighten it you have to keep both pads molten at the same time.

If there is an easier way anyone, please let me know. I'd like to learn more about this cookie sheet baking method. I've been soldering one part at a time.

I bought myself a few pairs of reading glasses from Longs Drugs in San Jose varying in diopter strength from +1.0 to +3.5 in 0.5 diopter increments. You gotta be able to see what you're doing! If you are short sighted (minus diopters) and wear glasses like me, you can remove your regular glasses to see closer. Adding the reading glasses gives even more focus range.

It's the first time I needed to use tweezers, and for rework with Kynar wire I needed the fine tipped "nippy cutters" and fine tipped non-magnetic long nose pliers. All the tools in miniature including the smallest solder and iron tip you can find. I use stainless hemostats, straight and bent tip.

Flux? Use a flux pen. It just wipes on.

If the IC lead spacing is less than 0.05", the fun definitely goes away.

[jwax]

A handy device for SMT work is a hot-air gun. Not the one you use for shrinking heat-shrink tubing, but a little tiny one. The tip is small, and you control the amount of heat and the airflow for the job at hand.
Main use is to wave it around a multipinned surface-mount part, and heat all the joints uniformly at the same time in order to remove the part.

All the solder companies have most of their alloys available in paste form, you only have to pick the flux of choice- no-clean, water clean, etc.
Posted shelf life is in terms of months, but if stored properly will last a year plus.

A pneumatic syringe dispenser is great for doing any kind of production. Solder paste is icky. To get good, consistent joints, you need to apply the correct amount of paste repeatedly. A pressurized dispenser will do that.
Then you have available all the different size nozzles, syringes, etc. to take on most any task. Paste can be bought already in syringes.

Good luck!
John