BQ79616-Q1: BQ79718 chip failure during battery pack integration soldering process

Hey Wu,

I'm still looking into this issue. I will get back to you tomorrow.

Best regards,

Abdel

Dear Abel, 

Thank you for your support and looking forward to your professional response.

Best regards,

Kubo

Dear Abel, 

Sorry to bother you again, may I ask when you are available to help reply? I am very much looking forward to your guidance.Thank you very much.

Best regards,

Kubo

Hi Abdel, 

May you kindly help take a look and provide some suggestions for this customer issue? 

It is urgent for customer since they are already started the DV test. 

If any questions, please let me know. 

Thanks!

Ethan Wen

Hey Ethan,

Thanks for noting the urgency of this problem. I will be working with the customer to figure out the issue. 

Regards,

Abdel

Hey Kubo,

Sorry for the late reply. I asked my colleagues about similar issue and the all suggested to check the wire harness connections as similar failures could be found with wrong connections were cells could mistakenly be plugged incorrectly. Could you provide me more details about the process of connecting the battery pack.

Best regards,

Abdel

Hi Abdel, 

Thank you for your reply.

I think there is some deviation in our understanding of the key points of the problem. We have identified the production step of the battery pack that caused the BQ79718 chip to burn out, as shown in the following figure:
Prior to this step, the voltage sampling board with BQ79718 chip had already been welded together with FPC (already conductive, without connectors), and the aluminum bar had already been welded together with the lithium battery.
The main purpose of this step is to integrate the integrated FPC, nickel sheet and voltage sampling board, with integrated battery cells and aluminum bars  through laser welding.
The main work content of this step is to first lay the entire FPC (with nickel sheet and voltage sampling board) on the flat surface formed by the aluminum bar and the top surface of the battery cell, press the copper sleeve of the laser welding equipment on the nickel sheet to complete the welding, (the copper sleeve is used to press nickel sheet  and aluminum bars tightly, while limiting the distance between laser welding and welding points, and the copper sleeve will conduct with the shoulder of the cell) , and weld the next point in the same way along the direction of voltage increase.
Abnormal point of this step: In this step, the laser welding copper sleeve will contact the nickel sheet and conduct, then conduct with the BQ79718 chip on the voltage sampling board through the wiring inside the FPC. At that time, there was a situation where the insulation resistance of the copper sleeve during laser welding was relatively low (about 100kΩ), which was equivalent to the BQ79718 chip pins that had completed the previous step (laser welding) being connected to the battery cell normally, while the corresponding pins of the nickel chip undergoing laser welding were connected to the ground through a 100k resistor.

In this condition, two BQ79718 chips were broken in the 8 sets of battery packs welded. And after strengthening the insulation level of the copper sleeve, we have produced more than 10 sets of battery packs without experiencing similar chip failures.

Can you please provide some key points or reference cases to help us gaanalyze the failure mode of this type of failure?

Best regards,

Kubo