tqfp packages and capacitive coupling

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brandonb
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tqfp packages and capacitive coupling

Postby brandonb » Sat Jan 26, 2013 11:08 am

not sure if this fits in the v5 section so please move or delete if it doesnt :?: really i dont know where on the web to ask this question :lol: designed a new board that i can use for communiction projects and such, i've built two of these boards, the second one i used 10k pulldown resistors on everypin and still have capacitive coupling issues, it appears to be traced back to chip pins, here are two examples
if i enable pwm 2 on E0 i get with a 10k pulldown 100mv of pwm signal on E1 and A5
when using i2c with 1k pullups and 10k pulldowns the clock cuts into the sda line at certain periods
in the picture the traces dont appear be a problem in the way they are layed out it appears to be at the chip legs
i have 6 .1uf caps spread around the board,
adding additional grounds, powers and caps of different values do not change condition
1k pulldown resistors pretty much eliminate phenomenem but thats to low of a resistance
was wondering your thoughts on this if it is normal or are there special measures that i should do to prevent this error on next pcb run
the last board with a 28 pin soic didnt have any sign of these issues and had longer traces that are just as close together?
https://dl.dropbox.com/u/10367218/100_0992.JPG
https://dl.dropbox.com/u/10367218/100_0993.JPG

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Re: tqfp packages and capacitive coupling

Postby brandonb » Sat Jan 26, 2013 11:24 am

heres a close up of pins with light behind board to show chip legs
https://dl.dropbox.com/u/10367218/100_0997.JPG

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Re: tqfp packages and capacitive coupling

Postby kersing » Sat Jan 26, 2013 11:53 am

brandonb wrote:it appears to be traced back to chip pins, here are two examples
...
when using i2c with 1k pullups and 10k pulldowns the clock cuts into the sda line at certain periods


The I2C pins are RC3 and RC4? Those are 'miles' apart on the chip, so it seems unlikely to be related to capacitive coupling at the chip pins for this pair.
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Re: tqfp packages and capacitive coupling

Postby brandonb » Sat Jan 26, 2013 11:56 am

The I2C pins are RC3 and RC4? Those are 'miles' apart on the chip, so it seems unlikely to be related to capacitive coupling at the chip pins for this pair.

i'll run the shop and get some high res scope captures

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Re: tqfp packages and capacitive coupling

Postby medelec35 » Sat Jan 26, 2013 12:43 pm

brandonb wrote:i have 6 .1uf caps spread around the board,

Just a thought, are any of these caps on signal lines like ICSP or I2C?
Martin

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Re: tqfp packages and capacitive coupling

Postby brandonb » Sat Jan 26, 2013 10:33 pm

alright, got my pico and did some testing.... this looks alot different when using the pico with a different power supply on the board! looks to be ok, there is a slight amount of noise from chip pins that are close to each at the chip, t his is still there even if the traces are ran in complete different directions... the i2c problem is no problem at all (my bad), i tested on the 40 pin dip version of the 1939 and i have the same consistant pulse, it hit me this is believed to the ACK responce :lol:
here are the waveforms during test, im running 20Mhz sample rate but using software lowpass filter set to 1Mhz so the waves appear to be cleaner than what they actually are
Attachments
pwm 2 E0.jpg
pwm 2 E0.jpg (63.25 KiB) Viewed 4444 times
i2c.jpg
i2c.jpg (64.92 KiB) Viewed 4444 times
uart tx rx.jpg
uart tx rx.jpg (57.06 KiB) Viewed 4444 times

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Re: tqfp packages and capacitive coupling

Postby dazz » Sat Jan 26, 2013 10:53 pm

Hi Brandon

thats a neat board are you making your own dev type system

Regards
Dazz
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Re: tqfp packages and capacitive coupling

Postby brandonb » Sat Jan 26, 2013 11:14 pm

thats a neat board are you making your own dev type system

yes, i create alot of stuff for my automotive job and give my designs out to the public to make auto techs lives easier and cheeper, auto tools are way over priced

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Re: tqfp packages and capacitive coupling

Postby dazz » Sat Jan 26, 2013 11:33 pm

Hi
i had a Rover 75 conniseur tourer a while back and looked at a diag box, almost choked when i saw the price, but the motor trade has always been like that, remember in the early 80's boss got a tuning machine in basically an oscilliscope with a co2 meter(you know the ones :lol: ) cost was ridiculous(price of 3 or 4 new cars) , it's seen as a closed market, but with more 3rd part and open sourse stuff hopefully the prices will get more realistic

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Re: tqfp packages and capacitive coupling

Postby brandonb » Sat Jan 26, 2013 11:49 pm

in the early 80's boss got a tuning machine in basically an oscilliscope with a co2 meter(you know the ones :lol:

yep, a over glamorized sun scope, a good example of what im talking about is the ASNU fuel injection machine, i made a controller for it thats too large to post here but it does alot more and is more flexable than the real thing, just a couple advantages is saving complete flow sessions for later recall, giving horspower and flowrate numbers for tflowed injectors, ect, i posted it on the autonerdz form and sell the boards at cost to techs that need one in their shops, have many other projects on that network as well

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Re: tqfp packages and capacitive coupling

