No, I do not chemically tin any of my boards. They are "just" protected by the green paste. I don't know why, but I can't find chemical tinners in Spain in low quantities (yeh, checked pro stores, I live 100 meters away from the pro'st electronics store of Madrid), and what is available is quite expensive. Much more expensive than the green paste for the same results. A syringe of the green paste lasts a lot of time, you'll spend about 1/10th - 1/8th of the syringe to coat a double layer 100x160mm board. The layer required is very very thin, and 10ml's will last quite a bit. I'm starting my second syringe, and my first one lasted a bit more than a year.
The thing about the chemical tinning metallurgy isn't made up, it's a real problem. Sure it's a bit paranoid and most times it won't be a real problem at all, but happens. it was just written for informative pourposes. Another problem that might happen with high frequency signals is losses due to skin effect, because tin doesn't conduct electricity as good as copper. Again, paranoid but it happens :P
I honestly think that protection provided by soldermask is better than protection provided by chemical tining. In fact, industrially made PCB's aren't chemically tinned (they actually use a chemical tin coat as etchant resist, but it's then removed, and the soldermask is directly applied over bare copper, and then the uncovered areas are finished by HASL, ENIG or other coatings). It doesn't only protect against oxidation and corrosion, but it also provides mechanical protection against scratches, and it facilitates soldering a lot, as your solder stays confined to the pads and doesn't spread everywhere, and it's a "must" for QFN's, LGA's or other packages like those.
[quote author="arhi"]you need to add a fotoresistive foil with your solder mask on top of the green gue, after you ligt it up, the part that was lit hardens and the part where you had your mask stays gue so you can then clean the board and remove green gue from the board.
The green stuff is really mostly for presentation, if you want to really protect and "better" your home made pcb's use the chemical tinning compound to apply layer of solder to your copper. It will protect your copper + it will strengthen it and make soldering easier. You can find some info here: http://www.aliatron.pt/download/Sur-Tin_manual.pdf .. also multi compound chemical tinner are better then ready made stuff as they all degrade in time so with multi compound (usually 3 parts) you can make smaller batches as you need them.
No, it's not only for presentation. In fact, the aesthetical part isn't important for me. The green stuff protects the board from oxidation and corrosion, and it's a really good protection. It's the same protection that industry uses for PCB's. And also, the biggest advantage you get with soldermask is that soldering gets much easier, specially for SMD's, as solder doesn't spread everywhere and gets confined to the pads, making things much easier and reliable. And I must say that this stuff is kinda "fragile" right after you apply it (with fragile I mean easy to scratch), but as it absorbs more and more light it gets harder and harder, and after a day or two it provides a very very good protection to the board.
I was trying to find chemical tinning compounds for a while, but they aren't easily available in Spain, at least not in small quantities. I wanted them for finishing the pads, but chemical tining isn't as good as people tends to think. It has some disadvantages. Tin melts at a higher temperature than solder, so the solder joint is no longer between copper and solder alloy, but between solder alloy and tin, which is chemically bonded to copper. Surely diffusion occurs and the intermetallic layer eventually diffuses through the solder joint blending with the alloy, but metallurgically speaking it isn't the best bond you can get.
That's something similar to what happens with electroplated gold finishes. As you can't directly plate gold to copper because it quickly diffuses, an intermediate barrier layer of nickel must be applied, so finally, what you get are solder joints between soldering alloy and nickel (gold diffuses almost instantly), and that nickel is plated to copper, and that's a weaker bonding.
Anyway, I'll post pics and document the process when I make my next board, which will be as soon as I finish my final exams.
Yeah, that's it. But what they don't say is that the stuff sticks to nylon, polyester, acetate and polyamides (learnt it the hard way) and most transparency films are made of any of these materials. Also, as transparency films are quite thick, it gets difficult to spread the paste correctly. So an intermediate layer other kind of plastic is required (say polyethilene or polipropylene film). There comes the paper sheet pack wrapping plastic trick ;). I screwed the first 2 or 3 boards I tried to soldermask because the stuff stuck to the transparency, and then screwed one or two more for not spreading the paste correctly. After that, all the PCB's have gone just fine. Don't worry, the PCB's I used for testing were small and cheap ;)
Okay, i'll post pics the next time I make a board. You'll have to wait a bit because I'm on my final exams period and I can't play with boards in this moment, but I promise I will deliever.
How are the fumes of this stuff? I have 0.5mm solder which has kolophony core that smells really really bad, It's a nasty smell, like if it were toxic or something. I have however 1mm solder with synthetic flux core that is a pleasure to solder with, it smells much much better. Also, doesn't it smell like isopropanol? Isopropanol fumes can lead to dizziness and vertigo if you smell them in high concentrations.
