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DRV8846: What is the role of PPAD

Part Number: DRV8846
Other Parts Discussed in Thread: TPS61160,

Hi Team,

The customer uses DRV8846 to drive the motor and TPS61160 to drive the liquid crystal. In each production, the LCD screens of several machines are not bright or the motor rotation noise is very loud. There is no problem with the soldering of the pads around the chip visually, and it is impossible to judge and prove whether there is any problem with the PPAD soldering. The chip can be repaired by heating the chip with a hot air gun every time, and it can be seen that the chip is indeed not soldered well. Customer would like to know:

1. What is the role of PPAD and how it is connected to other parts of the chip?

2. Does the QFN packaged device require a special soldering process?

Thanks,

Annie

  • Hi Annie,

    Thank you for the background, I am always happy to help. 

    1. The PPAD part is not an electrical component per se but rather the heat dissipation solution. It allows for the chip to dump all of it's heat unto the board avoiding of overheating and overtemperature shut down. If it is not attached correctly then the chip will quickly overheat and will stop functioning. 

    2. The QFN package, due to having its contact points in the bottom of the chip, requires reflow soldering to properly solder the pads and ensure good contact for both the thermal and electric components. 

    Please let me know if you have any further questions, also if you do please elaborate on how the customer is fixing the chip by heating it with a hot air gun every time. Is it shutting down and then the hot air gun allows it to work again? Does the hot air temporarily allows it to work properly?

    Best,

    Pedro Arango Ramirez. 

  • Hi Pedro,

    Thanks for your answer.

    The bottom pad is ground. Is the bottom pad inside the chip connected to ground?  What is the function of the connection between the pad and the ground on the circuit board?

    1. If it is only the heat dissipation, could the heat be dissipated without soldering?

    2. Reflow soldering is used for each production. Only use a hot air gun to correct defective products. The current situation is that hot air can make defective products work normally. 

    3. The problem now lies in the LCD driver chip TPS61160. The machine has been used by the customer for 1 year, and the LCD backlight flickers or the backlight does not light up at all. 

    4. The current direction of doubt. (1) The circuit board pads are made in accordance with the design recommended by TI.  (2) Welding process: only know the welding temperature and solder parameters.

    Thanks,

    Annie

  • Hello Annie,

    In most QFN designed PCB's the landing spot for the PPAD at the bottom of the chip is connected to the grounding plane in the PCB. Indeed this would make the bottom PPAD connected to ground, nevertheless I believe that the ground reference is also taken into account on pin18 of the package. Heat dissipation requires a proper solder connection between the PPAD and the circuit board, without proper soldering there is no way for the heat to be dissipated into the circuit board. 

    My guess is that if hot air is fixing the problem, then it is most likely a PPAD soldering problem with the circuit board. I would guess that solder connecting the chip and the board through the PPAD did not properly melt and distribute along the bottom surface, and once hot air is applied the solder is reflowing and covering a larger area. This would be confirmed if the hot air is only applied to the QFN chip. While my knowledge is limited in solder reflowing I would troubleshoot the process by increasing the amount of oven time and double checking that the amount of solder for the pads is adequate. 

    Please let me know if you have any other questions I might assist in.

    Best,

    Pedro 

  • Hi Pedro,

    I appreciate that it points out the problem sharply. May I know is there any recommendation for welding parameters, such as welding time, welding temperature, solder composition, etc. What is the welding success rate of this type of chip?

    Thanks,

    Annie

  • Hi Annie,

    I believe that the following App note will be very helpful for soldering troubleshooting:

    https://www.ti.com/lit/an/slua271b/slua271b.pdf?ts=1629298087918&ref_url=https%253A%252F%252Fwww.google.com%252F

    It details the different kinds of solder that can be used as well as welding time, to temperature curves on page 14, be sure to adjust for the max temperature of the DRV8846. I do not have the precise figure on welding success rate of the chip. The welding success rate may be a better question to ask for the packaging team seeing how vQFN is ubiquitous across many ICs. 

    As always, let me know if you have any other questions. 

    Best,

    Pedro