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AVSS current increases as temperature rises

Mike Ihm
Prodigy 120 points
Part Number: DAC7716
Other Parts Discussed in Thread: DCP020515D, , REF5050, ADS8883, OPA4990, DAC8234, DAC8734, PGA855

Tool/software:

Hello,

I am using two DAC7716 on our board and they are being powered by a DCP020515D part.  Through testing at around 75C we found that the DCP020515D would go into what looks like thermal shutdown because the output rails of the part are rock solid +/-15V until all of a sudden at around 75C they have huge amount of ripple at around 150KHz approx and I take that to mean that the part is going in/out of thermal shutdown?.

I removed the DCP020515D and used an external power supply instead.   As the temperature rose, I saw the negative supply current rise from 50mA to about 200mA by the time 85C was reached.  This indicated to me that the issue is not with the DCP020515D shutting down but instead there is a load problem.

Setting the board on the bench and continuing to use the external power supply, and now this time rather than heating up the entire board  I used a reflow pen at its lowest setting (100C) and kept the pen around 0.5-1 inch away from parts and move it around the board looking to see which semiconductors would cause an increase in negative supply current.   Moving the pen over the DAC7716 would increase the current by 50mA at the elevated temperature

Normal negative supply current is around 40-50mA at room temperature.  At around -30C the negative supply current is around 30-40mA..

Any suggestions?

  • 0 in reply to Mike Ihm
    TI__Mastermind 33188 points

    Hi Mike, 

    Yes, CTE = Coefficient of Thermal Expansion.

    "Work" in this context is related to the forces acting on the joints during temperature cycling. 

    As the system goes through temperature ranges in a cycle you get thermally induced stresses in variable magnitudes and interfaces due to the different materials in the system expanding at different rates (PPM per degree C). The QFN is held down by solder joints and the exposed center pad. The leads are small in this package type, and the exposed pad is very large compared to the leads and their total area.

    So as the temperature cycles the differential expansion rates create forces on the joints which are technically work because it is moving solder on a small scale (grain scale). That work stresses the joints, displaces grains of solder and creates deformations which eventually separate the alloy (crack). If you remove the exposed pad you will have a higher work (and therefore higher stress) on the leads than if you kept the exposed pad.

    Either using any sort of epoxy, or leaving the thermal pad unconnected altogether pose risks on stressing the solder joints for the reasons described above. We do not have a type of epoxy that we would recommend to replace the solder down connection of the exposed thermal pad. 

    Best,

    Katlynne Jones 

  • 0 in reply to Katlynne Jones
    Prodigy 120 points

    Hi Katlynne,

    Would the Loctite 3621 SMT adhesive material work in place of the solder in the center of pad?

    next.henkel-adhesives.com/.../68087IB.html

    datasheets.tdx.henkel.com/LOCTITE-3621-en_GL.pdf

    -

    Is it acceptable rework to perform the following to reclaim a few boards due to tight timelines:

    U17:  (1) mill rectangle to create a gap in the center pad, (2) drill out all holes through first layer, (3) solder mask the outer region of the center pad (shown in green), (4) solder part as normal.  Below is before on the left and after modification on the right.

  • 0 in reply to Mike Ihm
    TI__Mastermind 33188 points

    Hi Mike, 

    We really can't suggest any adhesive material being used in place of solder. We don't have any insight into what the long term mechanical performance would be. Our packaging team doesn't recommend any material besides solder being used without characterization and qualification testing on your end, and it sounds like you don't have the time for that. 

    Your second idea of milling out the ground connections and still soldering the device as normal would be preferred to adhesive or not soldering down the center pad at all. 

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Prodigy 120 points

    Are there any versions of the DAC7716 or other resolution parts that does not have the center pad connected?

  • 0 in reply to Mike Ihm
    TI__Mastermind 33188 points

    Hi Mike, 

    All of the devices in the same family as the DAC7716 (different resolution devices, DAC8734, and DAC8234) have the same center pad connection. There are no other p2p devices.

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Prodigy 120 points

    Can you tell me the CTE of the plastic encapsulant used on the DAC7716 QFN package?

    Mike

  • 0 in reply to Mike Ihm
    TI__Mastermind 33188 points

    Hi Mike,

    Below are the details for the epoxy mold compound properties, which technically is an encapsulant:

    Alpha 1 = 9 PPM/C

    Alpha 2 = 36 PPM/C

    TG = 130C

    Best,

    Katlynne Jones