Step I: Taking an impression
I like to work in amounts of 2.5-2.8g. It doesn't matter if you don't have a scale that will reliably measures in the 0.1g or lower range. There's nothing magic about this number. It's just the size that I started doing this kind of micro-impressioning in. It happens to be the mass of a block the exact size and shape of a Scrabble tile. (In fact, at one point, stuck in a VT cabin with an incomplete Scrabble set, I made a whole set of tiles out of PMC. It can be a little dangerous to be around me when I'm bored.) It also makes an oblong about the size and shape of a peanut M&M, and is about 1/20th of a standard 2 oz (58g) PMC block.
My first example will be an adapter for a 14 pin SSOP-narrow chip (SSOP-narrow is just one of many SMD form factors) 2.5g of PMC should easily produce 12 or more finished adapters of this size, but aim for 10 (it's a pain to try to use the last bit. Save it for your next batch.) A single $1-2 block of PMC will make hundreds of adapters (depending on size)!
Here are the dimensions of our first example:
Though you won't use these numbers directly, there are a few key dimensions to note:
a) the width of the pins (note the wide variance: this is why I try to take an impression of the exact kind of chip, from the same manufacturer, as I will be using in the circuit)
b) the gap between the pins (the PMC that oozes up through this gap will guide all our work)
c) the full outside distance, end-to-end and pin-tip to pin-tip
d) the center-to-center pin spacing on the datasheet for your particular manufacturer can be important because the same chip can be made in metric (.600mm) or English (0.250" = .635mm) spacing, and once you've made a batch of adapters, you'll want to buy the same spacing in the future. For short chips, like this one, an English spaced PCB or adapter will work with a metric chip, and vice versa, but longer chips may not work, because the .035mm difference in pin spacing, times the number of pins in a row, becomes a substantial fraction of the pin spacing, and the pins will bridge the pads or vice versa.
Note that .035mm is 35 microns (the width of five red blood cells), which is far smaller distinction than, say, a brain surgeon would trust themselves to do with their bare hands, you'll be amazed to find how much we can get away with, if we just lay our process out right
The first step is to condition the PMC
. This basically means "squishing it until it is uniformly soft. I personally prefer to roll it into a ball, a 1:1 (length:width) cylinder and a penny-to-nickle sized disk, and repeat at least 10 times -- squishing, not folding.
I prefer to avoid folding it at this stage, because sometimes folds can produce weak seams that can split or crack later. PMC will pick up skin oils, any dust on your work surface, lint from you clothing ... anything, and this stuff can get in your way. You'll end up folding and merging loose bits plenty of times as you work, so why add inclusions now?
Uniformity and not softness is the most important quantity, and the clay usually needs a bit more squishing even after you *think* it's uniform. Softness is not a good guide; it depends on temperature, humidity, age, storage conditions (both before and after you buy it -- and you never know how it was stored at the factory, warehouse, in shipping, etc) Besides, too much softness and the accompanying stickiness isn't good thing when casting on this scale.
When you think you're ready, roll the PMC into a thin cylinder a little narrower than the size of the chip you will be impressioning. Length or width? It depends on how you are going orient your impression. Each approach has its merits, but it mostly depends on how your PMC is behaving today, rather than any planning on your part (if it's too soft or sticky, try putting it in a sealed plastic bag in the refrigerator for a while. Allow it to come back to room temp *before* opening the bag and squishing it (not as much, this time) -- you want to avoid moisture condensing on the cold clay, which can can make softness/stickiness worse.
You have a LOT of leeway in the width of the cylinder. It'll squish anyway as you take the impression of the chip, and you'll be trimming away a lot of excess from he final impression before baking it. In fact, you don't need to be very precise in most of the steps (esp. with a short chip like this): the chip itself will take care of the key dimensions.
There are two basic ways to take the impression: axially or transversely. The choice mostly depends on how your clay is behaving, whether you like a really deep impression or really wide pin slots, your wire wrapping technique (in a later step), and what adhesive you use. You'll work out a suitable compromise with a little practice.
In the meantime, just cut each impression off off the cylinder with a razor, decide if it's a keeper, and make a new impression near the fresh end. When you've gone through the entire cylinder, trim your keepers to a nice rectangle, ball your rejects and trimmed bits together and roll them into a new cylinder and make some more until you've got 10 keepers.
If you find the new cylinder starts cracking or flaking in layers, condition (squish) it more
The first picture (yellow Craftsmart PMC) illustrates a couple of things.
The impression on the left is too shallow for a beginner. The impression on the right is actually better, even though there is bulging and cracking (which you'll trim away later anyway). It has nice deep grooves that will be easy to follow when you wrap the wires. (but you will be able to use eitehr depth, with a little practice
But the real reason I included this picture is because the first impression (on the left) bowed back inward (due to the springiness of the clay) when I tool the second impression. Noticing this can help you "fix" slightly skewed impressions, especially in very soft PMC. While this is useful info once you get some practice, I don't advise trying to "fix" a bad impression (not now, anyway). Besides, flaws are easier to fix with a razor after baking
When you trim you keepers to rectangles (which may look satisfyingly factory sharp), you should leave a thin margin beyond the first and last pin of each row (i.e you want a 'wall' on both sides of each pin groove). You can either leave a thin margin beyond the ends of the pin grooves or trim them flush so the grooves are "open". I suggest making five each way, so you can learn which you prefer, based on your other tools and materials, later.
While I encourage you to strive for perfection during your first batch, just so you get used to the material and its properties, it doesn't matter if the grooves are slightly slanted, if some are narrower than others, etc. As I said, the chip itself will makes sure they're "good enough" to fit, and you can get away with a lot.
The second (green Sculpey III PMC) illustrates two other tricks.
The first is: if you cast you impression transversely (pins going across the cylinder), you can create longer grooves by pressing the chip in a little at a time, and gently pulling the cylinder, or pressing and spreading the clay just beyond the end of the grooves. Again, don't worry about a little cracking.
The second trick is: running a loop of Metric 1 UEW wire under each row of pins can make it a lot easier to pull the cip out of the impression. In fact, removing the chip without ruining the impression may be the hardest part of the entire process of making an adapter (but it's really quite manageable after a little practice
I use Metric 1 UEW wire for a lot of micro-electronics and even "nanotech". You probably won't find it in your local electronics shop, but you can get it at Dealextreme
or PM me and I'll mail you a roll or two. "Metric 1" means it is 100 microns in diameter and UEW means "(poly)Urethane Enameled Wire" UEW insulation is thin and burns off when soldered, so you don't need to strip the insulation -- a generally nifty property, that I'll make good use of in these adapters. It's also great for making millimeter motors, tunneling electron microscopes and other nifty tools of the 21st century tinker. It's really worth having
If you can't find metric wire, Metric 1 is between 38 AWG (101.6 microns) and 39 AWG (88.9 microns) so anything in that range should work. I'll later show you how to use 30 gauge insulated wire (often sold as "wire wrap wire"), that is sold at Radio Shack
and is also a very good thing to have in your workbench. Though it is technically possible to use the common 28 AWG wire (320 microns) or even 24 AWG ((510.5 microns), the commonest sizes used in hobby electronics, that's a bit advanced for a beginner: the center-to-center pin spacing on this chip is .635mm (635 microns) so you have to be more careful when using such "fat" wires
Not that 24-28 gauge (AWG) are really "fat". Those are the sizes of the individual solid wires inside CAT5/6 or telephone cable, which are often the cheapest way to buy hobbyist wire, since you get 400ft(telephone) or 800ft(CAT5/6) of wire in every 100 ft of cable