Monday, August 8, 2011

How to estimate the amount of gold in electronic scrap.

When dealing scrap, knowledge is king. Although the method is only ballpark, it's better than having no idea of the value.

(1) Make measurements and calculate the total gold plated surface area. You want to know how many square inches or, how many square centimeters of surface you have. You need to break the plated part down into it's geometric shapes.

Fingers are easy because they are rectangles. You just measure the width and length of one finger, multiply these together, and multiply this answer times the total number of fingers. If you have 50 sets of identical fingers, multiply that answer times 50. All this gives the total surface area on these particular 50 sets of identical fingers. For a round thin pin, you multiply the diameter times pi (3.1416) times the length. For a thick round pin, you also have to calculate the area of the end. For a square thin pin, multiply the width of one side times 4 times the length.

Type in - surface area formulas - in Google. Some sites will give drawings of the different geometric shapes and their surface area formulas. I will look for a good one and post it in the Glossary section.

You can measure in several different ways. The cheapest way is to use a triangle shaped plastic architect's ruler. You can get plenty of accuracy using one of these rulers. Get the one that measures in tenths of an inch and learn to estimate reading it in between the lines (marks). Don't ever measure in 1/16ths, 1/8ths, etc. This would make the calculations much more complicated. For small measurements, you can place the ruler on the object and use something like a 10X eye loupe to read it.

My favorite is a cheap set of calipers that measures in a decimal part of an inch and, not in 1/16", 1/32", etc. If you can pay more, you can get one that has a dial for the last digit. The cheaper ones have a vernier and that is a little more difficult to get used to.

Probably, the best is a comparator. This is an eye loupe that has a scale built into it. You can get a variety of screw-on scales for it.

You could use a micrometer but, they are cumbersome.

(2) Estimate the gold plating thickness. This may be the most important factor. You have to have an idea of how much is needed for different types of parts. Gold plating is measured in millionths of an inch. Another name for this is microinches. I will refer to them as micro". First of all, you must understand that this is a pure educated guess. You must understand that the thickness of gold plating has changed over the years. Also, the plating on such things as fingers or pins can run all over the map. I have seen fingers that ran from 15 micro" to 80 micro". It depends on the usage requirements and which company made the parts.

On the average, Catfish has been using 30 micro", with very good success, for things that plug into other things, whether male or female, or for those things that rub against other things, such as those tiny gold plated balls in small multi-switches. This includes such things as gold plated connector pins and many fingers. This has also been pretty well confirmed by lazersteve's yields.

For things that have to be heated, in order to solder an integrated circuit (chip, die) to a package, you can figure 50 (40 to 60) micro". This includes many CPU's, sidebraze IC's, etc. It also includes hybrid packages (microwave, etc) that often contain hybrid circuits. The gold thickness on all these types of packages have run quite consistent over the years, unlike fingers.

For everthing else, you are on your own. There are hints, however. For things that just sit there, on run-of-the-mill electronic equipment, figure 15-20 micro". The gold is there only for mild corrosion protection. It doesn't need wear resistance. If you run across some rare, high tech, military part that has to withstand high current flow or a highly corrosive environment, the thickness could be out of sight. Most common modern items run between 12 micro" to 35 micro".

Two ways to really know the real gold thickness, on specific parts, is to assay them or to refine them (the whole batch or, just a few of them, as a sample). You can then measure and calculate the surface area. From this, you can calculate the gold thickness. There are specific types of equipment that measure plating thickness. The Microderm and Betascope utilize radioactive isotopes to do this. The Kocour machine actually dissolves a small spot of the plating. There is also X-ray thickness measuring equipment. Most all of these need standards in order to set them up. You can also plate backup nickel on the gold plate, make a metallurgical mount, polish and etch it, and measure the thickness using a metallurgical microscope. I have lots of experience using all of the methods and equipment (except for x-ray) covered in this paragraph. If you want more info on any of these, let me know.

A special category is gold brazes. They probably run from 500 to 1000 micro" thick. A 80/20, gold/tin braze is used around the edge on gold plated lids used to seal IC packages, such as some CPU's, side braze packages, or all-gold plated hybrid packages. Also, on many of these same parts, a 96/4 (I think), gold/silicon braze is used to attach the chip.

The mil specs and other official thickness charts, concerning electronic parts, aren't that helpful unless you know what thickness Class Number was called out when that exact part was manufactured. If you assayed parts, you could probably get some correlation between the data, however. The official decorative gold plating charts are a little more helpful, since some plated jewelry is marked. If you see a marking of H.G.E., e.g., you know it should be 100 micro" thick, when new.

(3) Calculate the gold plating value.

First, for us in the US, who have measured things in square inches.
(a) Get the spot market price of gold, in dollars per troy ounce. Divide this number by 100,000 (ten thousand) on the calculator (Note: This is the only change I made in my method). This gives the dollar value of one square inch of gold plating, one micro" thick. Let this sink in your brain. For example, if the gold price were $660 per tr.oz., the value of one square inch of one micro" thick gold, would be 660 divided by 100,000 or, $.066. Please note that this was a little discovery of mine and you won't find it anywhere else. I just noticed that, when I calculated the value of one square inch of one micro" thick gold, using the density of gold, etc., it just happened to be within 1.5% of dividing the spot price by 100,000. Pure coincidence.

(b) Multiply this times the gold thickness in micro" and multiply this times the number of square inches you have.

Dollar value of gold plating = Spot price divided by 100,000 X thickness in micro" X area in square inches.

Example: The gold spot is $650. You have 9.58 square inches of gold plate that you estimate to be 30 micro" thick.
650 divided by 100,000 X 9.58 X 30 = $1.87.

For those measuring in centimeters and calculating area in square centimeters:

Spot price in US dollars divided by 100,000 X thickness in micro" X area in square centimeters divided by 6.452.

You'll have to convert other currency values on your own.

Play with this. It's easier than I've made it sound. As you probably know by now, I'm a detail guy. Just remember it's limitations. At best, it's a estimate, which will often (but, not always) be a bit on the low side.

Todd Klimson
Pager & Cellular
Recycle All Electronics

1500 SW 17th Ave
Building 300
Ocala, FL 34471

352-502-4449 - Ext. 126
352-426 2716- Cell

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