I have about 200 custom boards designed around the PN7150. A majority of the boards exhibit an issue where the PN7150 reports a tag detected that cannot be activated because it does not exist. This only occurs under narrow temperature ranges as measured on the surface of the PN7150. In the worst case scenario a device will end up within this range during use in the field and the problem will manifest so heavily that an actual RFID tag cannot be read. In other cases an RFID tag may take 1-2 seconds to be read.
Here is a capture of the device operating normally picked up with a simple loop.
Here is an image of the problem appearing
In the second image you can see that after recovering there is an opportunity to read an actual tag, but in some cases it would get so bad that no tags will be read.
For a given unit the temperature range where this occurs is always the same.
Applying hot air or cold air directly to the PN7150 corrects the issue as soon as the temperature is outside the offending range. Similarly bringing the chip into the range causes the issue to appear.
Applying pressure to the PN7150 via a C clamp directly on the chip or flexing the board in certain ways prevents the issue from occurring.
I have not been able to remove the issue by reflowing a board, but I have seen a shift in the offending temperature range after reflowing.
I took these findings to imply I should improve the thermal coupling between the PN7150 and the ground pad. I added many via-in-pad and chose non-conductive fill. Two more boards were produced with this configuration.
Board A does not exhibit any issue for any range of temperature it looks like this under XRay.
Board B exhibits the issue when the surface temperature of the chip is between 61C and 64C. Board B looks like this under XRay.
I got them XRayed in the hopes that voiding beneath the chip would explain the issue but puzzlingly Board A appears to show voiding to a greater extent than Board B.
The sample size is small but adding these vias does seem to have an effect. Boards without vias tended to show the issue in ranges centered in the mid 40s C and had wide ranges. This Board B is the only board to my knowledge to show the issue at such a high temperature in the 60s C and its range is very narrow at only 4C wide.
I'm basically at a loss here. I don't know what exactly to ask other than if this behavior is understood by NXP or other users.
I saw one user who had this issue from -5C to 0C and they said cleaning flux from the board fixed it. I have vigorously cleaned my boards many times to no effect, and they go through a DI Water wash in assembly. No visible flux is present on any unit. That user also said they switched the solder they are using but I'm not sure which properties of the solder I should be looking for. Here is their post: https://community.nxp.com/t5/NFC/PN7150-vs-Temperature/m-p/1043162
Does it make sense that voiding beneath the chip can cause these issues? Any recommendations on what to try?