nRF52840 Lead Finish of the BGA Balls

RE: nRF52840 Lead Finish of the BGA Balls

Kaja — Tue, 05 Nov 2019 12:02:34 GMT

Hi Israel and John,

I understand that Simon is helping you with this in the other case 238114.

He just updated the case.

Best regards,
Kaja

RE: nRF52840 Lead Finish of the BGA Balls

Israel — Mon, 04 Nov 2019 15:40:51 GMT

Hi Kaja

We have tried both ways

NRF_POWER->DCDCEN = 1;

and

NRF_POWER->DCDCEN = 0;

They give the same results.

RE: nRF52840 Lead Finish of the BGA Balls

John — Mon, 04 Nov 2019 15:35:49 GMT

I'd like to bring our FW engineer into this discussion but I don't see how I can add him. I'll try converting this to a public question. By the way, we have tried the internal LDO and DC-DC 1 and we get the same failure rate.

RE: nRF52840 Lead Finish of the BGA Balls

John — Mon, 04 Nov 2019 15:31:04 GMT

Hello Kaja,

Thank you for getting back to us. We are using the DC-DC 1 per Figure 212 in your product spec. We don't use USB or NFC. We have made some code changes that seem to be helping this problem significantly.

RE: nRF52840 Lead Finish of the BGA Balls

Kaja — Mon, 04 Nov 2019 15:09:34 GMT

Hi John,

sorry for the delayed response, I was on business travel last week.

But a question are you using LDO or DC/DC?

Best regards,
Kaja

RE: nRF52840 Lead Finish of the BGA Balls

John — Fri, 01 Nov 2019 13:18:13 GMT

Hello,

Haven't heard from you folks in a couple of days. Would really appreciate some help. I have some new information. Turns out we only use the HFXO when we use the BLE radio. We ran a test where we turned off BLE so we are never using the HFXO. The product goes to sleep and wakes up using the HFINT only. Will still see resets on our boards when they are running at -20C. If we switch to constant latency mode, we never see any resets. So I don't think the issue is with the 32MHz crystal. I think we are running into an issue when we are waking up from sleep. Perhaps we can run a test where we sleep/wake-up a lot more often to see if this increases our error rate. This is a huge problem for us. We have delayed the release of this product for two months while we've been trying to fix this. We thought it was a soldering issue and we have recently upgraded from a 7-zone to a 10-zone reflow oven. I'm thinking now the problem isn't a soldering issue. Hope to hear from you soon.

RE: nRF52840 Lead Finish of the BGA Balls

John — Wed, 30 Oct 2019 16:58:16 GMT

Hi Kaja,

We are still trying to resolve why our nRF52840 design is having some resets when we run the baord at -20C. For a long time we thought we were having a soldering issue with the BGA, but now we are not sure. If we run 50 boards in our environmental chamber at -20C, after two days about 1/2 of the PCBs will have experienced a reset. The reset reason reported by the nRF52840 is a power on reset. We have been monitoring the 3.3V supply and the 1.3V supply and we don't see them ever going away.

Typically, our design utilizes low power mode so we can get the longest battery life. We tried a test where we switched to constant latency mode, and now we never see a reset on any PCB! We can't ship out product with constant latency turned on because that increases our power consumption too much, but perhaps this is a clue to what is going on. As I mentioned above, we only see this problem when we soak the PCBs at -20C or lower. Does the 32MHz crystal stay on at all times in constant latency mode? If the 32MHz crystal is taking a little longer to start up at -20C, could that cause a reset? Typically, our device is in low power mode 99.9% of the time. We do wake up every second to do some housekeeping tasks.

Thank you,

John Araujo

RE: nRF52840 Lead Finish of the BGA Balls

Kaja — Fri, 30 Aug 2019 13:15:44 GMT

Hi John,

I'm trying to set up a meeting with our package expert, but his in Asia, so we have to find a day where you can join the call in the evening (your time), because we want our production house to join as well.

But they would like some more information from you:

1) What kind of solder paste are you using ? SAC305?

2) Can you return failure samples so that we can conduct solderability test to check pad status?

3) Is this your first time using this device?

4) Is this your temperature setting in anyway different from the reflow profile?

5) Could you share pictures of the failing boards?

6) X-rays?

Best regards,
Kaja

RE: nRF52840 Lead Finish of the BGA Balls

Kaja — Tue, 27 Aug 2019 11:44:34 GMT

Hi John,

we are waiting for a reply form our production house at the moment,to see if they have an updated reflow profile for us.
If they have an updated one it would be great if you could test that before sending us boards, to see if that fixes the problem.
Could you send some pictures of the failing boards?
Have you preformed some X-rays? - If yes, tt would be great to have a look at those as well.