Postby kersing » Sun Jan 27, 2013 12:42 am

What kind of solder have you used for the board? Could it be flux from this solder creates capacitive bridges between the pins?
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Re: tqfp packages and capacitive coupling

Postby brandonb » Sun Jan 27, 2013 1:15 am

What kind of solder have you used for the board? Could it be flux from this solder creates capacitive bridges between the pins?
thats the first thing i considered, and by the picture where the light is behind the board it looks like flux is present still, i used a new tooth brush drenched with iso alcohol and scrubbed fairly hard from center out, then i took a coffee filter and did the same with a tooth pick to clean between the pins, ("holy molars batman,good thing for dential hygene")
is there anything that you would advise to cleaning?
on the i2c waveform im not sure what the spikes are, are they ack? i can download that program to various chips/packages and i get that so it appears normal, just doesnt make any sense as it gets read in the middle so it wouldnt effect the read
i have a 30watt el'cheepo soldering iron so i use thin 62/36/2 .015" diameter silver bearing solder, i also use water soluable flux (kester fluxpen #2331-zx) which is known for the issues you describe, thats the reason why i went through aggressive cleaning of board after build
i would figure that a 10k resistor would take care of any possible shorts of any left behind flux residue
what do you think?

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Re: tqfp packages and capacitive coupling

Postby Mikat » Sun Jan 27, 2013 7:23 am

Hi.
What is the chip? I have playing with the 30F series, and with the higher clock those are quite power hungry.. Is there 100nF cap NEAR the chip, I can't see it, is it other side of the board (at least in 30F series those caps are must, without the vcc fluctuates really bad)?
Seems that even the board is 2 layer, there isn't very big ground layer (top is +5V?), so how is the return paths of current? Even those chips don't draw much current at average, they take quite huge amount spike of current when the i/o:s change state, because the i/o:s are really fast.
Ok, now I seen the bottom side of the board, the pictures was loading veeeryyy slowly that I was able to write reply before seen the bottom side of the board.. I seems fine, so hard to say whet causes the problem..
Does the problem occur when the i/o:s are "loaded", or is the problem present when nothing is connected to i/o:s?

Mika
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Re: tqfp packages and capacitive coupling

Postby brandonb » Sun Jan 27, 2013 10:18 am

Does the problem occur when the i/o:s are "loaded", or is the problem present when nothing is connected to i/o:s?
the are always loaded as every pin has pull down resistors, when load the pin down with an additional 1k resistor the noise does go away, i have tried jumping power,ground and various cap rating to different locations on the board and the only thing that remedies it is either putting a 1k resistor to ground on top of the 10 thats in place or putting a .1uf cap on the pin which i dont want to do, its 100mv with 10k pull down on pins that are side by side so its not gonna be a problem, i dont see this on the comm pins
one thing that has been bugging me is the i2c
there are asynchonous pulses that occur at intervals depending on what i send, funny thing is i have tested different chips and chip packages all with the same results, look to i2c waveform to see what im taking about
when i use software mode it doesnot occur, to go further i wrote code im microC for i2c and have the same results so i think its
irregularities in the i2c hardware module in chip, can anyone confirms this? if do this it will occur three times at the end of each packet, try sending below and see if SDA has the same effect on different micro controller in the 16f family
( start 5 5 5 stop)

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update: tqfp packages and capacitive coupling

Postby brandonb » Sun Jan 27, 2013 10:25 pm

i did some internet searching and kept running into this

@Brett – Imagine my surprise being shown the high-tech board cleaning system when touring an assembly house I hired. Kenmore.

I shouldn’t have been surprised because when I was just a young engineer and had a board come back from the prototype house non-functional, an older engineer took all my boards into the company lunchroom and ran them through the dishwasher. They worked perfectly after that.
by kersing » Sat Jan 26, 2013 4:42 pm
What kind of solder have you used for the board? Could it be flux from this solder creates capacitive bridges between the pins?
right on the money!
i had that int he back of my mind from the close up picture with the light behind it, sure did look like suspicious at the time,it works perfectly now, heres a close up veiw of the chip after cleaning with above method
https://dl.dropbox.com/u/10367218/close%20up%20after%20dishwater.jpg
thanks guys
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Re: tqfp packages and capacitive coupling

Postby Mikat » Mon Jan 28, 2013 6:59 am

So it was the flux which cause the capacitive coupling?

Mika

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Re: tqfp packages and capacitive coupling

Postby brandonb » Mon Jan 28, 2013 5:32 pm

So it was the flux which cause the capacitive coupling?
yes, hot running water, polmolive dish soap, a tooth brush, and a hair dryer fixed the problem
low tech solutions, alcohol did not remove the flux no matter how hard i scrubbed

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Re: tqfp packages and capacitive coupling

Postby Mikat » Thu Jan 31, 2013 6:52 am

brandonb wrote:
So it was the flux which cause the capacitive coupling?
yes, hot running water, polmolive dish soap, a tooth brush, and a hair dryer fixed the problem
low tech solutions, alcohol did not remove the flux no matter how hard i scrubbed


Good to know, I have been playin with the tqfp 64/80 pagages lately (thanks to open pcb service), but never bump to that problem...

Mika