Reflow TQFP without stencil?

[cyberfish]

Has anyone tried reflow soldering TQFP (or similar small pitch footprints) without a stencil, and just apply the paste with syringe?

How did it turn out?

I already have my PCB made, and trying to decide if I want to get a Pololu plastic stencil for it.

Thanks

[djsb]

Just hand solder it. You can apply tacky flux, place the part and line it up. Then solder one of the pins on each corner. Apply some solder to a suitable bit and drag solder the remaining pins. Remove any excess solder with solder braid. Clean up.
Or use hot air with a small jet. ( with temperature/flow adjustment capabilities)
- Use flux to coat all of the ic's pads on the pcb.
- Put some solder on the tip of the soldering iron and drag it regularily across the ic pins.
- Use flux again to coat all of the ic's pads on the pcb.
- Place the IC on the board on top of the solder and fix it at 2 corners
- Use the hot air and heat the pins while pressing down the device weakly



David.

[TopherTheME]

I dont even use flux when soldering TQFP chips. I just align the chip and drag a tinned iron across all the pins. Then do a little clean up with and braided copper and the chips good to go.

[mikeselectricstuff]

Although you can do without, Flux really does make things easier when using solder wire (as opposed to paste).
My normal way of hand-mounting a TQFP is to put a line of paste along each pin row on teh PCB, place & align the decive on top,  tack a couple of corner pins with a fine tip, then use a full-width blade bit to flow each side in one - place the the bit on the pins, then drag it away from the chip body to draw excess solder off with the bit. 

[cyberfish]

Thanks

I guess it comes down to solder wire and solder paste then. Anyone done both? How do they compare?

[djsb]

Paste needs to be stored properly and as I don't use it much it goes off quickly. I find it difficult to use but thats because I'm not used to it.
Solder much easier all round I find.

David.

[mikeselectricstuff]

Paste needs to be stored properly and as I don't use it much it goes off quickly.

This seems to depend a lot on brand - I have some EFD Solderplus that's 3 years old & still fine. I do keep it in the fridge though.

[djsb]

Mike,where do you buy buy your paste from? Might try experimenting with it again.

David

[mikeselectricstuff]

Last lot came from ebay...

[cyberfish]

I've decided to get a stencil, mainly just to try it out.

[Cheshyr]

If you want to get super-fancy, you can use a heat-gun and some flux.  It's overkill for QFP parts, but it also works for QFN and LCC parts, and getting the practice on parts where you can examine the solder joints afterwards is helpful.  A good heat-gun can be expensive, (>$100 USD) but that's still cheaper than stencils are over the long term.

• Lightly tin the solder pads.
• Heat up the pads with a heat-gun until solder melts. (preheat)
• Place component on pads.
• Drag a little flux across the pads.
• Reheat until solder reflows. (Should be quick, since it should still be hot from preheat)
• Do a quick continuity check from the trace to the top of the pin. (Not as necessary as you get practice)

As I said, a bit overkill.  All the above suggestions work well too.  (Tack the corner pins, flux, drag solder across pins, solderwick the excess).

Regarding your original question, I've tried applying paste directly to the pads, and then reflowing.  The results weren't too bad, but there was a little bit of splatter on the pads where I put down too much paste.  Splatter can turn into short-circuits.

[Mechatrommer]

@cyberfish: you are learning pal! just as i am
i'm waiting for my $170 soldering accessories/consumables (paste, flux etc) from Zyphertronics.... mainly just to try it out... an expensive try out.

[cyberfish]

If you want to get super-fancy, you can use a heat-gun and some flux.  It's overkill for QFP parts, but it also works for QFN and LCC parts, and getting the practice on parts where you can examine the solder joints afterwards is helpful.  A good heat-gun can be expensive, (>$100 USD) but that's still cheaper than stencils are over the long term.

Thanks! That sounds very interesting. Maybe I will try it next time. For boards with just 1 or 2 SMD components, stencil would be very expensive.

