With all these new Mega Drive releases lately we've had a lot of people complain about cartridges being of low quality and potentially putting consoles at risk. Shouting at a dying forum or something is not going to help any vendors realize what they're doing wrong, so I'm quickly putting together here what to look out for. This probably deserves a proper page with strict specs to follow some day.
5V vs 3.3V components
Yes, this thing again.
Mega Drive is a 5V machine and everything on the cartridge slot is 5V. Modern components are usually 3.3V instead (or even 1.8V). Now, using modern components is great, you can get cheap Flash memory that way! But you absolutely must not mix 5V and 3.3V signals as-is.
If you're using 3.3V (or 1.8V) components, you must use a voltage regulator and level shifters (the former generates 3.3V VCC out of 5V VCC, the latter converts signals between both voltages). Connecting 5V directly to 3.3V components risks damage long term, in particular to the latter.
The hot topic of the day!
Something that may be easy to overlook is that the bottom of the board should be beveled (i.e. thinned down). This is important to reduce risk of damaging the cartridge slot: if the board isn't beveled then there's little room for margin to maneauver and you risk pushing down the pins when inserting the cartridge, potentially damaging it permanently. With beveling the board is easier to insert and pins will be at most pushed a bit outward instead, which is much safer.
Here's how the bottom of the board should look like (based on that PDF
GEN-CART-BASIC-fabnotes.pdf that keeps circulating
around), note that 0.063 inches is a common thickness for boards:
Related to the above, the bottom corners of the board should also be chamfered in a similar way, making them more rounded instead of hard corners. This one isn't as critical at least (failing to do this simply makes it a bit harder to seat or unseat the board) but you should still try to do it.
This one is easy to miss if one isn't putting much thought into it, but ideally you should cover up any empty space in the board with a trace to GND. This acts as a RF shield and helps reduce risk of interference with other nearby electronics.
Bringing this up since I've seen a few boards without it.
One last note
Check the actual boards please. I'm seeing a lot of people going "they make shitty cartridges!" over stuff that was done wrong long ago but nobody bothering to confirm if their newest releases still do it, or what they are doing wrong (because without details we have to assume the absolute worst). It starts getting hard to trust those comments if we can't tell for sure if they're still true nowadays (they may be, but we can't tell).
This gets even more important as new issues are found. Board beveling is something that people only started paying attention like last year or so. I'm sure that we'll find yet more issues in the future that we're currently ignoring, and we're probably going to be slamming manufacturers for getting that wrong back when nobody was paying attention.
I suppose there's also some mea culpa for not having put together yet a page giving full specs for cartridges to follow. It's very easy to get everything wrong when there isn't any checklist to follow, after all!