Here's the deal: I'm changing out caps, and I am touching nearly all the factory solder joints on a 30 year old amp — heating the solder to molten state and being sure I am on the pin at the same time while adding a tiny spot of fresh solder. I am physically removing flux both before and after the touching.

After this sort of work, the board never looks as nice as the original. I mean — no matter how much I practice of how carful I am the board always looks like it has been worked on with scratches and tiny bare spots on the traces and so on. In one spot I have to repair the traces. SO — I bought a can of spray-on solder mask, or 'board protector' (the rest of the language is in Japanese).

The QUESTION: Should I spray the whole (underside -duh-) of the board — solder joints and all. Or should I mask off the joints with say little blobs of blu-tac before I spray. What do YOU do?

Does anyone brush on brush on some shellac ... or ...?

I've not used that particular product, however I do find it helpful to touch the circuit board with tools that are less apt to scratch the surface, such as something with a dull plastic tip. By contrast, bending the new leads with the sharp tip of a small metal screwdriver will definitely increase your chances of scratching the hell out of the board.

The QUESTION: Should I spray the whole (underside -duh-) of the board — solder joints and all. Or should I mask off the joints with say little blobs of blu-tac before I spray. What do YOU do?

Does anyone brush on brush on some shellac ... or ...?

Definition
Solder Mask: A dielectric material used to cover the entire surface (except where the solder joints are to be formed) of the PCB primarily to protect the circuitry from environmental damage. Solder mask also helps to reduce bridging.

Solder "MASK" is used at the factory to cover or "Mask" the areas where solder is not required. Solder is only applied to the areas not masked.

You should not mask the entire board. If you are doing repair of a trace that was masked, you can re-apply mask in that area.

You're probably leaving some of the old flux, which may cause cold solder joimts in due time. I used to have some kind of flux remover for stubborn old solder. Also, your iron may be too hot, or too cold, kind of like a brown leaf thing for houseplants, the problem is generally is too much or too little water. Try cleaning the pads with denatured alcohol once you think that they are clean. I usually only use 15-25 watt Weller irons, and get in and out quick. Don't forget heat sinks where applicable.

Thanks for the advice. It's making me rethink what I do, which is a good thing. Strawman wrote:You're probably leaving some of the old flux, which may cause cold solder joimts in due time. I used to have some kind of flux remover for stubborn old solder. I am surprised at how much solder flux Sansui left on the six boards used in this amp. Not good! :yuck: :pistols: So yesterday I took a dental pick and physically removed all of it. I have some short bristle brushes designed for cleaning boards that I bought on a visit to Canada, and I used them to wisk away the detritus. Moreover, they are stiff enough to gently polish all the soldered pins and vacant pads. They don't damage the mask. I followed this up by spraying a light wash of CaiKleen 41 on the board and moping around with Q tips. I must say that this approach has left a pretty good looking board. I hope that it is a worthwhile process. What do you think? By integrating comments like yours into my work, I hope that I can be doing the right thing. More comments most welcome .... Lorne

I use Techspray isopropyl Alcohol (1610-P), and a tooth brush to clean up the circuit boards, after recap and solder joint retouch. The manufacturers generally left a gross amount of flux on the boards, which, after 30 years, gets to be a bit hard to get off. Keep a clean paper towel or rag on hand, and frequently, wet the brush and clean it on the rag. You want to make sure you do not get the dissolved flux into bad places, like trimmer pots and caps, and other places.

Be careful to ensure that each joint has a nice, slightly concave fillet. If it does not, remove the old solder and put on new. Note that, if there is a fracture, or a cold solder joint, there can be a solderability problem. You may need to remove the solder, and work to get the oxidation off the component leads, then reapply the solder.

Be careful to not overheat the component leads, especially to semiconductor devices.

Rich P

Rich P. wrote: Be careful to ensure that each joint has a nice, slightly concave fillet.

I’m glad you brought this point up. And BTW, the soldering on these Sansui boards DO look very nice — well, most of it. Lovely Mount Fuji-like shapes to the solder with a good concave surround. But here is a question that has been dancing around on the head of a pin in my head for some time:

Virtually all the work I do is on boards that do not have through-hole copper. It has always occur to me that if you just stick the part through the holes with a bit if a bend to secure it for soldering, the lead is not really making direct contact with the pad. It appears that you have bend the lead over at least 45 degrees to get direct contact with the edge of the hole. Without direct contact with the pad, we must be relying on the conductivity of the solder alone to transmit current. No one seems to address this, so perhaps I’m all wet (no pun intended). But I can’t help but think about how solder is typically tin and 40% lead.

Now the rub. If I just have a moderate bend or just stick the pins through the hole, the concave cone shape is easy to achieve. If I make a more radical bend, the solder may come out very nicely, but the concave shape is not nearly so good. In fact it is even harder to get the ideal cone shape. The concave, cone shape must be attributable to three factors: flow, wicking and gravity. With a sharply bent pin, I can’t see how we can get the shape. And without the bend, I can’t see how we can make a direct mechanical contact

I don’t mean to convey that I’m a ham-fisted solder clod. I’ve practiced a lot of soldering. It looks like a lot of soldering on factory boards. Nevertheless, I'm less than 100% confident. Can you comment?

Without direct contact with the pad, we must be relying on the conductivity of the solder alone to transmit current.

I dont think this is a bad thing. In a good solder joint, the solder should alloy to both parts, so there is an unbroken connection. Yes, its not copper, but the connections are very short. I think a fine job of soldering is much more important than trying to get the conductors to touch inside the joint, which they wont anyway if the solder is flowing properly. Just my $.02, othes might differ :)

John

Thanks John: What you say makes sense to me. Why is it that soldering primers often tell novices that soldering is simple? There seem to be so many factors that embrace mechanical, thermal, and chemical considerations.

Solder conducts just fine for the distance the signal must travel through it. As far as flux, I've seen trouble with the heavy flux coating on some boards conducting a signal. Acetone is cheap, and does a great job of removing the heavy flux coating on boards. After it dries, I spray the board with WD-40 and scrub it with a toothbrush, then dab the WD-40 off with a paper towel. Makes the foil side of the board look like new, and you can really see the quality of your soldering.

This may help.

Sounds like you're doing just fine. With time, you'll be an old hand at it. I've worked with some older boards that were just a bear to do anything with, others where a breeze.

With time, you'll be an old hand at it. This thread is really important to me personally. I hope that it helps others too. For example, I have THOUGHT about the solder connection and the minimal distance it presents, but I really need the opinions from others.I've worked with some older boards that were just a bear to do anything with, others where a breeze. in my limited experience, this Sansui is not the most difficult to work on. A couple of mid-70's Pioneer tuners were awful. I could lift the foil by just looking hard :D. ... and yet another use for WD/40!

At least you're trying, and that's what holds water around here buddy. :thmbsp: