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Author Topic: Solder Paste  (Read 4514 times)
N0PKG
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Posts: 6




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« on: September 25, 2017, 09:33:14 AM »

I am looking to learn SMD soldering by doing a complete capacitor replacement on my new to me Kenwood TS-50. This radio has 46 caps total that I will be replacing.

I have a hot air soldering station and various other tools & supplies ordered, but holding out on soldering paste. I am not sure how much I will need for this. I plan to order from KD5SSJ, his syringes are 0.5 cc each. Can anyone give me a rough idea here? I would imagine 1 or 2 syringes should be plenty, right?

(Hopefully this is the right sub forum, if not, please let me know)
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VK2TIL
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« Reply #1 on: September 25, 2017, 01:56:08 PM »

One will do the job, I'm sure.

I buy these syringes from Cash; his price is low, even with shipping to Australia, and the quality is high.

A flux pen is useful; I use Kester #952-D6.
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VA3VF
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« Reply #2 on: September 25, 2017, 04:25:50 PM »

I am looking to learn SMD soldering by doing a complete capacitor replacement on my new to me Kenwood TS-50. This radio has 46 caps total that I will be replacing.

I hope you don't mind me asking. What happened to the radio that requires all caps to be replaced?
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N7EKU
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« Reply #3 on: September 25, 2017, 07:02:58 PM »

Sorry to say,

But that sounds like not the best idea.  And certainly not the best way to learn SMD soldering.  You might start with some small kits that use SMD parts like the KD1JV digital dial kit, or some of the QRP rig SMD training kits.

Why would you wholesale replace all the capacitors?  You may risk damaging the radio by doing such extensive replacement.  If you are worried about them, the best thing to do would be to make, purchase or borrow an ESR meter and check each one first.

Also, the boards may contain parts with plastic that are not intended to be exposed to extended high temperature heating from a hot air gun.

I have done SMD capacitor replacement when needed (Kenwood dual-band HT that had some leaking ones) and found a fine tip soldering iron and fine solder worked perfectly.

73,


Mark.
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Mark -- N7EKU/VE3
N0PKG
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« Reply #4 on: September 26, 2017, 06:17:08 AM »

Thanks for the input everyone.

The reason I am looking to replace all the caps is that the radio currently has an issue with some front buttons not working, and I can see that the large capacitor on that board has leaked and there is some corrosion around it. This seems to be quite common. I have read that during the time period this radio was produced, electrolytic capacitors tended to have issues with their chemical composition that made them predisposed to leak. The reoccurring theme is "It's not a question of if, but when they will leak".
I follow the Kenwood TS-50 group on Yahoo and the consensus among people who are quite experienced with these transceivers, and also a recommendation from the trusted repair shops who frequent that group, that the first thing they do when they buy a used one is replace all capacitors.
I figure it's better to just replace them all now, than to just replace ones that already leaked, only to have some other mysterious issue crop up 6 months later, and then having to do a bunch of trace and via repairs and who knows what else that gets damaged when the cap sprays its corrosive stuff over the innards of the radio.

Actually, I was thinking about getting some SMD kits to practice on first. Since that was recommended here, too, I think I will certainly do that.

Regarding heat exposure to the plastic components, yes, that could be an issue. However, it's likely that the radio was initially produced using solder paste and a hot air oven. I also purchased some Kapton tape to cover any components that I do not want to work on while I replace each cap. I think with that and just taking care and using common sense when doing this, I should be able to keep stuff from melting. From what I have read, using the hot air method to remove components is recommended over soldering iron since it reduces the risk of damaging the pads dramatically. The other options would be using soldering tweezers for removal.
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KI7LGC
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« Reply #5 on: September 26, 2017, 07:30:59 AM »

Most SMD capacitors can be replaced with a soldering iron. Use a small tip on the iron. In the hand that doesn't hold the soldering iron, use a pair of tweezers to gently lift the part as you heat one side of it. Just be gentle, you wont ruin the pads.

When re-installing the parts, tin the pads with a bit of solder. Use the tweezers to hold the part to the pad, and heat one pad at a time.

This will help with heating other parts with a heat gun. Heat guns can de-solder nearby parts, and even blow them off the board. They can also melt other parts and connectors if you're not careful. Most of the parts likely were put into a heated oven to flow, but an oven is easier to control the heat, than a heat gun.

