4MB RAM PCB ASSEMBLY NIGHTMARE

As the cost of assembly was so high in the 4MB RAM upgrades, I have been plotting for a long time to assemble them myself.  I did try a couple several months ago and it worked out well. Though I lost my 17pin header strips and had to source some more :(  Not only that, I have been having troubles with various brands of SMT paste also :(

First up I tried the new SMT paste..

This paste is is supposed to be better in all ways (won't go into it all). So I thought I would give it a try.

Overall it stuck the resistors on just fine. Though I had a lot of shorts between the DRAM IC's. I was using a SMT stencil so the paste on each pad was pretty small really.  Even so, the last paste I used was just plonked on anyold how and it worked great. So this paste appears to have more "meat" in it. Which is great for caps and resistors, but not so good for DRAM it seems.

The problem with a lot of paste is the high temp needed to melt. The past I use melts at about 150C, most paste needs 250C and higher. I can't use the temp that high as around 200c the plastics start to burn and melt. So it limits my paste to basically 2 or 3 brands. Not like there is a lot of paste anyway to chose from.

But it gets better...

My new header pins melted to buggery.

Actually the melted part was pretty neat really. Though the pins had just bent all over the place. So I thought, why not just put 2 PCB's on the bottom to hold the pins, and let the plastic melt. It wouldn't really matter as long it was holding the pins straight anyway..

Pins stayed straight, though the plastic melted all over the place and looked like a total mess :(

I also tried manually soldering the pins like I had done in the past. Though even with the soldering iron on a low temp, using low temp paste, 2 seconds of heat and the pin just bent and melted anyway. So in the end I gave up with it.

I did get some samples from harwin of the header pins so I tried those..

Thankfully those worked great! They look a bit yucky in the image, but that was actually some rouge flux and they didn't melt at all :)

The pins are unfortunately pretty expensive at around 40p per strip. So it will ramp up the cost of the boards. Though I also have about 400 pins I can't actually use as well.  Not to mention the amount I spent on SMT paste these past few weeks.  I can only get the pins from the USA (im a UK guy!) and the paste will likely have to be imported direct from the manufacture in the USA also.  So a lot of costs in postage and import taxes etc.  All these costs have to be factored into the "cost" of doing the next batch of these and the cost just keeps escalating!

I had a idea that since the plastic melted "neatly" on one test, but the pins dropped down all over the place, and the 2 PCB's on the back to hold the pins straight worked, but "squashed" the melted part, I thought of doing a mix of 2 things.

Basically use the pins as before, let them melt, but only use 1 extra PCB on the end of the pins. This way the plastic can melt without it getting squashed by the second PCB I had on the back previously. This actually worked really well.  The plastic melted "neatly" and all the pins remained straight and level.

While "melted plastic" seems like a bad thing, I actually think its better doing it that way. Because the plastic is hardened onto the PCB and "glued" onto the base of the PCB.  So not only are the pins soldered into the PCB, but now actually glued on the back making them a lot more secure.

At least using this "cheap" pins means I don't potentially have to raise the price of like £2 per board just for the sake of using the Harwin pins. Overall I think as the pins are fragile, then "gluing" them to the PCB can only be a good thing.

UPDATE
I created a new stencil which turned into a nightmare. I thought I would make the IC pads about 30% smaller to use less paste and reduce the shorts. 

Well, turns out Eagle (PCB software) doesn't like me drawing on the Tcream layer manually. In fact when export to gerber, the parts I draw by hand totally vanish! I can turn on/off the Tcream layer and the whole layer vanishes as expected. Though exporting, nope. Not gonna happen it seems.  So that was a waste of a stencil.  Well, now the 3rd one in fact due to various "issues".

It seems near impossible to get help from cadsoft now. Farnell took over, and all I can find is a forum for help. Where, well, wasn't much help.

So I decided to do a new IC lib with smaller Tcream areas and export that instead. At least then they would export as "normal" parts. Nope, not gonna happen either it seems. Eagle seems to automatically create the Tcream layer based on the copper pad size. So I had to change all the pads, which renders it useless for the actual PCB. So long story short, I ended up with 2 libs, a PCB fabrication lib, and a solder stencil lib. How annoying!

So it finally came and I pasted it up..

I only pasted the IC's and pins this time.

Into the oven..

The pins looked pretty decent, just 1 small short.  Probably could do with some more paste overall actually.  The IC.. well, still shorts :(  It also looks like some paste is actually missing of some pins, so no idea where that went.So pretty much another epic fail there.

The paste I am using is class as "slump free". I'm assuming it means just that. Great for caps, resistors and such, but for close pitch IC pins.. nope.. I need it to "slump" , or well, actually flow on the IC pins and pads. So I think the "slump free" is actually hindering efforts as I have not had so much trouble with paste before.  On the plus side, its a very clean board, no "splatter" anywhere.

So my choice of paste so far , are paste which splatters everywhere which is impossible to clean, but doesn't short.  Or a paste which is very clean and shorts everything out.  So either spend time trying to clean the boards, or trying to fix them. 

Considering the amount I have spend in stencils and SMT paste so far, It makes me wonder if I had should have just got them assembled in the first place.  Even so, I still end up fixing 60-80% of boards anyway. So basic SMT soldering is a nightmare for fine pitched stuff.

My only real option is to go back to the paste I am happy to work with and try importing it direct from the manufacture in the USA.  I think the solder splatter issue only happens when the paste is going "off", and since its basically 6-12months out of date buying it in the UK, then I don't really have much choice.

The hunt continues...

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