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Post by exxos » Tue Oct 31, 2017 3:10 pm

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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..
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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.
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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.
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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..
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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..
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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!
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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.

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

Into the oven.
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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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I have been trying for days to work out why the assembly of these are failing. It seems half the time the IC doesn't solder to the actual board. Though if I use a hotter profile, the PCB itself seems to burn up and break :( I even had one IC which seemed to bend up on one end which is madness. I have tried various SMT pastes, at various heat profiles.

I have even tried just solder pasting the PCB and making sure no shorts on the PCB after heated, and then applying the IC's on with plenty of flux. The IC does not seem to "sink" into the solder like it should. It would seem the PCB isn't getting hot enough to melt the solder properly. Though heating more seems to cause even more damage.

I also wonder if the PCB itself is warping as it is heating. I think I have seen something like that happen before as the copper heats, it can bend the PCB. As the IC does bend the same (basically remains straight) then the PCB isn't level to the IC and causes parts not to solder correctly. Its possible that could be another problem.

I think there is also a possibility that because I am adding flux to resolder with my gas soldering iron, that the flux leaves a residue making it hard to see solder bridges between pins. It is possible once a bridge happens, its blowing the IC up. I have managed to kill 2 MMU's so far :( Though its also possible that the 5V and 0V copper tracks were not large enough to cope with a short circuit and the tracks act like fuses and "blow". It seems the more I try and fix these boards, the worse they seem to become.

Simply re-heating the IC with flux to "re-flow" it shouldn't cause more damage. So I have come to the conclusion that the PCB sux. Not only that I went for a 1mm thick PCB because of height issues in some machines. Though overall, a extra 0.6mm isn't going to matter. More to the point, the PCB is actually thin and pushing it into the socket in itself could lead to damage after a while.

The previous boards were auto routed, this new board is 100% manually routed. This makes routing a lot neater and I can control better how close things are to pins and pads. Not only that, I used copper fill on top and bottom layers for 5V and 0V to make sure the power rails were "un-blowable".

Of course it seems unlikely that out of probably 20+ boards that each and every one failed to solder correctly. Its just a 100% failure rate each and every time. Even by chance there should be one good board, but there never is.

I know originally I made a batch of these myself and hardly had any failures. Though I think the current batch of PCB's are just to fragile to solder by hand. The last batch (as in the ones sold in my store) were manufactured by machine and mostly they turned out fine. I did have to re-flow several boards and I think 5 ultimately failed beyond repair. Even so, for some reason that batch of PCB just seems to be very problematic to assemble.

There is also a issue that the header pins themselves are so tiny, solder bridges become hard to spot and hard to solve. The new PCB has smaller solder areas to try and space out the gap between pins making it easier to see shorts. Also I had used square pads, now I used round ones , again to help prevent and spot solder shorts.

Now the new prototype PCB is on order. I paid for the faster service so I hope they will be here in about a weeks time. This PCB should be a lot stronger and be a whole lot less prone to assembly issues.

While I can get them made by machine, the cost is very high in producing a batch of 30. I don't want to invest huge sums of cash into its which sell very slowly. Fingers crossed the assembly goes better on these new boards else it likely will mean the 4MB MMU kits will no longer be produced.

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Revision 7C has arrived. I need to order a new solder stencil for this. Hopefully will be here in a few days. Then I am going to try my regular SMT paste but mixing in a bit more flux as this seems to work the best so far. I can't really optimise the PCB anymore than it is now for assembly. So its down to getting the right mix of flux & paste, and getting the right amount of paste on the PC with the stencil.. hours of fun...
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