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 Post subject: Finding an open solder joint that looks good
PostPosted: Jul Fri 07, 2017 2:19 pm 
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Joined: Mar Tue 18, 2008 12:15 pm
Posts: 161
My new old Fluke 77 had a problem I posted about a few days ago, and the root problem was a broken solder joint on a SMD microchip.

I looked very carefully without seeing anything wrong with the solder joints but one WAS OPEN.

I found it by:
1. Assuming it was a cracked pcb trace and gently flexing the board with the meter powered up, and seeing that it worked intermittently.
2. Probing the board with a chopstick until it began to work,
3. Localized it to the chip and then under a magnifying glass used a dental pick to find the loose joint,
4. Applied a tiny bit of liquid flux and reflowed the broken solder joint.

Hope this helps anyone new to SMD (for that matter, to any troubleshooting). I'm retired now but I spent many years working on SMD antilock brake systems, airbag control modules, and infotainment systems in the automotive industry and this one was a tough one.


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 Post subject: Re: Finding an open solder joint that looks good
PostPosted: Jul Fri 07, 2017 8:57 pm 
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Joined: Apr Wed 24, 2013 10:08 pm
Posts: 925
Location: Highland, MI
I'm lucky that I have stereobinoculars available to me at work. It magnifies from 10X to 40X. I found many a bad solder joint this way.


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 Post subject: Re: Finding an open solder joint that looks good
PostPosted: Jul Sat 08, 2017 9:38 pm 
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Joined: Feb Thu 24, 2011 3:13 pm
Posts: 732
Location: Ironwood, MI
Well done, Premiumplus! So often the breaches are mechanical and not visible with even
magnifying lenses. Three such cases I experienced recently were an internally-cracked resistor with no
evidence on its outer surface, an electrolytic capacitor with its lead broken just inside the body of the
capacitor and the darnedest one was a pinched solid conductor inside the insulation of hookup wire that had broken. Thank goodness for those chopsticks and back-to-basics troubleshooting. You're not too dissimilar from a good doctor isolating a problem by saying to your PC board, "Does it hurt there?" "No." "How about there?" "No." "How about here?" "YEOW!" Problem found.


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 Post subject: Re: Finding an open solder joint that looks good
PostPosted: Jan Wed 10, 2018 9:47 pm 
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Joined: May Fri 01, 2009 3:53 am
Posts: 1339
Location: Glendale, California
I had a Fluke 77 that had a broken solder joint on one of the SMD legs on the backside of the PCB. I found it under the microscope at work. From what I can see, it seems to be a common problem among the Fluke 75/77 series.

_________________
Senior Analog Meter Technician @ Instrument Meter Specialties. Simpson 260 specialist.

John, KK6WHY


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 Post subject: Re: Finding an open solder joint that looks good
PostPosted: Jan Thu 11, 2018 2:20 am 
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Joined: Sep Tue 30, 2014 6:08 am
Posts: 2198
Location: The Old Dominion VA 23518
cwr56 wrote:
I had a Fluke 77 that had a broken solder joint on one of the SMD legs on the backside of the PCB. I found it under the microscope at work. From what I can see, it seems to be a common problem among the Fluke 75/77 series.



No need for a microscope here. I've fixed many 77A/N's by simply dragging the corner of a sheet of paper across the legs of both SMD chips - the loose leg will "wiggle" and give itself a way. Loose legs, dirty elastomeric conductive rubber strips, and the open 1K fusible/current source resistor are 99% of the problems with the early 70-series (73, 75, 77) meters. I've also seen one bad (heavily worn) function switch, bad /dirty gaps in the spark arrestors ( a business card soaked in alcohol cleans them nicely), and a few other oddities.

The 23/25 meters also are served with the same tips.

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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