Impedance of Solder Bridge vs. Daisy Chain Dummy IC - Page 1
Forum section: electronics / projects

author: Delta2Wye
author_rank: Newbie
timestamp: October 14, 2012, 01:19:42 am
content: Hi all,I have been trying to determine the best way to determine the difference between an automated way to detect a solder bridge on daisy chained dummy ICs.The dummy IC I'm using is the following:http://www.topline.tv/drawings/pdf/TSSOP_Daisy_Chain/TSSOP30_Daisy_Chain.pdf. The rated impedance of the IC is <50m?.I was thinking that I might be able to measure the difference in impedance between a solder bridge and the daisy chained IC.What are your guys' thoughts?Thanks!

author: robrenz
author_rank: Super Contributor
timestamp: October 14, 2012, 01:38:11 am
content: Can you give a little more description of exactly what you are trying to do? I am currently confused

author: Delta2Wye
author_rank: Newbie
timestamp: October 14, 2012, 01:52:38 am
content: Sure thing.Essentially what I'm trying to create is a fine pitch SMD soldering practice system. The person would have a "father" board which connects to a daughter board on which a dummy IC is soldered. The "father" board would detect if the IC was soldered correctly, hence using daisy chained dummy ICs. My issue is that I need some way to differentiate between a correct solder joint, in which two neighboring pins are shorted through the dummy IC, and a solder bridge.

author: robrenz
author_rank: Super Contributor
timestamp: October 14, 2012, 02:45:00 am
content: A micro ohmeter with 1 micro ohm resolution will easily tell the difference between the internally shorted pins of the IC and the pins plus a solder bridge. You could put all the shorted pins on one side of the IC in series so that they would have a "good" resistance value. Lower resistance means a solder bridge, higher resistance or open circuit means poor or missing solder joints. having each side of the IC separate would tell you which side is bad. The resistance of a leg just touching the pad without solder will be much higher so it would show up.A constant current supply of several amps and a high resolution DMM will allow you to get 1 micro ohm resolution but you need to make your connection to the daughter board a 4 wire or Kelvin connection at these low resistances. Also for real stable reading your leads need to be shielded.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 14, 2012, 07:10:11 am
content: The problem of that approach is that a cold solderjoint of forgotten joint can compensate for a solderbidge. I do not know if you have a choise in the dummy ICs but if you could make or get them with the Pin 1,3,5,7 ect connected and 2,4,6,8 ect connected you can use a simple multimeterMeasure the even numbers, this resistance must be equal to the uneven. And then measure between even and uneven, this must be very high, if there is a bridge it will be almost zero Ohm.An othe solution is a thing Bob Pease designed, it is in his book troubelshooting analog design. A sort uOhm meter. He used it to find tracks with shorts. It sources a current and sounds a beep that has an increasing freqeuncy if you reach the short. In you case the sound should be equal for both rows ( you can measure the frequency using a uP and direct give resistance on a display instead of the speaker)If you have a good board you measure the resistans at the end of the chain and a the last pin of every IC. If total is wrong you measure half way. Then you know if you need to go right or left. Then half wY the bad site ect until you find the short.An other way could be optical. This involves mechanicl construction. The dummyboard must be single layer.you make a arm like a pen plotter or X,Y writer bu then only on dirction controlled, tne other can be done by hand or by moving the pcb, a stepper motor moves a U profile that moves over/under the board. A bright led on the component site and a Phototransistor fat against the other site.The Photorresistor sees the light between the pins. If there is a brigde you will have three non transparant spots in a rows. Or more steps of the motor a dark spot as the widht of one pin and the uP stops the senor or goes on but puts the pinnumber on the display.

author: robrenz
author_rank: Super Contributor
timestamp: October 14, 2012, 01:59:46 pm
content: I gladly stand corrected

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 15, 2012, 12:17:54 pm
content: If it wasn't as fine of a pitch, you could put a pad or trace(with no solder mask) in the middle of each pad, if it was a bridge, the middle pad/trace would also show low resistance.