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Author Topic: Best Practice: Trimming resistor leads before or after soldering to board?  (Read 8250 times)
AA4PB
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« Reply #15 on: April 16, 2013, 01:31:19 PM »

This is what I use:

http://www.amazon.com/Xuron-170-II-Micro-Shear-Flush-Cutter/dp/B000IBSFAI/ref=sr_1_1?s=industrial&ie=UTF8&qid=1366144199&sr=1-1&keywords=Micro-Shear+Flush+Cutter
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K1CJS
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« Reply #16 on: April 18, 2013, 04:01:22 AM »

Having the leads uncut is also supporting heat dissipation. While this is not so critical for resistors. Diodes or transistors will appreciate the advantage.

Trim after soldering--and the above is the best reason for doing so.
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KE3WD
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« Reply #17 on: April 18, 2013, 06:38:11 AM »

Worrying about heat dissipation can be rather misleading for the noobs. 

Consider this:  I've been working with SMT components since day one of their introduction, hand soldering prototypes, repairs, etc. -- and SMT resistors have NO LEADS on them. 

Modern, regulated, temperature-controlled soldering iron.  Which is available and affordable for all today. 

Those warnings concerning the damage of components with too much heat weren't ever meant for resistors and caps anyway.  That only came about when the Transistor and other solid state devices became prominent - but many were still using the old fashioned nonregulated large soldering irons or the transformer guns.  Even so, the 100 -150W gun would not likely destroy a resistor unless the user was absolutely incompetent.  Well, they're out there...


73
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G8HQP
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« Reply #18 on: April 18, 2013, 07:15:21 AM »

Quote from: KB3TXH
Many of the solder joints on commercial pc boards are "re-flowed".
That is because the very many components are tack soldered in place by hand,
the excess lead length is cut off, then the board is run through a solder machine
in which the entire bottom (lead side), makes contact with molten solder.
So the joints that were tacked in place, now have the solder re-flowed.

I find that very surprising. I had always assumed that components were placed by machine with their leads already cropped to the correct length, then the board went through a solder bath.

Tack soldering can be a useful technique for making quick temporary joints during fault-finding, or for the occasional awkward component which refuses to stay in place during construction, but it is best avoided as a routine construction method.
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K2GWK
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« Reply #19 on: April 18, 2013, 12:45:59 PM »

I have gotten into the habit of fitting the part, trimming and then soldering. I do this especially on something that will go outdoors. I want the whole joint to be covered with solder to protect the connection. When I worked as an engineer for an aerospace contractor our designs were required to work over temperature, humidity, altitude and in a vacuum so if we used through mounted components the production people would fit the part and then run the board over a solder flow machine so everything was covered with solder.
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Guy
Lawn Guyland, New York

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KG4RUL
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« Reply #20 on: April 18, 2013, 02:13:51 PM »

In production facilities, back when PCBs were populated with leaded components, it was not uncommon to insert all leaded components in the PCB, wave solder and then trim them using a fly cutter.  Then, the PCBs were sometimes sent through a re-flow machine, which made all the solder joints neat looking.  These day, SMD devices predominate to the extent that leaded components often are inserted last and hand soldered.
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K1CJS
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« Reply #21 on: April 19, 2013, 04:24:38 AM »

Worrying about heat dissipation can be rather misleading for the noobs.... 

Please look at the entire quoted piece.  Right there it states not so much for resistors and capacitors but more for diodes and transistors.  And especially for the 'noobs' since they probably don't have the soldering experience that long time technicians and builders have--to just heat the joint until the solder flows on both sides of the connection--and no more.
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K1CJS
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« Reply #22 on: April 19, 2013, 04:32:26 AM »

...I had always assumed that components were placed by machine with their leads already cropped to the correct length, then the board went through a solder bath....
 

They are.  As you said machine constructed boards.

Quote
Tack soldering can be a useful technique for making quick temporary joints during fault-finding, or for the occasional awkward component which refuses to stay in place during construction, but it is best avoided as a routine construction method.

Hand construction where the board is moved, tipped and otherwise disturbed is a different story altogether--and is still 'routine' construction.  So is repair to a printed circuit or a breadboard.

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KE3WD
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« Reply #23 on: April 19, 2013, 07:26:24 AM »

When's the last time you actually encountered a diode or transistor that was damaged by heat from soldering? 

73
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K1CJS
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« Reply #24 on: April 20, 2013, 10:48:56 AM »

When I was doing the soldering?  A long time ago--AAMOF, when I was a newbie.  I think you'll agree that newbies are the ones that can make mistakes--such as putting too much heat to a solder joint!  Yes, It can happen.  No, it doesn't happen very much at all to techs who have been in the business (or hobby) a long time.  But if it does happen, the chances are that the person doing the soldering hasn't had that much experience, doesn't it?  Another reason failure could occur is that the part could be failure prone due to manufacturing stress or missed tolerances, something that happens especially with parts cheaply made, but I think that has been gone into enough on this board. 73!
« Last Edit: April 20, 2013, 10:51:21 AM by K1CJS » Logged
AC5UP
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« Reply #25 on: April 20, 2013, 11:09:46 AM »

When's the last time you actually encountered a diode or transistor that was damaged by heat from soldering? 

Rare, as I've taught myself to allow the iron enough time to come up to full temperature so I can go in hot and come out fast. Heat exposure involves both temperature and time and IMHO limiting time is the better way to limit the risk of smoking a component.

BTW:  A worn out diagonal cutter will likely require more pressure to cut a soldered lead with potentially more stress on the solder joint. Point being that a good soldering job followed by a low-stress trim is probably the combo of choice.
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W2EM
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« Reply #26 on: April 24, 2013, 10:43:35 AM »

Many of the solder joints on commercial pc boards are "re-flowed".
That is because the very many components are tack soldered in place by hand, the excess lead length is cut off, then the board is run through a solder machine in which the entire bottom (lead side), makes contact with molten solder.

Years ago I had a friend who had an electronics firm. He mentioned something about "Waving his boards".  I asked him what he meant by that and he showed me how he soldered up his run of boards. The solder machine consisted of a very large tank of molten solder and a track above it where the circuit boards were slid across the solder tank. The boards were about an inch above the solder. He turned on a switch  to the air pump and a wave of solder rose up far enough to touch the bottom of the boards.  He just slid them across the wave and the whole board was perfectly soldered in a couple of seconds.  Sounds like something you had at your job.
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KA5IWO
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« Reply #27 on: April 27, 2013, 10:55:18 AM »

You have got to be kidding me! Grin If the resistor in a circuit runs normally cool or hand touchable warm when the circuit is operating properly, then place it horizontal and flush to the board, if thats the way the original was. Some require you stand it up vertically. Others that produce alot of heat should be raised above the board to help dissipate the heat and prevent burning the board. Again, depending on the board layout it will either solder in horizontal or vertically, either flush or raised.

Leave the leads long and bend them a little to help secure it in place, then solder it.
 
Then ALWAYS TRIM THE LEADS after you have soldered it in the circuit. Leaving them long or uncut on new parts can cause it to short to other parts on the board or the radios chassis.

For the comment about diodes and transistors:
"When's the last time you actually encountered a diode or transistor that was damaged by heat from soldering?"

I see it often when inexperienced techs overheat transistors/diodes with too much heat from an excessive high wattage iron for the job at hand. Saw a brand new Icom hf rig destroyed from a guy using a butane pencil torch trying to freeband it for 11m. What a waste.
He even tried to claim "warranty" with Icom. He now owns a nice looking paperweight.

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