TPS22916: Reliability

Hi Gordon,

Could you send your schematic with external circuitry?

Do you have a picture of the damaged part?

What kind of damage are you seeing, and what are your failure conditions? 

One possibility we've seen could be due to your solder mask opening:

Can your PCB fabrication house maintain a solder mask width of 0.07mm? If not, it is possible they reduced the solder mask opening to create more space for the solder mask strip. 

Best Regards, 

Elizabeth 

Hi Elizabeth,

thanks for getting back to me and apologies for taking a while to sespond - I wanted to do so properly. Picture below shows layout with one of the devices - I've measured dimesnsions on the PCB and it seems OK in terms of the footprint specified. Blue is bottom side copper, pink is bottom side solder mask, green is just component courtyard.

I don't have any photos of the failures as I've removed them all and replaced by hand using hot air gun (I'm sure you can imagine how fun a job that was!), but I can say that there was no sign of any visible damage.

The symptoms are varied -

1 - sometimes it appears that the enable is pulled to ground (measures close to a short) and as expected, the device just doesn't turn on in this situation.

2 - sometimes the output is constantly fully on.

3 - by far the most common failure is that the device is not turned fully off when enable signal is at 0V, and it draws much higher quiescent current that it should do. Sometimes it's obvious straight away, sometimes it takes several hours to be seen. The excess current is sometimes as low as 50uA (which still swamps my board's normal Iq of about 12uA) but it's often as high as a few mA. After all the debug I've done, I can now find this failure quite easily because when this happens, the voltage measured on the output side of the switch (in 'off' condition) is always notably above 0V. When the switch is working this voltage is always <1mV (and whatever non-zero value is measured is probably just the error in the meter). When the switch fails, I've seen voltages at the output of 10mV, 100mV with the highest being about 700mV. I didn't record values of Iq against output voltage when off, but anecdotally (and I'm fairly sure this is right), I'd say that higher values of ourput voltage correspond to higher Iq values.

Of these, I can accept that for (1) and (2) it's possible that the chip has been misaligned when placed on the board, though when heating up the chip and poking it around slightly on its pads, I can never remove the failure. With (3), I don't think it can be due to any short between pads, and leads me to think the internals of the chip have somehow become damaged, or were not properly functional to begin with.

If you're able to make any further comment on this, I'd certainly be interested in trying to understand this. I'd really like to keep this device in my design as the small footprint is fantastic in such a tight design.

Thanks,

Gordon.

Hi Gordon, 

Because of this device's fragile package, any sort of rough handling conditions while being placed onto the board could cause damage that would cause unexpected functionality, therefore manufacturing and handling is going to be very critical for this device. 

We do test all of our devices to ensure they have full functionality before they leave the lab. 

The fact that we're seeing 30% failure is not likely due to anything on the chip level and is likely due to a soldering/ handling issue or a limitation with your PCB manufacturing process. The situation explained in failure (3) could also be caused by a soldering or handling issue. 

I see your board design lines up with our datasheet recommendations, but can you confirm that your PCB manufacturer is adhering to those dimensions?

Is this device intended to operate in water or salt water/ high humidity environment?

Best Regards, 

Elizabeth

Hi Gordon, 

Do you have any updates for me here?

Best Regards, 

Elizabeth 

Hi Elizabeth,

sorry about the delay. I'd agree that it seems by far the most likely that the damage is causes somehow when the devices are fitted to the PCB, rather than during chip manufacture. I've been trying to find out from my PCB manufacturer what might have happened, but as yet I don't have any useful info back from them.

I'm due to meet them in a couple of weeks, so if the thread can be kept open til then, I'll see if I can get any further info by then.

Thanks,

Gordon.

Hi Gordon, 

Thanks for the update! I can keep it open for a couple weeks. Best of luck with your PCB manufacturer; please let me know if there's anything I can help with in the meantime. 

Best Regards, 

Elizabeth 

Hi Elizabeth,

I've now had some info back from the PCB manufacturer. It seems that solder mask clearances have been adhered to and that the board was produced as per design intent.

One thing the PCB manufacturer has commented on is that reflow profiles are not specified for TI parts as well as they would like. They are using the generic document (https://www.ti.com/lit/an/spraby1a/spraby1a.pdf?ts=1636031374132&ref_url=https%253A%252F%252Fwww.google.com%252F) as guidance, but wonder if this is appropriate given the nature of this device. They comment as follows:

"Our maximum zone temperature is set to 255 degrees with a typical delta of -5 to -10 degrees from zone max temp to device max temp.

If you are seeing such a high fail rate then we could look to reduce the max temperature by 10 degrees which would still be in the reflow parameters of the device."

Are you able to give any guidance as to what the ideal temperature profile for this device would be? Obviously there might have to be compromises given other devices are present on the board which might have competing needs, but the ideal profile for this device would be a good starting point, if such a thing exists.

Thanks,

Gordon.

Hi Gordon, 

Thanks for the update; I'll reach out to packaging to see if we can supply any specific considerations for this device with respect to maximum zone temperature. 

Foremost, we recommend to the manufacturer to reduce the nozzle pick and place force. 

Also, is this device intended to operate in water or salt water/ high humidity environment?

Best Regards, 

Elizabeth 

Hi Gordon, 

Here are some resources in regard to WCSP handling: 

https://training.ti.com/troubleshooting-tips-ic-wcsp-handling?keyMatch=WCSP

https://www.ti.com/lit/an/szza013b/szza013b.pdf?ts=1636060585272&ref_url=https%253A%252F%252Fwww.ti.com%252Fsupport-packaging%252FSMT-application-notes.html

https://www.ti.com/support-quality/faqs/wafer-level-chip-scale-package-faqs.html

Best Regards, 

Elizabeth

Hi Elizabeth,

thanks for those links, will have a good look over them and also pass to PCB manufacturer.

Fair point about the pick and place force - is there a particular force that would be most suitable (apologies if it's in the documents as not read them yet!)

On your question about operating in water or in high humidity - these devices do go in the sea, but boards are solid potted. In reality there will be some water absorption by the potting compound but it's quite small. From what I've seen so far, when I can get a board working on the bench, it continues to work in the real environment.

Gordon.

Hi Gordon,

Apologies for the delay.

With regard to pick and place force recommendation, we do not have specific threshold but the least possible will be most beneficial. 

The WCSP package has no protection against cracking or chipping, so delicate quality manufacturing needs to be implemented. 

Was your manufacturer able to resolve the issue? 

Best Regards, 

Elizabeth 

Hi Gordon, 

Since this thread is going on two weeks without a reply, I am closing/locking it. If there is another update here please make a new thread. 

Best Regards, 

Elizabeth