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Questions relating to designing PCBs
By voxmagna
#154127
I am climbing the Eagle learning curve and have mastered laser toner printing to the extent of producing single and double sided boards confidently with 10mil tracks without retouching and can do 8mil with luck. Even my method to get the top and bottom side print registered seems to work consistently. I am using a modified laminator for the toner transfer.

I have made a few single sided boards OK and used the liquid tinning approach which has worked ok so far.

Now I am taking the small step of a double sided board with wired components topside and some SMD caps and a couple of SOICs on the reverse. Time to try out some reflow, even though I could hand solder these few components.

I modified a toaster oven which is driven from a PID controller. After 'learning' the PID controller and testing the oven, I can get very close to the recommended ramp times.

Liquid tinning does not seem to like reflow! Rather the tinning goes from shiny to nasty grey so I guess I will have to give that up. My biggest problem now is the solder paste and its finish after reflow. I was expecting my test pieces with paste blobs to come out nice and shiney just like hand soldering. I am using a paste code SN62 RM89 AAS 85V. After reflow my solder blobs have a darkish gray film over them which does not clean off well with alcohol. It is thicker than I expected for a flux residue and 'peels' away from my test blobs like a conformal coating. By adjusting the ramp soak time I can get the coating to be harder or softer, but it still seems well stuck. the solder underneath has flowed and looks bright. I chose this lead based paste for its 210 deg C lower reflow temperature than non-lead alternatives

My question is am I using the wrong kind of solder paste, does everybody else get this kind of 'coating' and is it something I need to find a solvent for? I was hoping my test boards would reflow the solder nice and bright like manufactured boards so what am I doing wrong? I have read of others watching solder during reflow go into the 'coalesc' phase and run into a joint, but all I see is a grey spot with the solder phase hidden underneath.
By kirash4
#154157
I use a solder paste with the following chemistry: Sn 96.5 Ag 3.0 Cu0.5 (DigiKey part# SMD291SNL-ND)

It reflows well and I don't get the "residue" you're seeing. It reflows into a nice shiny connection.
By voxmagna
#154166
Thanks, I did a bit of reading and the type you are using is the lead free no-clean type with a higher melting point (216). Although my oven can easily do that , my hobby component stock has components pre-reflow techniques being used and I was trying to use the lowest of the reflow temperatures. I think I will have to change the way of doing things. Either use all smd or if mixing types, drill the holes, do the smd first then the leaded components last. My aim is to eventually move away from as many leaded components as possible if I can get better at making smd boards.

However, even with a PID controlled oven it is easy to get overshoot around the profile peak. I will try some of your paste and see how the finish comes out. The one thing that bugs me is the limited shelf life of solder pastes. I probably only do 4 or 5 small board projects a year and even in a fridge 6-12 months is recommended. Has anybody tried adding liquid fluxes to pastes to increase their use by dates? I know the pcb fab shops will only work with fresh paste, but hobby projects are a bit more tolerant.

What do you do about tinning? The proper way is to have a template mask made and squeegee over the paste. For one off quick hobby prototype boards I want to avoid having anything made. If I were to get the templates made I might as well have the whole board made - then find out afterwards I need to change it! I can register laser transfers on both sides of a board quite well now, but I think adding in the template mask would increase positioning errors. I looked into etching my own from 5mil brass sheet, but it seems hard to justify on first off prototypes, where mistakes or changes can be made. I think that is probably a step I will consider when my board reaches 80-90% smd. I have realised that since I am using a lot of copper pour at the moment to reduce demand from the etchant, etching the template increases load and cost on the etchant solution which makes the case stronger for having the template laser cut professionally.

I am thinking of getting one of those paste/glue droppers as I have an air comp. in my workshop. Although my boards would not look so nice, I was thinking of cleaning the copper sides well and just dropping the paste where needed, even tinning over some of the thinner more fragile traces. Perhaps there is some coloured acrylic lacquer I could air brush over the board to give it come protection. I have used clear coat acrylic lacquer in the past. It dries quickly and has no adverse losses up to the 100Mhz or so where I have used it. It also burns off easily for hand solder repairs.
By kirash4
#154225
You know, I have yet to use a reflow oven. I put my stuff in a skillet. After that's done, I will do the through hole parts, if any. Usually those are headers.
By voxmagna
#154238
Thanks. I have one of those and I will hook up the PID controller to it with a ceramic tile in the bottom and see how the results come out. Good point about headers because they need a lot of solder around the pins anyway. I think my skillet maxes out at 220 deg C on its own stat, but I can deal with that.
By kirash4
#154243
Myah, that's one way. My skillet is just that, a skillet. It goes on the stove. I have an electric stove top. I turn the stove on, put skillet on and let it warm up. Once warm, I put the PCBs in and let them come up to temperature. I watch the smoke from the evaporating flux (in the paste) and once that clears and the paste is a dull dark gray color, I ramp up the heat a bit which will cause it to reflow within seconds. As soon as that's done, I remove the skillet from the heat, fish out the PCBs and let them cool down.

