You could have issues over time with this setup.Cheap audio gear used this principle in the eighties, and this was quickly abandoned because (small tact) switches get bad (resistive) over time.Better to use a port expander, with the switches in a matrix.Leo..
Cars still use resistor ladders for many steering controls and radio input. It work well, it shouldn't drift if you use good tactile switch from reputable brands , use a precise ADC with a good clean vref and don't forget less than 1% tolerance on the resistors.
Some questions about your design. What material are you using to make the contacts with the circuit board pads? Notice the board that works has bare copper. The one with problems has tin coating on the copper. Perhaps scrub the tin really well.The size of the contacts seems to be extremely small based on the keypads I have seen in the past. Those had serpentine traces from both sides of the connection that provided a huge area for the contact. The keys from years ago used carbon filled conductive foam. The foam would die from ozone and probably other stuff in the air.Paul
HASL - Hot Air Solder LevelingHASL is a type of finish used on printed circuit boards (PCBs). For hot air solder leveling finish, the board will be dipped in a bath of molten solder and then passed through a conveyor of hot air knives that brush the excess solder off. Basically, the copper pads are pre-tinned with the thinnest layer of solder, and the tinning protects the bare copper underneath. which can provide the board with a good shelf-life, and the existing layer of solder makes the molten joints much easier to join. It is a commonly used and cheap finish for most DIY projects. However, it is not suitable for fine pitch components since HASL leaves uneven surfaces.
Lead-free HASLLead-free HASL is variant from HASL, which is lead-free and qualified for ROHS compliance products. Please note that higher soldering temperatures are required for lead-free solder.
OK, so the car is 16 years old. Are you saying they didn't use a "good tactile switch from a reputable brand"? I don't think it has anything to do with 1% tolerance (which is quite ridiculous in this application)!
Sorry for the delayed response, went out today for the first time since March.Paul The solder is "0.8mm Lead Free Solder Wire with Rosin Core Flux Sn99.3 Cu0.7 " ( https://www.amazon.co.uk/gp/product/B07XQ9XTV8-and the actual physical connector is a 2.54mm pitch pin. The other end of the pin is a female Dupont connector which is attached to a Cherry mx at the other end via 2 inch wires. I'm using Cherry MX because I had a handful of them floating about.The coating (as JLCPCB state it) isAlthough I opted for the lead free version which has some additional notes, but not a lot.All of the traces I used in Kicad are 0.25mm. (9.84mils). It's interesting that you mention pad sizes, would trace sizes also have an impact? I found an old keypad and the traces are about 3mm wide on that.Do you think upping the trace width and creating a larger pad footprint would help?
reading = analogRead(keyp[i]); reading = analogRead(keyp[i]);
delay(2); reading = analogRead(keyp[i]); delay(2); reading = analogRead(keyp[i]); delay(2); reading = analogRead(keyp[i]);