PCB Kit soldering issues and using series resistors for testing - Page 1
Forum section: electronics / beginners

author: kalel
author_rank: Frequent Contributor
timestamp: September 10, 2017, 09:14:46 am
content: Being a beginner, I haven't perfected soldering with cheap Chinese kits yet (I'm sure some get it right the first time and that I too should have by now, but it's never late to learn).I haven't worked much with SMD yet at all, for good reason (if you check below to see the through hole issues).The only exception are relatively large (the largest I could find cheap) SMD LEDs, which can be soldered to stripboards/veroboards relatively easily.I might dare soldering one or two of those on a PCB with proper pads, but nothing else. Maybe in the distant future, when I also have found a good/cheap magnifying solution.A few issues I've noticed so far:1) IC rotation:Sometimes it happens that I turn the IC the other way around, and/or possibly other issues (of course if the IC is the wrong way around, it's hard to tell about other components).I believe it's mostly the IC, as the capacitors (plus polarity), resistors, etc. are all well marked, and I test every single component before placing it. But I don't always find the IC rotation obvious.Once I resorted to seeing an image on the internet where the kit seems completed, and following that same rotation, which... might have been wrong.I'm not sure how to address this point, but finding schematics on the internet (IC datasheet + kit schematics) and then using continuity check to find which contact on the PCB goes where might help prevent me repeating the mistake.2) Dry solder or VIA contacts getting scraped off:Sometimes I'm not happy with how the solder joints look, and try to add more or remove the dry looking solder.Sometimes the contacts around the VIAs (if that's the right word) even come off during the process, but it seems to only happen on some boards.On those same boards, it's very easy to remove solder mask (very light pressure will do it, even the soldering iron can free the copper - but maybe that's normal), which is also a good thing since it can be used to solder those pins that don't have good VIAs anymore.Dry joints might be related to the solder used (as a contributor, not sole reason):I recently changed solder. I had some German solder which worked really well (which was actually used up), and now I have two formulations of cheap solder from China. Supposedly 60/40 (larger diameter) and 63/37 (smaller diameter). Flux as written is either 1.5% on both or 2% on the first one (would have to check).So, I might have to learn how to work with this solder. I can change the power on my soldering iron, but not an exact temperature. But there's always the human factor (melting it longer or shorter) which can influence the result even with no temperature control.-- Testing PCBsOnce a PCB kit has been soldered, it might help to have a proper current limited power supply.Lacking that, would you recommend using a series resistor?Something like this:The goal being to prevent short circuits and limit the current.In some cases, hope is that it might even prevent a wrongly positioned IC or component burn out, but I'm not absolutely sure if it can.The trouble would be getting the "right" resistance value for each circuit.Using a potentiometer, it would be difficult to know the exact resistance value presently used (without measuring it after each adjustment).In your experience, is this something to do or not to do? Obviously, without a resistor, the current is limited only to the power supply. Even with batteries, that can be a good amount. With most power plugs or even USB power banks, it can still be enough to cause damage.

author: Ian.M
author_rank: Super Contributor
timestamp: September 10, 2017, 01:51:29 pm
content: Buy some polyfuses (with leads) in a selection of current ratings. They are basically fast acting, self resetting thermal over-current trips and are ideal for protecting low to medium current low voltage (typically <30V) DC circuits from burnups caused by accidental idiocy + a heavy duty PSU. You'll find them with rated holding currents from 50mA upwards.

author: kalel
author_rank: Frequent Contributor
timestamp: September 10, 2017, 03:31:05 pm
content: Quote from: Ian.M on September 10, 2017, 01:51:29 pmSounds like a good idea, thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: September 10, 2017, 03:40:33 pm
content: N.B. once tripped it doesn't take much current to keep them tripped, so unlike a self-resetting bimetallic strip thermal circuit breaker, they wont keep cycling back on till the short is cleared. If sized appropriately for the expected load current, you will usually need to cycle power or disconnect the load to reset them.

author: macboy
author_rank: Super Contributor
timestamp: September 11, 2017, 04:39:01 pm
content: Orientation of IC's should not be a big issue. Normally, pin 1 is marked on the board. This is often a "1" , a dot, or an arrow printed in the silkscreen near pin 1, or the silkscreen may depict an outline with a notch, which is above pin 1.Look at this PCB for an example, it has both a "1" and a notch. You may need to look at both the top and bottom of the board. Often the pin 1 copper pad is also shaped differently, for example a square pad instead of a round pad for other holes. Pin 1 is always marked on the IC package, usually a mark like a notch, dot, or arrow at/above pin 1. It might be printed or embossed in the plastic. For DIP, SOIC, etc. that have two rows of pins, pin 1 is going to be the bottom left corner when the IC is "right side up" (so you can read the printing on it). Orient the IC so you can read the markings, then verify pin 1 is where you know it should be by finding the mark.The fact that you are lifting copper pads off the board while soldering means one or all of: too high temperature, too much dwell time (time spent heating a pad), or too much pressure. When a pad is hot, the copper plating is very easy to damage. Just pressing too hard can slightly deform the copper, and it will lift off the board. Higher temperature and longer dwell time make this even more likely. But copper will rarely if ever just fall off the board without mechanical force applied. Use a gentler touch on your iron. You shouldn't need to press hard. Setting correct temperature without a temperature controlled iron is difficult. You'll need to experiment and figure it out, or get a better iron.Finally, get some good name brand lead/tin solder with a rosin based flux. Kester 44 (60/40 alloy, 0.030", 66 core which is 3.3%) is my favorite, but I also have some Kester 285 (a less active rosin), and some Multicore/Loctite brand that are all excellent. I've reworked some joints on cheap boards from China (like Arduino shields) and the solder they used is garbage. This is probably the same cheap spools of stuff you bought on ebay/ali. The alloy is probably just a mix of whatever scrap they threw in the pot and managed to melt, and who knows what the flux was. You can't solder properly without proper solder.

author: kalel
author_rank: Frequent Contributor
timestamp: September 11, 2017, 05:22:06 pm
content: Thanks. I did have a decent experience with quality branded solder, but I can't judge the Chinese stuff just yet (very very little use so far).I was able to remove the IC on a board and re-solder it correctly. Since it's a few component kit (mostly for practice) and the particular IC didn't mind (I'm sure most do) it is now working correctly. But the trace stuff seems to depend on the specific PCB quality too. With this board there's absolutely no issue with traces at all, using the same equipment and methods, even after re-soldering components. In the future, I'll simply apply a few of the methods (since those boards are simple, usually a continuity checker should be able to confirm how to place the IC - so that e.g. pin 1 goes to VCC or etc).I'm still curious about your opinions on limiting current while testing such little boards (small cheap electronics kits in my case) with a resistor, or as mentioned above, perhaps adding some type of fusing. Either than the circuit not working if the current is too little, I assume it can't hurt.