[quote user="JohnAraujo"]I was wondering, however, if the VSS ball (B7) becomes unsolderd from the PCB, will that cause the internal 1.3V or 1.1V regulators to fail? We are using the on-board buck regulator to generate the core voltage as shown in Figure 212 of the product spec.[/quote][quote user="JohnAraujo"] We don't believe the reset issue is due to the power circuitry, but we will tripple check this. [/quote]

- Difficult to say, but not having proper grounding of the IC, not specifically B7, can probably contribute to the issue.

Could you also share the design files? - So that I can have a look at the design as well.

Best regards,
Kaja

RE: nRF52840 Lead Finish of the BGA Balls

John — Mon, 26 Aug 2019 16:01:32 GMT

Hi Kaja,

Is there any chance we could have a conference call with your package expert for the nRF52840? Our process people have done everything they can but we are still seeing many PCBs where the nRF52840 is resetting. We are temperature cycling the PCBs in our environmental chamber from +70C to -30C. All resets are occuring very close to when the chamber gets to -30C. We've never seen a reset at any other temperature. We also noticed that the reels have an expiration date. We've made a lot of these boards with ICs that are a few months past the expiration date. Not sure if this is an issue because we had some newer reels, made some PCBs from those parts and we are still seeing the problem. We could send some of these ICs back to you for your inspection. We don't believe the reset issue is due to the power circuitry, but we will tripple check this. We operate the PCB from two AAA alkaline batteries and VCC is generated from an external boost supply with a 3.3V output. We use the nRF52840's internal buck regulator to generate 1.3V for the core. There are many DECn pins on this processor and the product spec doesn't get into a lot of details on their function. Our circuit is basically a copy of Reference Circuit 5 shown in Figure 212 of the product spec. If we short out any of the DEC caps, the processor resets. Thank you!

RE: nRF52840 Lead Finish of the BGA Balls

John — Fri, 23 Aug 2019 14:17:43 GMT

Hi Kaja,

Thanks for getting back to me. We solder the nRF52840 right here in our facility, so I work with the folks that are trying to resolve this soldering issue. We've tried different stencils and oven profiles but we don't have the process good enough yet.

I was wondering, however, if the VSS ball (B7) becomes unsolderd from the PCB, will that cause the internal 1.3V or 1.1V regulators to fail? We are using the on-board buck regulator to generate the core voltage as shown in Figure 212 of the product spec.

Thank you!

RE: nRF52840 Lead Finish of the BGA Balls

Kaja — Fri, 23 Aug 2019 13:40:42 GMT

Hi John,

hm.. their must be something wrong during the production, soldering, the failure rate is most decently form the IC not being properly soldered to the PCB.
Have you given an feedback to the production house, that the IC is not soldered properly? Then they will most likely adjust the amount of solder paste, to get the IC to sink deeper and then get better soldered to the PCB.

- This is not my area of expertise, but I have sent a request to our package expert, so he might have some more suggestions.
I will also hear with one from our test group, she's back on Monday.

Best regards,
Kaja

RE: nRF52840 Lead Finish of the BGA Balls

John — Thu, 22 Aug 2019 13:12:54 GMT

Hello Kaja,

We think we are still having issues soldering the nRF52840 to our PCB. We have been temperature cycling our PCBs from +70C (2 hour soak) to -30C (2 hour soak), and after a few of these cycles we are seeing nRF52840 resets on about 1/2 of our PCBs. These resets always occur near or shortly after the temperature gets to -30C. We tried a destructive test to evaluate our soldedring. We actually pryed the nRF52840 off the PCB with a couple of small flat-blade screwdrivers. When we look at the PCB and IC afterwards, we note either the PCB pad stood with the IC, or the IC pad stood with the PCB. We think this means the solder joints were really good! The one exception is the VSS pad. When we look at the VSS pad, we notice the pad on the PCB is still there and the pad on the IC is still there. We think this means this joint isn't being soldered very well. If the VSS IC pad became disconnected from the PCB, would this casue the 1.3V core voltage or any other internal nRF52840 voltage to go away and cause a reset? The caps needed for the 1.3V buck circuit are still connected to VSS through a via, but does the circuit stop working if the VSS pad separate from the PCB? Thank you very much!

RE: nRF52840 Lead Finish of the BGA Balls

Kaja — Wed, 14 Aug 2019 07:03:07 GMT

Great, let me know!

Best regards,
Kaja

RE: nRF52840 Lead Finish of the BGA Balls

John — Tue, 13 Aug 2019 13:39:19 GMT

Thank you, Kaja. I will pass this information on to our Process Engineers and I'll let you know if they have further questions.

RE: nRF52840 Lead Finish of the BGA Balls

Kaja — Tue, 13 Aug 2019 12:49:23 GMT

And maybe this will help, some guidelines for aQFN:

Best regards,
Kaja

RE: nRF52840 Lead Finish of the BGA Balls

Kaja — Tue, 13 Aug 2019 12:32:39 GMT

Hi John,

we have this application note: QFN, but the the lead-frame is different for the nRF52840:

Maybe that will solve the issue?

Best regards,
Kaja