Friday, 21 July 2017

4MB RAM PCB ASSEMBLY NIGHTMARE - PART 2



Further to my previous post http://exxosnews.blogspot.co.uk/2017/07/4mb-ram-pcb-assembly-nightmare.html

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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