Also, Is it easy to clean?
If it smells properly and is easy to clean I might try to make a batch.
I think we all hate how oxidized homemade PCB's get over time, and I also think we all love how easier it gets to solder SMD parts when you have a soldermask.
I myself tried to find a source for low quantities of soldermasking ink, better if photosensitive or UV curable, but I was unsuccesful. All I could find were buckets of the pro stuff for €€€,€€ in strange distributors.
I found it like a year ago, and I thought I had to give it a try. Bough a syringe and... my life has changed since then. Once you know how to use this stuff properly, your homemade PCB's will never look the same.
It's a bit hard to learn how to use that stuff, however.
The recommended way to apply it is to squeeze some drops in the centre of the PCB, cover it with a plastic film and extend the product with a squeegee all over the board forming a thin layer. But there is a problem: this stuff sticks to nylon, acetate and polyamides, so you can't use transparency films with the photolite already printed on them, because you'll mess up the board, so I'll tell you how I apply it.
The first thing you must find is a sheet of any thin but still quite rigid plastic. It must be as thin as possible but it must NOT form wrinkles, and be as smooth as possible. And it also must not stick to the stuff. Seems easy to find such plastics, but it isn't. I get mine from paper sheet packages. When you buy 500 sheet packs, they usually come wrapped in a kind of plastic that is just perfect for this job. You have to remove the prints from the plastic, I do it cleaning the plastic sheet with common ethil alcohol, until it's totally clear and transparent. Don't worry, plastic sheets are reusable and you won't have to buy a new 500 sheet pack every time you want to make a board.
Then, I apply a few drops of the product to the board, I put the plastic sheet on top and I extend the product all over the board with a plastic squeegee . The layer of product must be thin. If it looks too green then it's probably too thick. Once I have all the board covered with the product as uniformly as I can (a sandwich of board, layer of product and plastic sheet), I place the transparency film with the mask printed on top of the stack, and I align it with the board. Then I sandwich everything between two layers of glass and insolate the board for about 30 minutes with a 8 watt blacklight tube. Once it's done, the stuff that has been exposed to UV light will be hardened and stuck to the board, while the stuff that was covered by the mask will still be liquid. I peel off the plastic sheet that shouldn't be stuck to the ink (the plastic sheet shoudn't have rests of hardened ink attached), and the liquid ink that remains in the board can be easily removed wiping it with a towel and some alcohol or something. Then I clean the plastic sheet (which will have some rests of liquid ink) so it can be reused.
The results are pretty amazing, achieving very very high resolution masks that are really useful to protect the boards and facilitate soldering. Usually the coating won't be as uniform as one would want, and the texture and appearance of this solder mask isn't the same of an industry applied mask, but does its job perfectly.
I'll post pictures of the process if requested, but here's a sample of some homemade boards I did with the stuff applied:
(I took the same image I used for my presentation thread, obviously the OLS and the BP boards aren't homemade :P) I don't have other pics right now. The robot you see has three LGA or QFN -packaged IC's (a buck voltage regulator, a dual H-bridge and an electronic potentiometer) and the mask was perfect for them.
[quote author="LisandroP"]I finally broke down an got a new 18F2550, which worked perfectly from the beginning. This must be the first time i ran across a broken PIC...
Anyway, this is a great little device. It's now receiving IR for my HTPC system and works great. Thank you for your hard work guys![/quote]
I've personally had 18F2550's with the very same issue, that ended up working after... heating!. Yeah, no VUSB, some heating, and off they go. Heating to 150ºC or so..., nothing close to or above reflow temperatures.That happened to me when I started making psgrooves for friends and that, and ordered a bunch of 2550's and like 3 of them had that issue.
I don't know what the hell happens to the 18FX550 series. I myself have broken like 5 4550's in TQFP44 in 3 different proyects in less than three months. With broken I mean that they were working, and suddenly they stopped. After checking the memory contents, the first 8 to 16 (or so) words were erased. It's magic. Some of them re-worked after reprogramming, some never started the oscillator again (literally). I first thought it was a USB ESD issue, but nopes. I also thought it could be something related to the "popcorn effect" while soldering, but nopes, I added some serious ESD protection. A """pre-baked""" chip also broke. I also had one that woudn't properly read the state of a pin, and guess what, worked after reheating. And poofed after a month.
Yeah, for me it also was the first time I saw a broken pic. In my early years I did all sorts of mistreat to those poor first PIC's that came to my hands, and all of them still work. I've only experienced that issues with FX550's, but never with any other PIC model, even with USB.
[quote author="mattcallow"]Thanks for all the advice. Do people use lead free, or leaded solder? (I have some 0.7mm SnSb solder that i was going to use. Is this any good?) Also, what sort of flux? I have a flux pen, and some tacky flux. Is liquid flux better?
