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soldering tiny surfmount pins

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kell

Fri Feb 24 2006, 08:05PM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

Are the old threads on the other board lost? Because I'm about to
start another one about surface mount soldering.

Searched all over for a diode array for a circuit
I need to build, found CMEDA-6I
SOT-28 package, 8 pins, four on each side
with a .026" pitch.
I never soldered surface mount stuff.
I don't think I would hesitate to tackle something
with a .05" pitch, but I look at .026" with some trepidation.
How should I go about it? I did a little reading on the web
and it looks like I have a couple of choices with surfmount stuff.
I can solder pin by pin, or I can drag the iron along all the pins,
or I can use the toaster oven technique.
This SOT-28 package, if I use it, would have the smallest pitch of any device in the circuit.
I can probably get everything else with at least .05" between the pins.
Tips on surf mount soldering appreciated.
Also, if anybody knows of a chip with an array of up to about
10 (isolated) diodes but with a .05" pin pitch, tell me about it.
I have other questions, but I guess that's enough for now.

craigsarea

Fri Feb 24 2006, 09:10PM

Registered Member #130 Joined: Fri Feb 10 2006, 08:12PM
Location:
Posts: 15

In the past I've used desolder braid to solder surface mount components. You do it like this (assuming you're soldering it to a PCB):

1) Secure the surface mount component in place (glue or a steady pair of hands).
2) Cover all the pins with blobs of solder. At this point all the solder joins to together so you basically end up with 2 big blobs of solder on each side of the component.
3) Use desolder braid to 'soak' up as much of the solder as possible. This removes all the visible solder but leaves just enough to make a connection between the component pin and the pad. You shouldn't have any short circuits between the pins.

I've soldered very fine pitch components like this without any problems. It can subject the component to a lot of heat though.

Avalanche

Fri Feb 24 2006, 09:20PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Yes I agree with the above, except use plenty of flux as well, it helps the solder separate properly when you're mopping it up and it is just a heck of a lot easier when it's drenched in flux!

ragnar

Fri Feb 24 2006, 11:34PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

And don't forget to meticulously wipe the flux off with something (e.g. alcohol/spirits) afterwards. =)

Carbon_Rod

Sat Feb 25 2006, 12:01AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

The folks at seattlerobotics show how the home solder paste works:

Most people use a simple small drop of cyanoacrylate (Krazy glue) in the centre to prevent the sliding problem.

Domo arigato Mr.Roboto

I thought it would be funny to try this with a toaster that auto-pops the board when done... lol he he he

kell

Sat Feb 25 2006, 12:06AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

Four traces will run underneath the chip, connecting pins on opposite sides.
I won't be able to get braid under the chip to soak up solder bridges.

How about putting a bead of solder on each pad, setting the chip down on top and then putting the board on a hot plate until the solder melts. Maybe prime the chip's pins with solder too, before setting it on the pads.

...

Sat Feb 25 2006, 12:55AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Most (if not all) smd chips have a coating of solder on the pins already, designed so that you just set the chip on the board and heat it up the the melting point of the solder, then cool it off (referred to reflow soldering).
On the other hand, I have yet to hear of anyone doing this successfully...

ragnar

Sat Feb 25 2006, 05:51AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Surface mount is easy - get a fine tipped iron, put little touches of solder on all the pads. with fingers/tweezers/pliers/tape, put the chip in place, solder pin 1, solder pin 16... now solder the rest =)

chip doesn't get hot, no flux, no solder brain, no pain... except you need to have good vision and fine skills...

alternatively (but requires similarly fine skills) you might make an SMD to DIP adapter from an IC socket... then socket the chip...

[edit] Ah.. fine co-ordination and good eyesight won't be any use on e.g. the 132-pin package processors in that link. =P

Dr. Slack

Mon Feb 27 2006, 12:03AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

See if you can get some optical aid - a strong magnifying glass or an eyepiece, I bought a 7x stereo microscope at a yard sale which is invaluable. You will be surprised how much manual dexterity you have when you can actually see what you are doing. Solder pin by pin, use a fine tipped iron, file a tip down if you dont have one fine enough, inspect the joint, wick away any excess solder, stop putting on solder when the joint is conductive, even if it still looks a little light.

Self Defenestrate

Mon Feb 27 2006, 12:53AM

Registered Member #87 Joined: Thu Feb 09 2006, 01:36PM
Location: San Jose
Posts: 191

Plenty of flux is key. I like to set the chip on the board and clamp it with something like an aligator clip, and then drag a drop of solder across the pins. A real hot tip fine tip iron works best, and any bridges can be fixed by heating a razor blade and melting the pins apart.

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