The Mac ROM-inator II is back in stock – get yours now at the BMOW Store. The ROM-inator II is a replacement ROM SIMM for Macintosh II series computers and the Mac SE/30, adding a bootable ROM disk, 32-bit cleanliness, HD20 hard disk support, and more. Read more about it at the project’s home page.

One small mystery has followed the Mac ROM SIMM project from its earliest days, even before BMOW took over the project from Doug Brown. Experience has found the Mac SE/30 to be much pickier about replacement ROM SIMMs than other Macintosh II series computers. On many SE/30 machines, the computer won’t boot with a ROM-inator II SIMM installed until the SIMM is shimmed or braced with clips or rubber bands. Why? Neither Doug nor I ever found a satisfactory answer. While it isn’t difficult to do, it’s an added step that isn’t needed for other machines.

Over the years, a few SE/30 owners have suggested the ROM-inator II SIMM is too loose in the SE/30 ROM socket, and one person had success manually tinning the SIMM pads to make them slightly thicker. I was never fully convinced about this theory. Why would it only affect the SE/30 and not other machines? And anyway the replacement SIMM was the same thickness as the original ROM SIMM (about 1.2 mm), to the limit of my caliper’s measuring abilities. The exact thickness is hard to pin down, since it varies slightly at different regions on the PCB, and there’s also a +/- 10% thickness tolerance from the manufacturer.

I recently learned something that made me reconsider the thickness theory. Back in 1970s and 1980s, PCB thickness was sometimes specified in mils (thousands of an inch) rather than in millimeters. 50 mil PCBs (1.27 mm) were a common thickness, but today these have all but disappeared. Now 1.2 mm is the closest standard thickness, and that’s what was used for all the previous generations of ROM-inator SIMMs.

Is it possible the original SE/30 SIMM was 50 mils (1.27 mm) nominal thickness, with a matching thickness socket, while all the other Macintosh II models used 1.2 mm nominal SIMMs and sockets? That would be a little surprising, but it could explain the behavior I’ve seen. With a 10% thickness tolerance, that means original SE/30 ROM SIMMs would be anywhere from 1.14 to 1.40 mm thick, and ROM-inator II SIMMs would be anywhere from 1.08 to 1.32 mm thick. It’s conceivable that the ROM-inator II SIMMs at the thin end of the distribution, under 1.14 mm thick, might have trouble in SE/30 machines.

It’s still just a theory, but it seems like a plausible one. I spoke to my local SIMM manufacturing partner about getting a thicker PCB, but unfortunately 50 mil PCBs were not available. To complicate matters further, if you’re concerned about differences of a hundredth of a millimeter, then you also need to consider the thickness of the PCB’s copper layer and any plating on the copper. Much of this calculation went over my head, but eventually my local partner was able to find a Korean manufacturer who could create a 1.27 mm stackup with 10% tolerance. The new batch of ROM-inator II SIMMs in the store today were made by that manufacturer.

So there you have it. If I’ve done this correctly, the nominal thickness of a new ROM-inator II SIMM should be a whopping 0.07 mm greater than before, an increase of about six percent. Will that do anything to help the SE/30? Especially when the thickness tolerance is still larger than the new increase in thickness? Honestly I’m not sure, but I’m interested to find out. Maybe it will behave the same as the ones from the previous manufacturer, which would be anti-climactic but fine. Or maybe it will provide some new clues for this long-running SE/30 ROM SIMM mystery.