TECH TIME

Helpful tips for the Avionics Technician

B  Y    A  L    I  N  G  L  E

Al Ingle, owner of Capital Avionics, A Tallahassee, Fla.-based service center, also serves on the AEA Board of Directors.

This month’s topic is how to successfully remove and replace surface mounted technology (SMT) devices in a new procedure using recently developed techniques. In the past, SMT devices have been removed by heating the component's leads to the melting point of the solder, which is about 400 degrees F, and then removing the defective component. The heat can be applied with either forced air or by direct contact with the leads with a precision milled cup that exactly contours the shape of the SMT component. The drawback to these methods is the heat required to make the repair. The printed circuit boards today are delicate, with fine lined traces and the typical copper pad or trace can only withstand an average of three heat cycles. If one heat cycle is expended removing the defective component, another to clean the residue, and a third heat cycle to install the new component, you have effectively used up all the heat margins for that circuit area. A second failure of that component for any reason and board damage is likely.

Enter the element bismuth, which has a lower thermal conductivity than any other element except mercury. When bismuth is mixed with tin and/or cadmium, alloys are formed that melt at low temperatures. Chip Quik, Inc. offers such an alloy that melts at only 136 degrees F. When this product is applied to the solder joint of a SMT device, the melting temperature drops to about 150 degrees F. When you consider that the TSO temperature rating of many pieces of avionics is 131 dgrees F, it is likely that many internal components are regularly exposed to tempeatures at or above this 150 degree melting point.

The new procedure to remove and replace a SMT device is as follows:

1. Apply flux to all leads of the SMT with a syringe.
2. Melt Chip Quik uniformly on all pins of the chip with a small tipped, low wattage soldering iron.
3. Maintain the alloy in a molten state long enough to release the chip. Note: Many of these chips are glued to the board before undergoing the solder reflow process so you may want to gently cut the leads with a sharp razor and separate the removal procedure into two parts - the leads and the body. You can experiment with the heating method. To heat the leads themselves, use a low temperature soldering iron, or low temperature heat gun, infra-red gun, etc. You will find that solder at a surprisingly low temperature.
4. After removing all the leads, if the body of the chip is still glued down, heat and gently pry with a dental pick.
5. Remove excess solder either by using solder-wick or taking a cotton swab and flux to move the molten solder to an unused section of the board. After it solidifies, it can be removed.
6. Apply heat to each pad and gently swab with flux until thoroughly clean.
7. The Chip Quik leaves an unsightly residue that fortunately is easily removed with alcohol. Let the board cool and when at room temperature, clean any remaining ash with alcohol pad.
8. Place the new chip onto the board and tack the corners with regular solder. A trick to minimize the soldering heat necessary is to preheat the bottom of the board. There are several methods. Suggestions include pre-heating with an infra-red lamp or using a small heat gun to raise the temperature to about 150 degrees. You want to mask off all parts of the underside of the board except the area of the chip being replaced. A board with a hole cut in it will work fine to isolate the heat. A solder paste applied to the leads, is then melted with hot air or by individually heating each pin, or by running the tip across a row of pins.
9. Let cool and clean up as necessary using alcohol or acceptable solvent.

There is no best way of replacing all sizes, shapes and lead spacing of SMT devices, so let practice and experience be your guide. Chip Quik, Inc. offers a free sample of their product for experimenting with, just contact them by phone or web site and ask. Below is a source list of products that have been mentioned. Also listed is a source for printed circuit boards loaded with dummy SMT devices that are great for practicing on. This is not and endorsement or recommendation of these products, only that we have tried them and been happy with the results. Please remember that while this new method described shows great promise in our work, it has not been recognized or sanctioned by the avionics OEMs. With time and education it may become the accepted method.