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BQ27546-G1: BQ27546-G1 hot issue

Star Xu
Genius 9645 points
Part Number: BQ27546-G1


Hi 

The customer design with BQ27546-G1, the problem is seriously hot. A lithium battery is connected between J20 and J2, and a load is added between J21 and J24. The load current is about 300mA, and U9 will heat up.

When it is hot, the battery is hot too. Disconnect the load on J21. But the chip will continue to heat up. Disconnect J20, that is, disconnect the connection with the battery, put it for a while, and then connect it again. it's normal.

The chip package is very small, so there is no hole punched at the bottom of the BGA, and it is directly connected with wires. As shown below:

They made a small move during the wiring. I don’t know if it will cause adverse effects on the chip. Can experienced engineers guide me? It is the C3 pin of this chip, which is useless to me, that is, when drawing the PCB package. , I removed the foot.

The reason for removing it is that this signal is not used in the circuit, but it blocks the routing of the internal TS signal. If the TS signal is brought out, it is necessary to pass a 3mil wire through the middle of the pad, so that the PCB production cost will be reduced. will be expensive.

After removing this pin, TS comes out from the original position of SE (the wire lit on the PCB in the figure below), so that all traces are larger than 6mil, and the PCB production cost will be cheaper.

Will the BGA solder balls not used here cause an internal short circuit after melting? For example, after the solder melts, because there is no pad underneath, it flows to the inside?

Please give some suggestions.

Thanks

Star

  • 0
    TI__Guru 65496 points

    The problem with SE is that it's a push-pull output so if that solder ball e.g. connects to Vss (next to it) because it may not flow correctly without a pad underneath, it can cause a current path between SE and VSS. You can try setting the configuration to make sure that SE is 0 (see Table 2-2. SE Pin State, https://www.ti.com/lit/ug/sluub74/sluub74.pdf). 

    But in general, it's a terrible idea to omit a pad for a solder ball on a BGA style chip.

  • 0 in reply to Dominik Hartl11
    TI__Genius 9645 points

    Hi Dominik

    Thanks for your reply.

    A version of the hardware has been redone, produced again, and added BGA balls to see if there is a similar problem. If according to the previous design, that is, a ball can be omitted under the BGA, and 6mil wires can be used for the wiring, and there is no spacing less than 6mil.

    Since the BGA ball is filled, a 3mil wire is required to lead out the TS pin, and the cost varies greatly. But this may not be the root cause.

    When the chip heats up, the D1 pin (VCC) will rise to 3.7V.

    Please give some suggestions.

    Thanks

    Star

  • 0 in reply to Star Xu
    TI__Guru 65496 points

    There is something very wrong with the connections then. VCC must not exceed 2.75V. 3.7V is more than abs.max. so this can permanently damage the gauge. The only connection on VCC is supposed to be a decoupling capacitor and a pull-up for TS. None of them can produce 3.7V assuming that there isn't a problem with the layout. Are they able to XRAY if everything reflowed correctly or if there are any short circuits?

  • 0 in reply to Dominik Hartl11
    TI__Genius 9645 points

    Hi Dominik

    Thanks for your reply.

    The customer do the XRAY and fins the  severely heated boards has BAT+ and BAT- short-circuited, and some heat-generating boards have VCC and BAT- short-circuited.

    Thanks 

    Star