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ken newbold
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Post by ken newbold » Sun Jan 08, 2006 9:06 am

A dry joint is when a component that is soldered to a printed circuit board looses electrical connection.

This is most common on TVs and other electrical appliances that generate heat as they're running.

The heater blower modules on your BX fail fequently with this fault, the heat sink on the PCB is there to cool the components but a combination of that and the cold damp air being drawn in causes bad electrical connections. Resoldering the joints will usually fix this.
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Vanny
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Post by Vanny » Sun Jan 08, 2006 10:22 am

Should fix it permanantly if its a dry joint as the normal cause is old solder. Old stuff used to have a lot of lime in it and the lime breaks down and hence the joint comes apart, modern solder uses less or no lime so shouldn't happen!

Anyone who's ever had a BBC Master or BBCB+ will know all about dry solder joints!

tom
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Post by tom » Sun Jan 08, 2006 1:34 pm

Lime? A new one on me but then I have only been soldering since I was seven years old and you learn bugger all in 40 years. Solder is made of tin and lead.
for the information of all, the heater modules rarely, if ever, fail due to dry joints. The most common problem is corrosion between the rivet and the pcb. See Ramblings October.

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Vanny
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Post by Vanny » Sun Jan 08, 2006 2:10 pm

Yep Lime! I always wondered about that but thats what i was told, and im no chemist so didnt bother to argue, but if you get a BBC have a go at the welds with a soldering iron and they fall apart before they melt, weird phenomina!


Now i've not ever bothered to pull onr of these things apart yet, but where is the rivet? Gonna have to go pull something apart arent i :twisted:

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DavidRutherford
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Post by DavidRutherford » Sun Jan 08, 2006 2:25 pm

Are you sure this person wasn't talking about mortar and housebricks? :D

If there's lime anywhere, there may be a little in the flux, and this is possibly one of the reasons for dry joints. Although, the main reason that I'm aware of is dirt/corrosion on the copper tracks before soldering, which means the solder doesn't "take" properly to the copper. You get a high-resistance connection, and repeated heating and cooling of the joint eventually breaks it.
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tom
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Post by tom » Sun Jan 08, 2006 2:55 pm

No, it is almost always due to the component leads not being clean. When a dry joint is examined, the loss of contact is almost always around the lead. Other dry joints, on wave solderd jobs are due to poor quality (drossy) solder, too high a solderin temperature, insufficient preheat of the board, or a failure to clean the wave soldering machine properly, The flux, a wetting agent, does not contain lime or lemon and does not promote dry joints. This is not idle speculation, take this as the counsel of somebody who knows of what they speak!