@cyberfish: you are learning pal! just as i am Grin
i'm waiting for my $170 soldering accessories/consumables (paste, flux etc) from Zyphertronics.... mainly just to try it out... an expensive try out.

Yeap .

I am spending less, though. $25 for paste (250gm no-clean), $30 tools (vacuum pickup, squeegee blade, temperature marker). Gonna use a skillet I already have for reflow.

[TechGuy]

Thanks! That sounds very interesting. Maybe I will try it next time. For boards with just 1 or 2 SMD components, stencil would be very expensive.

If you have and old stencil with a tqfp you can just cut it out and use it for a single chip stencil on your new board.

[cyberfish]

It has to have the same pin count and pitch as well.

[charliex]

I'm doing  tqfp 64's and 100's with leaded solder paste from zeph and a hot plate, applied with a needle and syringe. i usually get 1 or 2 small bridges per device which come off with a dab of copper braid.

it does work with the unleaded too, but its obviously a higher temp and the leaded just seems to work better

[cyberfish]

I'm doing  tqfp 64's and 100's with leaded solder paste from zeph and a hot plate, applied with a needle and syringe. i usually get 1 or 2 small bridges per device which come off with a dab of copper braid.

Hmm. I guess it just takes skill, then.

I have some paste in a syringe with needle, and I can't even do 0805 resistor without bridging everything...

[Cheshyr]

I'm doing  tqfp 64's and 100's with leaded solder paste from zeph and a hot plate, applied with a needle and syringe. i usually get 1 or 2 small bridges per device which come off with a dab of copper braid.

Hmm. I guess it just takes skill, then.

I have some paste in a syringe with needle, and I can't even do 0805 resistor without bridging everything...

Good magnification helps too.  I actually have a full-fledged workbench microscope for when I need to do really fine-pitch work.  I wouldn't suggest trying anything smaller than 0402 by hand though, even with a microscope.  I still have nightmares about some of my attempts at dust-parts.   

But yeah, practice and patience, and TQFPs will stop being difficult, and instead just be time consuming.

[Time]

You need very little paste on the pads.  Probably much less than you actually think.

[mikeselectricstuff]

You need very little paste on the pads.  Probably much less than you actually think.

Agreed, the minimum amount of paste you need on a pad is "Some".
If you can't see the pad for paste, that is probably too much.

[charliex]

i use the smallest needle i can to dispense paste without damaging my hand or the syringe with the force needed to dispense it, and its usually slightly over enough.

I do have an automatic feeder that helps a lot, but with a steady hand and pressure you can do it repeatedly, i taught two people, one who'd never soldered before to do TQFP 100's with a syringe and hotplate in about 10 minutes.

Its amazing how little paste you need.

[Mechatrommer]

mmmm, newbie question... whats the paste is for? i tried to order from Zeph, but they did not deliver.

[jahonen]

mmmm, newbie question... whats the paste is for? i tried to order from Zeph, but they did not deliver.

Paste is a fine mix of solder and flux, that is put to the pads before SMD components are placed and then reflowed.

Regards,
Janne

[Mechatrommer]

oo ok thanx, i thought it only for the component to stick to the PCB during reflow. hmm, guess i will miss that, since it was not in the Zephyr's box
so what happen when we bridge the paste? will it create a bridge as well among pins after reflow?
ok ok  i get it, the solder will tend to stick to metal

[alm]

Solder paste is basically an alternative for regular flux-core wire solder, that's used for reflow soldering (put solder paste on pads, place components and heat). The solder is kept from creating bridges by surface tension (and flux), plus proper amount and reflow profile I assume. The major advantage of solder paste is that you can print it on the PCB using a stencil, beats tinning all those pads by hand (or machine). I'd guess you can get something similar by tinning the pads manually and adding liquid flux, but it's a lot more hassle.

My issue with solder paste is that it usually has a fairly short shelf life, which sucks for a hobbyist not doing lots of soldering. Many packages are designed for industrial dispensers, and some are hard to use without one.

Sparkfun has written a fair amount about their experiences with printing solder paste and reflow.