25ns 5Volt DIP SRAM

The fastest DIP static ram in 512kB you can get is the Alliance AS6C4008-55P, which is about $5.00 quantity 1. It has an access time of 55ns, which is not really fast enough for the newest retro compatible CPUs like the z180-33.

There are, however 5 volt parts available in surface mount, and some of them have an access time down to 25ns. I'm going for broke, so the ISSI IS61C5128AS-25TLI it is. it's 75 cents cheaper, Q1 than the Alliance part.

I use the following tools in the build:

  • my new TS80P soldering iron, which I can't say enough good about

  • solder (the unleaded variety is a bit more finicky)

  • a 48 pin machine socket I use as a jig

  • desoldering wick. often not needed, but when you need it, you need it.

  • a 32 pin dip to SMT adaptor. i got these from ebay for 65 cents each, Q20.

  • 32 tiny pins. MILL-MAX 3128-1-00-15-00-00-08-0. 6 cents each, Q1000. you'll need a lot of them. If you the substitute the common square pins that are out there, the new device won't fit into sockets and it will be really TALL because of the carrier that the pins use.

  • a really good flush cutter. The best I've ever used is my Knipex.

  • A pin cutting jig made of 2 proto boards soldered together with a thin space between them. I used copper hookup wire to get about 1mm between the boards.

  • My eyes are not what they were, so I use a desktop 5x magnifier or some magnifier glasses that are easy and cheap.

Insert 32 little pins into the machine socket. This will register them exactly where you want them, and make the next bit much easier. This socket also gives a nice anchor for the work.

Next, put the boardlet firmly onto the pins. This one has two sides, one for 0.65mm pin pitch and the side we use, the 1.27mm pin pitch. if you don't use the one I am using, make sure that it is for this package.

Solder the through hole pins sticking out. it takes me about 5 seconds a pin, and I go through and re-melt all the pins, possibly adding a bit of solder if it does not look like a great joint.

You may notice that I put the machine socket on a breadboard. I find that this makes the handling much easier and steadies the work.

Ok, here's the fiddly part. Getting the SRAM aligned onto the boardlet is critical.

First, put a small solder ball onto ONE corner pin.

The idea is that you put the chip down right. Make sure the little dot is on pin 1, not the big dot. the little dot. I've done this wrong, and it will give you an upside down chip that you'll forever need to plug in reversed.

Then, with the tip of your soldering iron keeping the solder liquid, nudge the chip so that the one leg comes almost right up to the outside edge of the solder pad. There's not a lot of clearance with the chip centered on the boardlet, so you'll need to adjust this until it's spot on.

This is what it looks like with it's adequately aligned. It's not perfect, The chip's a little high.

while you are concentrating on this one pin, have a look at how the other pins on that side look. the package should be centered and each little leg should be more or less aligned with the solder pad. There is a little slop here, maybe a third of a millimeter side to side, so as long as you don't straddle pins between pads, you should be ok.

this takes a while to get right. we only do the one pin because it keeps the part anchored while inspecting all the other alignments. I find that the nudging is best done with my finger right on the package. if you find it's getting too hot, stop and let the chip cool down.

Solder the rest of the pins. The technique for this is like all other, heat the pad and pin junction with the tip of your iron, and jab the joint with the tip of the solder. Once it melts, remove the solder and iron and inspect the joint. clean your tip frequently, there's not a lot of room in there. If you've got solder blobs on the iron, you're doing it wrong. The solder goes on the pin, and pad, not the iron.

And this is what happens when there is too much solder, or if the placement of the iron is not right. I get one or two of these every time. Note the joints to the left; they're a little heavy, but entirely acceptable.

To remove the solder bridge, put the solder wick on top of the bridge, heat the solder wick until you see the wick get shiny with the solder that just got sucked into it, and remove both the wick and the iron. inspect the joints; you may need to add a little solder.

As with the pins, I go through and remelt all the solder joints once. It improves the appearance of the joints, and makes sure that there is a solid connection. Sometimes it is necessary to add a little solder or remove some. The wick will help the latter case.

Almost done.

Wow, those pins sure look long.

I was not able to find pins the right length; the next smaller ones are too short or too thin.

I'm going to cut them.

This is an edge-on view of the cutting jig. I threaded a copper wire between the 2 proto boards to get a nice pin length, and then soldered them together and ground the top and bottom off.

Snip, snip, snip with my flush cutters, and I only need to do 7 more boardlets to get 4Megs of S-100 that will never need a wait state, even with the fastest retro CPU