PIC16F18856 Microcontroller dry joints.....part too small

[Flyback]

Hello,

Our PCB assembler have told us that they are getting difficulties with soldering the PIC16F18856 [28 pin UQFN (4x4) ] on our board. They have actually asked us to make the copper pads smaller so that a solder resist bridge can be layed down between the pads. However, we are already using the microchip recommended footprint. (page 657 of datasheet)

Anyway, we are also getting problems with the lamp product that the micro is in. We configure the micro at production time so it knows what maximum dimming level to be at etc. Problem is, sometimes, (say in 5% of cases), the lamp doesn’t actually come back on after its startup routine. (the startup routine involves, whenever the lamp is switched on, the lamp comes on for 1 second, then dims down for 1 second, then goes off, then comes back on again, -just so’s we can see that the micro is compus mentus)….however, in say 10% of cases, the lamp doesn’t come back on after being configured.

We suspect this is due to dry joints in the micro pads. –eg the micro pin that connects to the light sensor may be dry and it may be seeing noise and think its light when its actually dark. Or the pin that connects to the temp sensor may have similar issues and think its too hot to turn the lamp back on etc etc.

Do you have any tips on ensuring no dry joints with this kind of footprint? Our footprint is exactly as on page 657 of the datasheet, but the assemblers want the pads trimmed back so solder resist can go between the pads……we currently have a 0.1mm solder resist bridge between each pad on the gerbers , but its so thin that the PCB manufacturer can’t produce it, and so we have no solder resist between the pads…the PCB assembler is complaining about this and reporting many failed boards to us.

PIC16F18856 datasheet
https://www.microchip.com/wwwproducts/en/PIC16F18856
(page 657 shows footprint of 28 pin UQFN)

We are also wondering about buying a cheap reflow oven so we can remove and resolder the micro ourselves so as to try and repair potential dry joints. Do you know of a cheap reflow oven.?
(The boards are 10cm by 8cm and 4 layer, components and leds only on top.)

 

[dknguyen]

You say you are using the footprint, but do you mean the file? Or just the drawing provided on the datasheet? Because you could try increasing the paste stencil size.

Your post is confusing though. Half of it is talking about dry joints and the other half seems to be talking about solder bridges (which would seem to be a result of excess solder rather than a dry joint).

 

[Rich D.]

I could never really grasp what is going on there with the solder quality, but my advice would be to follow the advice of the PCB assembler who is doing the soldering.
There are a lot of unknown variables that affect the quality of the solder joints, including the exact makeup of the solder paste, the process times and temperatures, board material and solder resist composition....and so on.

While Microchip may have a lot of experience with the general process on their machinery and suppliers, your PCB board manufacturer hopefully has the experience with their specific PCB maker and their process.
Hopefully they have gained their experience with another customer. This is assuming of course that your manufacturer isn't asking you to be their experiment. It sounds like they are taking the responsibility of fixing their own quality problems,
so I would do what they ask.

As for me, I don't trust a solder joint I can't see. I always opt for leaded flat packs over those "mystery" Q packages.

 

[Flyback]

Thanks, i was wondering if Microchip meant it for use only with leaded solder?
I realise that leaded solder gives far batter quality joints with such small pitch chips.
We might be able to ship these boards to our Customers in non-ROHS countires.

 

[dknguyen]

Thanks, i was wondering if Microchip meant it for use only with leaded solder?
I realise that leaded solder gives far batter quality joints with such small pitch chips.
We might be able to ship these boards to our Customers in non-ROHS countires.

That seems unlikely since they are all ROHS parts. OTOH, the footprint was probably never updated since transition to Pb-free.

 

[Flyback]

Thanks, do you agree that leaded solder could be the answer to getting these boards to be useful? Leaded solder will be better for this 28 pin UQFN (0.4mm pitch)
We have a number of unpopulated baords, and otherwise we will have to scrap them.....i mean, we have customers in non-rohs countires.

 

[dknguyen]

Thanks, do you agree that leaded solder could be the answer to getting these boards to be useful? Leaded solder will be better for this 28 pin UQFN (0.4mm pitch)
We have a number of unpopulated baords, and otherwise we will have to scrap them.....i mean, we have customers in non-rohs countires.

Beats me. I didn't really understand your post, as I previously indicated since I don't see what a smaller pad or increased resist would have anything to do with a dry joint. Those sound like corrections for solder bridges rather than dry joints to me.

 

[Rich D.]

I agree - I don't think smaller pads will solve the problem. I think narrower pads and therefore wider solder mask layer would help with shorts between pins, but shouldn't do much good for weak ("dry"?) solder joints.

I think a thicker solder paste stencil or longer length openings (perpendicular to the direction of the pin line) would allow for more solder on each pin and could help if you are getting a lot of poor high-resistance joints. That is assuming your boards are being soaked warm enough for long enough to get a good solder joint in the first place. Nothing could fix a cold board or a board with some cold spots.

 

[Flyback]

Your post is confusing though. Half of it is talking about dry joints and the other half seems to be talking about solder bridges (which would seem to be a result of excess solder rather than a dry joint).

Thanks, a few months ago, we had a batch of these boards populated, and we lost about 5 out of 300 boards to solder bridges between pic16f18856 pins.......that was ok, because shorted boards show up straight away and dont give intermittent faults...............the pcb assembler has reduced the amount of solder since then, to mitigate the shorted ones, but they have gone too far, and we now have a load of dreadful boards which give intermittent faults due to the dry jointed pic16f18856 pins.........we would prefer shorted pins to these dreadful drys which are intermittent.............we are taking products from this latest batch from stores, these have passed production, but then we take them and "shake" them, and they fail...due to this batch having dry jointed micro.