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Homebrew IIfx RAM - Did anyone finish making these?

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On 4/11/2020 at 4:28 PM, Bolle said:

I made myself some 2-layer SIMMs for the IIfx based on the 4-layer design joe used.

Couldn‘t get them to work yet but I also went for the cheapest non-gold plated PCB option so I suspect they do make very bad contact.

Stopped to care after a few tries because 32MB is actually enough for my IIfx.

Gotta dig those out and see if I can actually get them to work.

How closely do your PCB's match the "real deal" in thickness? Is there a difference between two layer and four layer boards in thickness? Six layer boards? Curious if the SE/30 RAM PCB thickness may be rearing its ugly head for IIfx SIMMs as well? Might tinning your contacts make for better physical fit in the slot? Un-plated edge card teeth might work to your advantage there.

 

Pressure sensitive vinyl washers (as cut for signs) for the socket connection was the other ideal I'd floated. Unmentioned notion percolating ATM would be that laser cut copper "combs" of a proper thickness to be applied with paste and hot air to the contacts of too thin PCBs might be an answer?

 

Quote

In the end the hard part won’t be making the PCBs but sourcing the dual ported RAM chips you need in large enough quantities. 

For obvious reasons, everyone's been looking for RAM to populate 16MB boards. Are RAM ICs more available that might make up 4MB-8MB SIMMs?

Guess here would be that tinning (electroplated?) gold contacts won't work?

 

Dunno, I'm happy enough with the 32MB I found in the pet IIx. That led me to return and snap up the remaining pile of them. They only had 8MB, but made for a lot of hacking fun back in the very early days before preservation took hold in the hobby.

Edited by Trash80toHP_Mini

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10 hours ago, Trash80toHP_Mini said:

How closely do your PCB's match the "real deal" in thickness? Is there a difference between two layer and four layer boards in thickness? Six layer boards? Curious if the SE/30 RAM PCB thickness may be rearing its ugly head for IIfx SIMMs as well?

IIfx SIMMs are also .050" thick, and so will have the same issues seen elsewhere if one settles fro 1.2mm thick boards.

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@joethezombie what's the thickness of your 4 layer PCBs and have you had any problems with reliability? Sourcing proper 1.27" PCBs would definitely the way to go, but if not really workable, tinning one or both sides of the edge connector contacts might help? What thickness might tinning one side add to the contact?

 

Alternately (crazy complicated?): soldered copper plating might be practical, depending on the alloy's properties? A quick search turns up 2 Mil Copper Foil where a thickness of 2.56 Mil thickness is required to achieve a proper .050"/1.27mm SIMM. What thickness might the sandwiched solder paste be, anywhere close to .00056 Mil? If the solder is a bit or much thicker, 1 Mil, 1.4 Mil and 3 Mil copper foils are available. Depending on the solder paste thickness, 3 Mil copper plates might be in order. Copper foil will be really soft as compared to the contact surfacess on standard SIMMs. 3 Mil plating might be better if not still too soft for the method to be workable.

 

Laser cut "combs" should be easy enough to apply via solder paste/hot air method. "Tree" waste would be sanded/filed away.

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Laser cutting copper would likely prove more problematic than aluminum given its better (worse for laser cutting) heat transfer properties. Dunno, gotta throw the notion out here to get this craziness outtta' my head. :blink:

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Oh yeah, the fabrication plant I used was able to create proper boards.

Quote

PC substrate: 1,27 ± 0,1 mm

 

I haven't had any problems with reliability.  I've built 8 complete sets of 128MB now, and all have worked fine in my two IIfx testbeds.  It's conceivable that I could have received some on the very low end of the scale at 1.17mm, which would be below threshold.  So far, out of the ones I have built, they are all .050" exactly on the micrometer across the fingers.  I did use 2oz copper, I wonder if that makes the surface copper layers thicker?

 

About Rominator,  I have 3 of them, and have never had to use tricks to get good contact.  SIMMs should have a tolerance from 1.19 to 1.37.  Standard fabrication tolerance is ±10%, so if BMoW used 1.2mm substrate, he must have received some batches on the bottom of that tolerance scale.  I should measure mine.

 

 

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I'm so glad to see this closer to becoming a reality for people who want some extra RAM! :-)

 

Just out of curiosity @joethezombie, which PCB fabrication place did you use that was able to do 0.050" PCBs? All my experience is with hobbyist-level services like Seeed Studio and PCBWay which both just do 1.2 mm. To get the correct thickness, did you have to go to somewhere a bit more "professional" and less "hobbyist"?

 

It still bothers me that I never quite got to the bottom of the SE/30 ROM SIMM thickness issue back in the day. At least, I think it was a thickness issue...

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Hi @dougg3,

 

Thanks so much for designing these.  [8D]

 

The only place that would do 1.27mm boards was allpcb, and it took a bunch of emails, a special setup, and it more than doubled the price of the PCB.

 

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Would love to be the first to purchase a 128MB set once you have them ready.  I have a bit of extra cash right now.

I'll most likely limit sales to 4x16MB per individual so everyone can at least get a bunch of RAM.  If there are leftovers, I'll do a round two.

Edited by joethezombie

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On the thickness problem - you know the orange tape that you sometimes find in electronics? Polyimide is the material, the translucent gold film. It is heat and distortion resistant and lasts forever, I have pulled it out of old electronics and it looks like new. It is not conductive which is vital since the SIMM sockets often have contacts at the rear also.

 

Anyway, you have PCBs that are 1.2mm substrate and you want 1.27. Adding .07mm of thickness means adding about 2.76 mil. Here's a tape that is 1/4" wide, 3 mil thick and since it is from 3M it is probably actually very close to the specification. Adding this to clean PCBs would give you 1.276mm thickness. 3M Polyimide Film Electrical Tape 1205

 

One strip of tape on the reverse of the contacts should make those SIMMs fit perfectly. Not cheap but better than throwing away PCBs, or paying setup costs for non-standard substrate?

Edited by aeberbach
conductive

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22 hours ago, joethezombie said:

Hi @dougg3,

 

Thanks so much for designing these.  [8D]

 

The only place that would do 1.27mm boards was allpcb, and it took a bunch of emails, a special setup, and it more than doubled the price of the PCB.

 

I'll most likely limit sales to 4x16MB per individual so everyone can at least get a bunch of RAM.  If there are leftovers, I'll do a round two.

Just wondering what kind of anticipated cost you'd be placing on 4x16MB modules.  Not going to hold you to it, just an estimate.  I'd very much be interested in 4x16MB for my IIfx pair.  Even if I could just upgrade one for now, that would be great!

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On 7/6/2020 at 6:46 AM, aeberbach said:

Here's a tape that is 1/4" wide, 3 mil thick and since it is from 3M it is probably actually very close to the specification. Adding this to clean PCBs would give you 1.276mm thickness. 3M Polyimide Film Electrical Tape 1205

Perhaps make this optional, and provide a kit of cut strips upon request for the user to apply as needed, since the tolerance of SIMM slots seems to vary (some work fine with 1.2mm SIMMs, whereas others seem to require 1.27mm exactly).

 

c

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Hey guys, anyone following this should check this out:

 

@hyperneogeo is building some of these modules right now, so please don't wait around... get 'em while the gettins' good!  I don't have a lot of memory chips, and my source has stopped replying to my inquiries after I thought we had made a deal...

 

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