While this method does not follow the proper heat, presoak, reflow, cool down ramp of any part, it's been a couple of years doing it this way and I have yet to encounter a problem. :)
By voxmagna
#154245
Yes I read the method. Not being USA I thought a 'skillet' was like a doubled sided electric hamburger cooker or my electric frying pan but I guess you mean a grilling plate you put on the stove. I have an electric frying pan and double sided burger/sandwich maker not doing anything. The sandwich maker has a top and bottom element. Although they connect them in series, I can separate the elements and fix that for top/bottom or both.Some methods need more skill and practice than others. I have already discovered you need to practice first on scrap test pieces before offering up your pride and joy pcb creation. SMD's are a lot easier to put on in one go than take them off! Do you use your skillet method for double sided pcbs? Will the smds hang on with their solder if you turn them over? Thanks
By kirash4
#154246
No, the skillet method is for single sided only. When I have a board that requires double sided components, I make sure the larger ones are all on one side so I can hand solder them. Pretty much anything larger than 0805. If you have an actual oven, you can use PCB glue to attach the components on one side, and put the others on the other side then bake the whole thing (with the glued parts on the bottom of course.) Search Google for SMD glue.
By voxmagna
#154248
Great help, good advice on the glue which I guess is high temp epoxy. Trouble is all these things have a shelf life shorter than what I would like. On the other hand I have used ordinary epoxies several years old and they have been ok.
By kirash4
#154251
I've used paste and flux as old as 12 months with no problems. I haven't gone past that though because by then I'm out of it and need more. I just don't buy a crap load. One syringe at a time gets me a long way. But yes, the epoxy is essentially just high temperature epoxy that actually hardens as it heats up. So the idea is that you do one side of the board first, reflow, then do the other side (the side without epoxy).
By UNTEngineer
#154298
If you want to do double sided smd work, you'll have to use two solder pastes with a high and a low melting point. Do one side first, then flip and do the other side.

Epoxy is too much of a hassle if you need to repair/rework/replace a part. Epoxy is useful if you have a cheap board, lots of boards and the circuit down to the T. Otherwise, no point in the headache.
By voxmagna
#154308
Thanks, good tip. I tried repairing a small commercial smd board once and could not get the component off the board. Then after deciding I would destroy it to use for 'practice', I levered it off under the heat gun and discovered a redish patch. I suspected they had used a conductive epoxy which made sense when you see how small some parts are and then consult the thermal specs. But like the use of Loctite for motor parts, I think glues are to be avoided for hobby work.

I will soon have both leaded and unleaded solder paste to experiment with. There is about 30 degrees difference in mp between the two and I can get the oven controlled to within a couple of degrees. Do you think that mp difference would work for double sided? I would not want to push up the mp so high that components failed or their reliability worsened.

Another question, I bought a small syringe of Sn96, 0.5Ag Cu0.5. I bought SN62 a while back and both pastes have the same consistency - the paste leaves the syringe ok but does not seem to have much viscosity to make it stick and stay on a board. I cannot see either paste going through a fine needle by hand unless in a glue dropper controlled by compressed air. Should the paste have 'stick', or is that the job of a flux which is dropped on first. Does paste lose its adhesion when it gets too old?

Thanks
By MichaelN
#154327
voxmagna wrote: After reflow my solder blobs have a darkish gray film over them which does not clean off well with alcohol.
That probably means not enough heat, and the solder isn't properly melting. For a start, I'd try something other than the lead-free solder you're using (I can't stand the stuff myself - just asking for trouble IMO).
By macegr
#154609
For stencils, I go down to TechShop and laser-cut my own out of 2mil mylar. There are various services around that will do the same thing in mylar or kapton, around the $20 range. These stencils are a bit messy and unstable compared to a good stainless steel foil stencil, but they are far better than nothing.

For paste, I currently use some really cheap Chinese 70/30 stuff with RMA flux. It works pretty well. I avoid thinning it if at all possible because it can get grainy or sludgy and not spread correctly, but I will sometimes thin it with rosin flux gel. Alcohol can be used, but only on small boards. It will dry too quickly if you try to spread it around a large board.

Reflow, I have tried all the options. I have done some reflow in a skillet on a stove, I have done reflow on a hot plate, I have done reflow in a toaster oven, I have done reflow using a hot air tool. Recently, I bought a cheap ($400) Chinese T-962A IR reflow oven, because my toaster oven was too small for some 9" square boards I was constructing. The firmware and interface is laughable, but it does work fairly well. All the reflow methods work fine, they just have varying yield and how much attention you need to pay to avoid burning your PCB. An automated reflow method is a huge time saver if you can work on placing parts while the soldering is happening. But, I've done a lot of complex boards in a simple toaster oven. I would say that a toaster oven is quite good for the average hobbyist.

As far as double-side reflow is concerned...I did not have luck with a toaster oven due to the position of the heating elements and the large air gap below the board. But on the new IR reflow oven, I can reflow one side, flip-stencil-place, reflow the other side, and the previously soldered components do not come loose. Most of the heat is applied to the top surface of the board. I could see a heavy component attached to a lot of through vias to the other side of the PCB possibly falling off, if enough heat is conducted through to reach liquidus.