I worked in a circuit board assembly shop for a while. I would almost always use a soldering iron to a hot air reflow station if possible. The hot air station was used almost exclusively for parts that couldn't be handled by an iron, like BGA components.
« Last Edit: September 26, 2017, 07:34:01 AM by KI7LGC » Logged
G3RZP
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« Reply #6 on: September 26, 2017, 09:46:46 AM »

Is the TS50 built with lead free or leaded solder? Lead free tends to have a higher melting point and they apparently don't always mix well.
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N0PKG
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« Reply #7 on: September 26, 2017, 10:07:30 AM »

I did order a kit now (the SMD Dummy Load from Kangas, ordered from KC9ON), this should give me some opportunity to try different methods. I will certainly give both the hot air and hand soldering a shot.

It looks to me like leaded solder, haven't looked under magnification, but it has smooth and shiny solder joints to the naked eye.
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N2SR
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« Reply #8 on: September 26, 2017, 12:14:23 PM »

Most SMD capacitors can be replaced with a soldering iron. Use a small tip on the iron. In the hand that doesn't hold the soldering iron, use a pair of tweezers to gently lift the part as you heat one side of it. Just be gentle, you wont ruin the pads.

When re-installing the parts, tin the pads with a bit of solder. Use the tweezers to hold the part to the pad, and heat one pad at a time.

This will help with heating other parts with a heat gun. Heat guns can de-solder nearby parts, and even blow them off the board. They can also melt other parts and connectors if you're not careful. Most of the parts likely were put into a heated oven to flow, but an oven is easier to control the heat, than a heat gun.

I worked in a circuit board assembly shop for a while. I would almost always use a soldering iron to a hot air reflow station if possible. The hot air station was used almost exclusively for parts that couldn't be handled by an iron, like BGA components.

Agreed.  Move the iron from one side of the SMT component to the other.  After a few times, the solder on both sides will melt and you can lift the part out.  Solder wick the solder left on the solder pads away.  Add a bit of solder to one pad.  Then bring the new part near the pad, hit the pad with the solder on it (to melt it), and allow the part to sit on the pad in the proper spot.  After the solder cools, hit the other side with the iron and some solder. 

Personally I like having two irons, since you can heat both sides at the same time and lift the part off. 

Hopefully you (the OP) has something to magnify the PWB, since SMT parts are very small.   It took me years to do it, but from scrounging, various hamfests, and Dayton, I was able to piece together a very nice binocular microscope with a variable light ring. 

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If no one is doing it that way, there is a probably a very good reason.
N0PKG
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« Reply #9 on: September 28, 2017, 06:29:46 AM »

I think the main advantage of the heatgun, or better, the main use case, is where the solder is corroded from the leaking electrolyte, and won't melt with the iron. In that case, having the hot air helps still melt the solder and then allows for easy removal of the part.

The technique with two irons is interesting. I saw that there is a tool called soldering tweezers. It's basically two irons joined together to make tweezers, with slightly inward-bent tips. This makes that technique a lot easier (less chopstick skills required).


Aisart (https://commons.wikimedia.org/wiki/File:Soldering_a_0805.jpg), „Soldering a 0805“, https://creativecommons.org/licenses/by-sa/3.0/legalcode
« Last Edit: September 28, 2017, 06:36:05 AM by N0PKG » Logged
KH2BR
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« Reply #10 on: September 28, 2017, 08:50:12 AM »

I have been using two soldering irons to remove SMD caps and resistors. Works great.
I googled soldering tweezers and found that if you don't order from eBay, your gonna get ripped off.
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KC4ZGP
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« Reply #11 on: September 28, 2017, 10:18:52 AM »


Here's how I solder SMT.

1. Tin the trace.

2. Lay your cap on the trace.

3. Hold the cap in place with a toothpick.

4. Touch the soldering iron to the trace near the cap. Heat will travel in the trace and viola, a soldered cap.

Kraus
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W3TTT
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« Reply #12 on: September 28, 2017, 11:41:45 AM »

...The reoccurring theme is "It's not a question of if, but when they will leak"....
... first thing they do when they buy a used one is replace all capacitors....
...I figure it's better to just replace them all now,...

Are you sure that you need ALL the caps replaced?  Huh  I am thinking that just the electrolytics and then, just the power supply caps need replacing.
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N0PKG
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« Reply #13 on: September 28, 2017, 11:46:02 AM »



Are you sure that you need ALL the caps replaced?  Huh  I am thinking that just the electrolytics and then, just the power supply caps need replacing.
[/quote]

Well, yes, I should have been more specific, just the electrolytics. But I am not sure about the power supply part. The one I know has leaked is on the display board. The main goal here is not replacing bad caps, but preventing future damage to traces etc. from leakage/corrosion.
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KC4ZGP
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« Reply #14 on: September 28, 2017, 12:15:28 PM »


SMT electrolytic?

They make them?

Kraus
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