SMD How To - 8

Skill Level: Beginner

by Nate September 02, 2006 | 8 comments

How does hot air rework actually work?

Be sure to click on the video window, and then click it again, to open it up in a new, larger window for easier viewing. Or just go directly to our YouTube videos here.

First you are going to need a hot-air rework unit. These are not too expensive. I highly recommend a mid to low-end unit for $200. The $1000+ units really don't do anything more.

Hot air flow in water

This demonstrates how powerful the airflow is coming out of the airgun. It's like a hair dryer on steroids - the air temp is about 425 to 450F and and nozzle puts out a semi-directed air flow.

Pretinning a footprint for hot air mounting

If you're not using a stencil to apply solder paste (this is almost always the case for prototype boards), you'll need to add some solder to the pads of the footprint. Here we pre-tin the pads with solder.

Removing dome from center pad

Some components have a center pad. 99% of the time this pad does NOT need to be connected to function. Usually it's just a GND pad. When pretinning a footprint, sometimes you'll get a little bit of solder on this center pad. Make sure it's not much at all otherwise this 'domed' pad will squish out molten solder during the IC mounting and short to all the surrounding pads.

Mounting an IC with hot air

This is a tight pitch (0.65mm I believe) CP2102 leadless IC. We've pre-tinned the footprint, now with a swirling motion from the gun, we heat the footprint until solder is molten-ish and place the IC down. You'll notice the IC jump into alignment on the footprint. This is the surface tension of the solder pulling the IC center. Very cool. Then I went and tapped it a bit to show how to align the IC if need be. Don't get greedy. You have a finite amount of time you can stress the PCB before things go south.

Air bumping the IC

Here we re-heat a board to adjust the IC. Be careful, you can only do this once or twice before the PCB starts to give out. Towards the end of the movie you can see where the air flow actually causes the IC to raise up a bit and center itself. It's common with novices and too high of air flow to blow the chip completely off the board.

Removing an IC with hot air

This board was faulty for some undetectable reason so we need to pull the IC off, clean up the PCB and the IC and then mount the IC back on. This video shows heating up the four sides with a circular motion of the air gun and slightly lifting on the IC. We use lead free solder paste so it takes a bit of time for the solder to release. Notice I am not lifting hard enough to raise the PCB - give the IC time to let go of the PCB on its own.

Mounting a cleaned IC and PCB

I've re-tinned the PCB with new solder. I've also cleaned up the bottom of the IC by re-tinning and removing any solder paste gunk that may have been shorting pads together on the bottom of the IC. Now I pre-heat the footprint a bit with a swirling motion to get the solder molten, place (throw) the IC down and it almost immediately goes to the center of the footprint. I bump the IC a bit (side to side and a bit of downward pressure) to make sure the IC is in contact with the footprint pads. A little more swirling of the air and the IC re-centers on the footprint and then I remove the air.

Blow away bits

Here is an example of the hot air flow turned up (heat is still the same). We can quite easily blow away 0603 components. With leaded solder, they would blow away on their own. Because we use lead-free paste, its a bit more tacky but with a little help the bits go flying!

How to destroy a PCB

So we've warned you not to sit too long on a given spot on a PCB while working with hot air. What happens? The glue that holds the copper layer to the FR4 laminate underneath lets go and comes out as black goo. The green soldermask on the top side also begins to come off. Because we don't have Smell-a-vision yet, we can try to describe the smell - it's horrendous. It smells like burning electronics (surprise) but the smell seems to cling to your skin and clothes. Not cool. So practice and don't be surprised if you smell bad things with your first couple re-work attempts.

We hope the videos help! Please be sure to look at some of the other SMD soldering videos on YouTube.

My goodness that was a huge tutorial. I hope you enjoyed learning everything we've discovered the hard way over the past few years. Please do more SMD soldering! There are far too many fun SMD components not to.

Sorry for the pictures without two-hand action. Try holding and using the right-handed camera in your left hand while soldering with your bad-right hand!

Written by Nathan Seidle

September 2nd, 2006