Tools and Supplies

Flux:

DONT TRY TO SKIP USING FLUX! Apply thick types with a hypodermic syringe and a thick needle or thinner stuff with a brush.  Mouser sells a flux pen. The secret to making this work is to use lots of flux. It's what keep the solder preferentially adhering to your PCB pads and IC pins instead of to each other. With thicker pastes, the boiling action can cause small balls of excess solder to fly off or bounce into other areas possibly causing shorts.

Solder:

Alpha Metals. There were three types of solder and two types of flux that were used most. The solders are classified by the types of flux contained in them. The are SMT Core, Cleanline 7000 (both no-clean), and Pure Core (core flux contains a type of acid - must be cleaned after use). The two types of liquid solder flux are 855 (used with the pure core solder - residue must be removed), and NR200 (can be cleaned or not, depending on application).

Magnifying glass:

Very bright lamp or lighted magnifying lamp. A 10x Hastings triplett is probably the most useful magnifier; 20x is pretty handy as well. If you've got lots of money, buy a Mantis.

A pair of binocular over the head and eyeglasses magnifiers--Harbor Freight sells a wonderful set for $5! Two powers via a second flip down lens. $30 for the same thing from any of the tool houses. Don't bother with the fluorescent lights with the big lenses--to cumbersome. Put it on your head and flip down when needed. If you can't see what you are doing...

Holders:

Precista 111SA tweezers. This little guy has an angled tip, with two flat surfaces that are grooved to grab and hold round diodes, chip resistors, and IC's very firmly with lots of control. May be about $40...

Dental Pic. Your local industrial liquidator is a good source.

Pamona makes a neat pair of tweezers with contacts on the ends and test leads. Plug it into your multi-meter and measure voltage drop across chip resistors with one hand. About $15.

Soldering:

Metcal SP200 soldering iron, which is a cheaper unit (around $300) and an SMT "hoof" tip ($25 ftp://ftp.metcal.com/pdfs/minihoof.pdf). (see www.metcal.com for an apps note with the "drag solder" style technique).

EDSYN model 951SX soldering stations (on sale at 1/2 price of $75 at radioshack.com as of 99/09/10).

Use the correct solder temperature. 63/37 solder melts at 361degrees farenheit. You'll use a slightly higher temperature. Some say that 400F is perfect. Others say crank the heat all the way up and leave it there. Either:

• apply solder to the PCB and no solder when actually soldering down the chip. Don't try to solder high-density QFPs one lead at a time, lay a bead of flux down across the solder pads and apply the iron to fix the part. Check to make sure all the pins are actual in contact as differences in the height of solder originally on the pad can cause leads to float just above the pad. Use a thin, non-thermally conductive tool (sharpened stick) to push the pin down while applying head with the iron to correct this problem.
  or
• apply tons of solder. Don't worry about bridging yet. After you are done, use de-soldering tape to remove exess solder and the bridges. The de-soldering tape will not pull out solder between the pins and the PCB. Be very carefull to ensure that all the bridges are removed. They can hide under the chip (especially when the traces extend under the chip and/or the entire area is fluxed) and can be so fine between the pins that they are invisible to the naked eye

Wave:

Use electric skillets for dip (wave) soldering and tin/lead plateing. The basic problem with home plating is that a roundness forms that causes the leads to slip off the pads. You can use wick to remove the the rounded tops. "Mopping" the solder over the board with an iron can result in uneven solder hight which can prevent all the chip leads from contacting the pads. Again, solder wick can remove "bumps"

One problem is that the package picks up moisture and turns to steam if heated rapidly as in wave soldering. This can cause delamination of the plastic from the die or leadframe. I've observed the problem appearing as blown output pins. Smaller packages are rarely a problem. Typically you can pre-bake the chip for 12-24 hours at low enough temperatures to prevent rapid generation of steam (Altera suggests 12hrs at 260F).

see also: http://www.cornwell.demon.co.uk/micro1.htm MELTING METALS IN A DOMESTIC MICROWAVE

Reflow:

Toaster ovens or hardware store hot air guns for smt reflow. Hakko 851 hot air pencil ($700 at Fry's) comes highly recommended.

Check out Rob Severson's UNDER $20 Hot Air SMT soldering pencil! Almost as good as a pro-reflow pen and it sure beats a toaster oven!

Preheat:

Preheat the board to about 100 C (212 F) with an electric pancake griddle, keeping everything except the area being worked on covered with cardboard to avoid burns. (Commercial hot air soldering equipment uses preheat from 100 to 150 C AFIK) The other reason for preheat is to drastically reduce the amount of time hot air must be applied to complete the soldering operation, significantly reducing thermal stresses.

BTY: You can use hot melt glue guns to make plastic injection molded parts.

Desoldering

Personal Experiences

Mark Neff

I've used several different Techniques for removing flat packs, all with varying degrees of success. The most obvious is to use a temperature controlled iron, solder-wick as much solder as you can off the pins, and then heat each pin individually, applying slight force from behind with a very fine dental-type pick until the pin pops free. In most cases you can do a couple of pins at a time, and if you're very careful, you won't lift a trace. It's not very pretty, and the chip is mangled when you're done, but it works in a pinch.

Another technique is to take a length of thin guitar string or piano wire and slide it behind and through the pins on one side of the chip. Anchor the string with an alligator clip or hemostats to the board, and then gently pull the free end away from the chip as close to the surface of the board as possible, while heating the pins where the force of the is applied. As the pins begin to come free, move the iron along with the string. It takes a little practice, and it won't work well if there are a lot of tall components around the chip, but in most cases it works great, and because the string is pulling each pin outward, instead of upward, the chances of lifting a trace is very slim. The pins around the chip are left relatively unscathed- I've pulled 84 pin chips from scrap units this way and reused them in emergency situations with no problems.

Getting more expensive, our shop has a special flat-pack desoldering tool that I found doesn't work very well. It has a variety of "tips", shaped like the outlines of popular IC packages. The tip fits over the IC and applies heat to all the pins by contact simultaneously. When the solder is hot enough, the technician pushes a button on the handle that applies suction to the center of the chip. The suction causes the IC to "stick" to the desoldering tip and then the tech lifts the iron and (hopefully) the chip away.

The problem is that the tips are so large that even heating is nearly impossible, and I've had a few cases where traces (several, not just one) got lifted when I thought all the pins were free. Lifting the iron with the chip in it just gives no "feel" for when the pins are free. It also takes a long time to warm up, and requires a lot of tinning to get the tip to work at all.

The latest gadget we are now using is a hot air desoldering station. This is simply the best, fastest, easiest way to remove a flat pack. It has a selection of tips with nozzles that direct hot air around the perimeter of standard chip types. The velocity and temperature of the air are adjustable. After about a 5-minute warm up, the technician holds the tip over but not touching the pins, and in about 5 or 10 seconds, with a slight lift from a dental pick, the chip comes easily free.

Jaws drop when I demonstrate this thing. All pins are heated at the same time, and the solder fully flows, so board damage is nearly impossible. The chip is in perfect shape (with tinned leads!) and can be easily reused. The only thing that can be a problem is the possibility of loosening and blowing away nearby small resistors and capacitors if the velocity of the air is set too high. Turning the velocity down on densely packed boards will minimize it, though. I simply can't say enough about this machine, and I highly recommend it if your shop does a lot of flat packs. The high cost will justify itself in the time saved with this method.

But if funds are tight or the quantity of flat packs is minimal, try the guitar string. I've done many that way with great success.