What approach should I use to measure this SPI Chip - Page 1
Forum section: electronics / beginners

author: HansGruber
author_rank: Contributor
timestamp: March 17, 2022, 09:05:16 am
content: I would like to measure a "8-contact WDFN (6mm x 5mm)" SPI Chip (see attachment) while it is soldered to the PCB. My main problem is the whole setup because I am not experienced with smaller sizes like this. The other problem is the conformal coating all over the PCB.My idea is to remove the conformal coating with my soldering iron ?In the next step I would hot glue a 4 pin male header pin to the PCB somewhere, so I have something solid to attach my probes ?Than finally add some extra solder to the SPI Chip pins and solder enamelled wires from the pins to the male header pins ?Does this sound like a good plan or what would be a better approach ?

author: jpanhalt
author_rank: Super Contributor
timestamp: March 17, 2022, 09:48:08 am
content: 1) Removing conformal coating with a soldering iron sounds risky. Unlike solder resist, conformal coatings are often soluble or more easily removed. If it feels hard, I would try a solvent first, like lacquer thinner or acetone/MEK. If it is soft/flexible (like hot melt), I would try peeling it. Isopropyl alcohol is a great help. It wicks into the interface between substrate and coating to facilitate removal. I prefer to use 99%. 91% also works on occasion.2) That appears to be a flash memory chip by Microchip. By "measure," I suspect you mean read it using the SPI interface. Its datasheet is available. So, I would just focus on the pins you need, and not try to remove all of the conformal coating.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 17, 2022, 12:27:06 pm
content: Usually acetone or some solvent will remove it with some brushing.I've never seen any damage caused by solvents except for plastic parts (Buttons, encoders... not ICs).What are you planning to do? Read its contents? That rarely works in-circuit because the pins are conected to other parts.A trick that usually works is holding the mcu in reset state by pulling the reset pin high/low (Usually active low), that way all pins will be in high impedance and won't disturb.Sometimes the only way is removing it. WDFN has bottom gnd pad, only hot air can remove it.

author: HansGruber
author_rank: Contributor
timestamp: March 17, 2022, 01:28:56 pm
content: I want to decode the SPI signal with my oscilloscope, that should work in-circuit, right ? I am not planning to read the existing content on the Chip, just observing the live communication on all 4 SPI pins. I am mostly interested in the timings, when the communication starts, how long it takes for one "package". I found the product "Chemtronics CW3500", it's a conformal coating remover pen. Maybe it's better than going to a hardware store and picking some solvent, especially since I have no clue what kind of conformal coating is used, I guess it's not silicone since it's relatively hard and brittle.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 17, 2022, 01:48:11 pm
content: You might take a look at the SDS for that CW3500.https://www.galco.com/techdoc/chem/cw3500_sds.pdfIt is mostly hexamethyldisiloxane (HMDSO) and acetone. HMDSO is a very poor solvent. Naptha would work as well with acetone. But then, I am a penny pincher.As for decoding with an oscilloscope, sure, it can be done. Have you done it before? Does you scope have a decoding function? If not, you might want to spend on a logic probe.

author: OM222O
author_rank: Frequent Contributor
timestamp: March 17, 2022, 04:25:15 pm
content: I'm not sure how to remove the coating, but you don't need to solder small wires to a tiny chip to measure stuff. The SPI lines must be connected to something else (resistors, probe points, other ICs, etc). Follow the traces and find an easy place to solder your probe wires.