First challenge: interface the Nokia N97's original keyboard.

I desoldered the 0.4 mm pitch FPC connector (X4401) from the N97 mainboard. With no microscope or stencil, I used a combination of:

• Tinning pads via drag soldering

• Reflowing with hot air to seat the connector

• Final drag pass with flux to clean and confirm joints

First try — no bridges, no lifted pads. All 20 pins mapped successfully.

From Nokia’s schematic, I confirmed the keyboard runs on 3.3 V logic and has 3.3 V LEDs (verified via testing). I repurposed a double row 30 pin 0.4 pitch breakout board to route the FPC pins to a 2.54 mm header for prototyping.

I also identified the resistive touchscreen's 4-wire layout by resistance probing — no onboard controller, fully passive. Will drive it via an XPT2046 SPI controller.

The original LCD (LTJ035L001A) uses RGB input, but I’m currently adapting a Waveshare 3.2″ HDMI panel to fit the housing. The long-term plan is to build a custom HDMI driver board once I finalize display fit and power routing.

First attempt at 0.4 pitch pad tinning

Salvaged and resoldered 20 pin FPC. No bridges on any of the FPC pins

Keyboard mapping.