MDM1. Itty bitty modem, itty bitty heatsink

Unscrewing the two tiny philips head screws and popping the lid off of it reveals the insides of this little modem, which turns out to be not that interesting as most of the insides are EMI shielded. Looking at the edge of the PCB it seems to be a four layer board.

The quality control sticker shows that it passed it in January of this year (2021). Looks like it’s not old stock they’re selling off for cheap, since I ordered it in February.

On the opposite end of the USB plug there’s the LTE antenna labeled PRX (Primary Receive) and a surface-mount RF plug (factory test plug?). I don’t want to force the plastic LTE antenna off of the board, but it doesn’t seem to be soldered. It’s held on by a plastic pin that drops into a recess in the PCB. My bet is that it’s got some pins underneath which slide onto the external antenna RF plug pads, allowing them to use the same PCB design for the two different configurations.

The left side of PCB has the WiFi trace antenna and the right side a secondary LTE trace antenna labeled DRX (Diversity Receive). The secondary LTE antenna appears to be disconnected, as the components on it’s trace aren’t populated.

The exposed portion of the board houses a flash chip and a ZTE modem SoC. The flash chip is a Fudan Micro 1 gigabit SPI NAND flash (part no. FM25LS01, the bottom left chip on the picture) and the SoC is a ZXIC ZX297xxxx (chip in the center of the picture). The SoC part no. is really hard to read and searching for the first portion that I managed to decipher didn’t yield any results anyways. We’ll probably manage to figure out the capabilities of this some other way (Note from the future: yes we will).

Something that I wasn’t expecting is that the SoC has a tiny aluminium heat sink that’s affixed to the upper portion of the enclosure. When the enclosure is closed then the heat sink gets pressed down on the SoC with a silicone thermal pad mushed inbetween. Since this is a mass-manufactured product then they must’ve done the math and found out that this tiny bent aluminium sheet is absolutely neccesary.

Both of the EMI shields were soldered on. Since I plan on using the dongle in the future I didn’t want to risk breaking it during lid removal. I don’t have a hot air gun, so it’d probably turn into coercing it loose with pliers ;). If there’s enough interest (or curiosity gets the better of me again) then maybe I’ll do a desoldered follow up on a second modem.