tools for SMT assembly and rework

One of these board preheaters showed up in the R&D area at work. And one of the guys has one at home, too, and he said that it (so far) hasn't burned the house down. So for $125, I sprung for one.

Why the board heater? You need it for SMT assembly. Take a look at how they do automated SMT reflow assembly at a stuffing house. You put some solder paste is put on the pads (a stencil helps if you have a lot of parts) (and you really want solder mask on the board), the parts are placed on the boards (get your tweezers and magnifying goggles), and then the board is put on the heater with the temperature set to ~150C. Let it "soak" for a minute. Then take your hot-air tool set to about 220C, with moderate air velocity (so you don't blow parts off the board!) and wave it over the parts. The parts will settle into position by the surface tension of the solder as the solder melts and becomes liquid.

After you finish all of the parts, let the board cool, and when it's cool, wash it. Or use solder paste with a no-clean flux.

It's a lot easier than trying to solder with the standard tool, and required for parts with thermal pads on the bottom.

Also the board preheater and the hot-air wand make removing parts from a board a lot easier. Done right, you won't rip SMT pads off of a board.

Andy,

did you notice a massive difference between just using a hot air wand?

Rochey said:

Andy,

did you notice a massive difference between just using a hot air wand?

Click to expand...

Oh, yeah, with the preheater the flux activates before the solder fully melts. Much better joint. (We're all lead-free here.)

I'm too old to hand pop smd boards but too cheap to pay somebody else to do onsey twosey  so I hand pop a few prototypes.

My most recent PCB had LEDs on top and bottom, when i tried to reflow the top LEDs with an air tool the bottom LEDs fell off.    :'( :'(

JR

John,

get an SMD paste dispensing glue thingy and put some of the magic red glue in it. you put a dot of it between the pins on the SMD led. reflow one side, they glue and paste on, then reflow the other.

JohnRoberts said:

My most recent PCB had LEDs on top and bottom, when i tried to reflow the top LEDs with an air tool the bottom LEDs fell off.    :'( :'(

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Kapton tape is your friend!

Andy Peters said:

One of these board preheaters showed up in the R&D area at work. And one of the guys has one at home, too, and he said that it (so far) hasn't burned the house down. So for $125, I sprung for one.

Click to expand...

Board preheaters can be cheaper than that, here is my SMD rework station, with "proper" SMD tools. Note that I only "pull", not "put".

Anyone try reflow in a toaster oven?

I have a DIY reflow oven. sometimes tricky, sometimes easy.... generally speaking easier then I thought. I made a temp chart of the oven heating up to get an idea of the reflow curve and how it matches the recommended profiles.

the slightly irregular heat distribution can be a problem, I have not found a way to solve that. and you have to visually control the process closely if your oven is not precisely  controlled. a pid controller was my first thought, but after seeing that it takes much longer to reach the reflow temp I decided to skip that. after reflow I need to open the door to cool the board - the temperature profile of my solder paste wants a much quicker descend. I tired (once) to take the board out, but i made the components move, bummer!

all in all not that bad, just a bit fiddly....

- Michael

Thanks for the info.

john12ax7 said:

Anyone try reflow in a toaster oven?

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I actually did some reflow in my kitchen oven (it worked), but later read that lead fumes can contaminate foods cooked later (at my age a little more lead might help).

These days I am hand soldering SMD with a tiny tip. but on a fairly small PCB with simple 2 lead or 3 lead parts (20 pops or so). For ICs I'd probably reflow.

I've seen some articles for rigging toaster ovens for thermal management. It seems controlling a temperature profile would not be a bad thing.. first to drive off moisture and then to hit the precise heat range when ready. Nor rocket science but a good application for simple control. You could probably use a diode junction as the temp sensor, but look out for soldered leads in the heat.

JR

the oven I bought has not enought ummpf to match the profile rise from soak to reflow, but for my boards it works fine. I'd like to add some low level airflow inside but I don't have a suitable fan....

PID controller would be nice indeed, quite some projects 4 that to look at.

- Michael

I was thinking of putting a thermocouple in an oven and then controlling it manually with a watch to try and get the profile right. After some trial and error could probably get it pretty close, or at least close enough for prototype purposes.

ah, missed an important point of course:

I have my digital kitchen thermometer in the oven to monitor temperature. to some extents you can see the different phases but of course it's always better to have some reference so that you don't overshoot badly and cook the components.

metal braided sensor (the one for low temp meat preparation), goes right next to the board. I don't think the fumes will be a health problem, we will see in the future...

- Michael