Through Board Desoldering High Melting Point Solder

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The power wires for a 500 watt ebike motor go through the board.  It there any off the shelf insulator designed to protect the caps and other components only 13 mm away?  This is asking for a torch.

Bret Cahill



Re: Through Board Desoldering High Melting Point Solder
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Two suggestions:  

Don't start with a torch!
Post a link to a photo (or two) to sci.electronics.repair

It would help to explain the reference to "High Melting Point Solder". 500 watts
isn't a lot of power. Normal solder and 12 AWG wire is sufficient under most
circumstances to carry it. An old Weller WTCP with a blunt 800 degree tip can
handle that sort of job. It has to be used quickly, the board material will
degrade fast.  

HTH,

bob prohaska


Re: Through Board Desoldering High Melting Point Solder
On 2020-09-14 14:08, Bret Cahill wrote:
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Try Chip Quik.  It's a bismuth alloy that should make the high-melting  
solder lower-melting.

Cheers

Phil Hobbs

Re: Through Board Desoldering High Melting Point Solder

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At first hard to believe amalgamin' can do that sort of thing.

But then I positively zuck at materials.

I'll give it a try.

Thanks.


Bret




Re: Through Board Desoldering High Melting Point Solder
On Sunday, 20 September 2020 05:38:57 UTC+1, Bret Cahill  wrote:

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There's a reason they used HMP, so you may want to be thorough about removing the bismuth.

Re: Through Board Desoldering High Melting Point Solder
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re any off the shelf insulator designed to protect the caps and other compo
nents only 13 mm away? This is asking for a torch.  

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I tried some with an 85 watt Weller and it didn't do much.  I bought a used
 260-200 watt Weller and it worked after multiple applications.    

I was wondering if this hack is common:  Since I didn't want to cut off mor
e than a few millimeters of wire for a nice clean packed end I was stuck ge
tting about 40 kinked strands, bits of solder still clinging to them, 3 mm  
effective diameter,  back into the 2.5 mm hole in the circuit board.  I wou
ldn't be surprised if there was a dedicated tool -- they got a lot of cool  
time savers in electronics -- for crimping all the wires into a nice tight  
round bundle.  I was going to try an insulation stripper tool but maybe it  
was in the storage locker.  It's really frustrating to have a tool, especia
lly one that probably won't work, and not be able to find it.  It's like wa
sting time^2.  I was still feeling unlucky when I got some nylon thread, wr
apped it around the frazzled copper about 15 - 20 times for mechanical adva
ntage and pulled, tourniqueting them to hexagonal close packing.  _It actua
lly werked_!

Anyway gearless high torque hub motors shouldn't be used on hills.  When th
e axle reams out the torque arm it spins, reeling up the wires, yanking the
m out of the circuit board.  I can't read any markings on the wires but I m
easured a strand at 0.3 mm dia.   40 X 0.07 mm^2 = 2.8 mm^2 for one wire.
  (This is a 38 volt system.)   Ultimate yield strength of copper is 210 MP
a or 210 N/mm^2.  The force on the board when each power wire ripped out co
uld have been up to 600 Newtons ~ 130 lbs.  It's hard to tell what actually
 happened -- the bike mechanic was trying to resolder the wires in the inle
t side next to caps, not through board -- but that's one rugged circuit boa
rd!

I'll add some safety features:  

An easy disconnect between the plugs.  The wires are two short now anyway.  
  

An easy disconnect shear pin for the torque arm on the chain stay to preser
ve the axle.  

A lower pedaling gear, 13 t instead of 12t.


Bret Cahill














Mystery Solved
The original torque arm was 11 mm between flat sides.  I've been using a 10 mm open wrench to turn the axle which isn't all that mangled.

The axle was designed for a 10 mm torque arm.  

The slack from the loose torque arm allows the motor to build up momentum before it engages.  The impact destroys the torque arm.

Someone put the wrong torque arm on possibly a lot of GO Swissdrive ebikes.  

In case you are wondering I'm a metrologist and measure stuff.

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