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Topic: A new PICqueño is born (PCB built) (Read 3224 times) previous topic - next topic

A new PICqueño is born (PCB built)

I don't know what the hell is happening, but MPLAB keeps failing at programing the bootloader. The FTDI works and enumerates, and the PIC has no solder bridges, checked 4 times already. I'm getting mad...

It lets me read the PIC but it won't get programmed, it says

Programming...
The following memory regions failed to program correctly:
Boot flash memory
Programming failed

Maybe it's fried but I don't know why....

EDIT: Oh well, a power problem. Seems that my PICKIT 3 was only putting out 2.8V instead of 3.3. I set it to 3.6V so it got up to 3.13 actual volts, and it programmed perfectly. Hoooray!!! Picture is coming.

EDIT2: here are the pictures. I soldered the board using the stencil I had made. I had to spread the paste twice, as the first time I put too little paste and not all pads were covered. The "spread quality" looked acceptable so I decided to continue with the mission (was much better on the first spread, I think that the paste just got hotter and more fluid between spreads). The plan B if the stencil failed was to just spread the paste manually, as I always do. This is how the paste spreading went:

http://i54.tinypic.com/2daln2a.jpg

I have two different soldering pastes, one that has superior quality and leaves no residue, and one that is pure crap and leaves a white, hard to clean residue. At least it gives quality solder joints. Guess which one I used for soldering this... (the high quality one is just too expensive to waste some by spreading it over a stencil, I use it only for manual application)

http://i56.tinypic.com/pyb07.jpg

It looks a bit messy because of the white flux residue from the paste. But the soldering quality is actually very very good. I will try to clean it better with a toothbrush and IPA, and I'll  post a pic after that if it gets much better. I'll try to get a nice supply of superior quality paste for the next times... I don't like white residues D:

Some shorts were formed in the PIC, but that was expected. They were easily removed with some flux and the iron. The rest was perfectly soldered.

Re: A new PICqueño is born (PCB built)

Reply #1
good work,

i have noticed that the pickit2 will give bunk voltage readings if your pc supply does not supply a perfect 5.0 volts.

i have a clone, and changed some resistors to 1% from 10% and used the calibration function in the pickit software.

Re: A new PICqueño is born (PCB built)

Reply #2
Some of the joints look dull (like the LDO tabs) are you sure the paste reacted completely? Looks good though. How many bridges were there on the QFP?

Re: A new PICqueño is born (PCB built)

Reply #3
[quote author="brian"]Some of the joints look dull (like the LDO tabs) are you sure the paste reacted completely? Looks good though. How many bridges were there on the QFP?[/quote]

Yes, the paste is completely reacted. It looks shiny just after soldering, then, when you clean it with IPA, the flux of the paste dissolves and when the IPA dries, it leaves a white, solid and messy residue all over the pads (look at the crystal's pads, they also look really bad but it's perfectly soldered). I've never been completely able to eliminate this residue, but most of it gets removed after brushing with IPA and a soft toothbrush. The paste I've used is low quality chinese paste, $4 for 50 grams. That's why it leaves such a residue, and it also smells like natural rosin while soldering. I have some CHIPQUIK branded paste too, that one costs $11 for 15 grams, but leaves no residue and it's completely odorless, and it is composed of smaller solder spheres. But as I used a stencil, I didn't want to waste the quality one.

About the bridges, one side of the chip got none, other side got just a single bridge, and the other sides got two each, one of the bridges being triple. But they were easily removed. Just flux a bit, touch with the iron and the bridge is gone.

Re: A new PICqueño is born (PCB built)

Reply #4
Okay cool. I pretty much always have bridging as well with stencils. The fellow who cut my last stencil suggested using a thin line rather than separate pads. I will just be making the paste smaller in the QFP areas from now on I think though, as I don't like having to do any rework. I might even try BGAs as they have the right amount of solder on them basically, though of course you can't rework them.

Re: A new PICqueño is born (PCB built)

Reply #5
[quote author="brian"]Okay cool. I pretty much always have bridging as well with stencils. The fellow who cut my last stencil suggested using a thin line rather than separate pads. I will just be making the paste smaller in the QFP areas from now on I think though, as I don't like having to do any rework. I might even try BGAs as they have the right amount of solder on them basically, though of course you can't rework them.[/quote]

There are at least three factors that influence bridge formation:
Paste's flux quality, amount of paste spread and temperature differences over a row of pins. I've seen that if I manage to get a row of pins uniformly heated, very few bridges, if any, are formed. But if a side of the row gets heated more than the other, the paste "flows" in the direction of the temperature gradient, eventually accumulating in some pins, which get a bridge. But those factors are very hard to control.

I think that you should use thinner stencil. You need thinner stencils for fine pitch, and thicker for passives and things with big pads. Also the shape of the stencil has lots to do. Some tricky IC's have stencil specifications on their datasheets and I've seen really strange apperture shapes. Definitely stencil design is serious business.

Re: A new PICqueño is born (PCB built)

Reply #6
I'm sorry I just got to this post now. Have you had a chance to do some testing? Is it working ok?
Got a question? Please ask in the forum for the fastest answers.

 

Re: A new PICqueño is born (PCB built)

Reply #7
[quote author="ian"]I'm sorry I just got to this post now. Have you had a chance to do some testing? Is it working ok?[/quote]

Yep, i've tested it. Not an intensive test, I just loaded an sketch with one of the less efficient algorithms available to calculate PI (an implementation of the Leibniz series) and tested how long it took to calculate PI to the 8th decimal place. Roughly 5.5 seconds, compared to de 86 seconds it took for a regular arduino to do it.

I haven't had the time to buy the double row headers to finish the picqueño assembly. I'm gonna do some IO and communication testings when the headers are mounted.

I've also tested the voltage regulator and I've powered the board from both the USB and the jack (@12V) with no issues.

The only issue I've had is that some time ago I bough 50 chinese 8Mhz crystals that should be special (in terms of load capacitance) or just crappy or something, because I've had problems of non-starting oscillators with them. That happened to the PICqueño, the day after I assembled it the oscillator (featuring one of those crystals) said goodbye and it wasn't starting. I just swapped the crystal with a higher quality 20Mhz one with the proper capacitors and reconfigured the PLL prescaler of the bootloader to divide by 5 instead of 2 and it's working perfecly since then.