Matt[/quote]
I do use a flux pen, more concretely the kester 2331-zx. That stuff however is pretty nasty, quite corrosive and you must take great care cleaning it. I learnt the hard way what that stuff can do to copper if you don't clean it... However it works perfectly and a drop or two are more than enough for most works. That 10ml pen has lasted me a couple of years and it's still half full, and it's the only flux I use apart from the one that comes embedded in solder and solder paste. I do prefer liquid fluxes over tacky ones for every soldering job (I mostly hate rosin based fluxes, synthetic forever!), but in this case I'd recommend liquid even more, or something that liquifies easily. People's gonna jump to my neck because in this forum most people love rosin fluxes but... just my opinion.
I'd use lead solder whenever possible, because it has a better flow ratio as far as I know. But with high quality flux it should work with lead free. Just practise a bit before doing the real job, it takes a while to get things to work as you want. Try different tips, different angles, different wiping speeds until you get comfortable with the results.
I solder those chips with solder paste. But when soldering paste is not available, there's a trick that always works for me. Just get some very high quality solder, for 0.5mm pitch i'd recomend 0.5-0.7mm, but 1mm will also do with some experience. If you only have cone shaped tips for the iron (which i think it's the case), choose the thickest one (1mm or so should do). Also get a PCB support that lets you hold the PCB vertically at a 60 degrees angle or so.
Now you must work quite quicky, and also be sure to take some diazepan if you don't have steady hands (disclaimer: medical tips not to be taken seriously).
After placing and securing the chip in place by soldering two opposite corner leads, position your soldering iron armed with the desired tip near the first row of leads you're going to solder. (Note: add some drops of flux to the row of pins). If your iron doesn't maintain temperature very well, set it to a higher temperature than usual (300-350ºC or so). Now touch the first lead of the row, immediately start adding solder and wiping the tip all over the row from top to bottom while adding more solder in the required amount (takes some experience to get it right). You must be quick. After the first pass, if you've too many shorts and flux is still not consumed, wipe the tip again quickly without making too much pressure, and this time without adding more solder. Remove the soldering iron and... hopefully you shouldn't have too many shorts. Using a conee shaped tip isn't the best idea, but most times works well enough for only having to remove a couple of shorts in the lowest part of the row with some soldering wick.
Rotate the PCB in the support so the next row is vertically oriented and repeat the process.
Depending on your overall skill and materials used, you'll either end up with very high quality solder joints or with a mess of tin and burned flux. I'd recomend practising first with spare IC's or things that you don't care about.
I had to register here to post a picture of a free pcb build. Now that I went through the registration process, I think i have to present myself. My name is David and i'm an electronics engineering student from Spain. I have electronics as my hobby and passion, and I also love lasers and related things. Over the years I've learned to code for microcontrollers and some stuff about analog electronics, and I'm pretty experienced in small switching PSU design, mostly for laser diode constant current sources (I have a quite powerful laser being pumped at 1.1Amps at 4.3V from a single li-ion). I also have some PCB making skills, and over the years I've developed enough of them for being able to easily make PCB's for tricky electronic packages and such. If I only could through-plate the holes... I'm equiped with a hot air soldering station, a temperature regulated soldering station with some different tips for the iron. I also have 2 boxes full of components that i've bough and accumulated over the years, mostly strips of passives and discretes, and a lot of leftover IC's from previous proyects. Now my goal is to learn some VHDL to develop CPLD and FPGA programming skills.
Here's a picture of my last electronic proyects along with the two gadgets I own from DP. I've ommited pics of the lasers for eye safety reasons :P [attachment=0] From left to right: A line follower competition robot, a 2-cell lithium battery charger-balancer, a MIDI based electronic battery controller (the rest of the battery is still to be built... someday) and a Nixie clock. All the PCB's are completely homemade.
Hope you like them. I hope I can contribute to this community!
Thanks ian! just redeemed the coupon for an XC2C64 dev board. I can't wait to assemble it and start learning CPLD programming! I have a couple of XC2C64's sitting next to me in a moisture barrier bag waiting for their moment... must hold the temptation to open it!
I got a free BPV3.5 PCB a while ago and I had to wait until today for the PIC to arrive (wasn't available at the store so had to sample it). I just finished the assembly. Note that I used 2K resistors for the LED's, as I have an OLS with 1k1 resistors and the LED's look too bright for me. The blue led's light just in the right spot but the green ones are just barely visible... I think 2k is a bit too much for them. The camera flash made the LED's barely visible in the picture but I swear it's working and 3 of them are ON ;)
Had to attach the picture because spamhammer doesn't let me post a link to tinypic. [attachment=0]
