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At A Push PCB Suggestions

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Stub Mandrel24/08/2011 22:22:59
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4318 forum posts
291 photos
1 articles
I make my own PCBs.
 
There are lost of ways of doing this, and I've tried several - dalo pens, rub-down transfers, laser-printer toiner transfer. I've even done a double sided board using a dalo pen in a pen plotter... all these methods can work, but they are crummy. The one route to success is to use boards with UV-resist.
 
The best way of making positives for UV-resist board I have found is laser printing (Samsung SCX-4200, Scotch laser film). The film, costs 1/3 the cost of the printer, but lasts ages.
 
Don't buy a light box, get some of the 'deep purple' disco-light tubes (I believe they call them clubs these days) and build them into a box. Calibrate your box and stick to he tiings that work best for you.
 
Keep unused boards in the dark and cool. They last 2years+ in a cupboard in my workshop.
 
Eagle PCB design software is free from their site, limited to PCB size and two layersarea limitation is only on part placement -so lay your tracks around the edges to get quite bit of extra area! Steep (very) learning curve, but once you have designed and eagle board and procesed it using UV resist you won't want to go back.
 
To do double sided, use two sheets of film separated by a bit of spare board, and expose one side at a time.
 
After some trial and error, I find I get MUCH better results developing UV resist with NaOH solution, than with the 'modern' solutions. But I use about 15g of NaOH pellets per litre, which is half the 'old fashioned' recommendation. Keep the solution cool.
 
On the other hand, warm up the ferric chloride in the microwave (carefully) to get it to etch faster and more evenly.
 
Use meths to remove the etch resisit, or a kitchen 'scratchy' or wet and dry. Don't use your garriflex blocks, it seems to ruin them.
 
Believe it or not I use my 5-speed pillar drill with the standard 'micro-box' type HSS drill bits with great success. Just adjust as miuch slop out of the spindle as you can. Don't try carbide bits at such low speeds.
 
If you do break a drill, I found (in desperation) that if you run it at high speed and gently stoke one of thoss cheap diamond slips across the end it eventaully gets a sort of tip on it that WILL drill FR4 epoxy board with very little extra pressure. Indeed the resulting 'stub drill' has much less tendency to wander.
 
I always use a bit of plywood as a backing when drilling PCBs.
 
If, like me, your PCBs end up as bodged as your drills, you will need to add holes, possibly at a point when the PCB has components on both sides. Again necessity being the mother of invention I discovered that expanded polystyrene is an ideal backing material, as you can push the components into the board so it is supported and flat while drilling.
 
I have successfuly made boards for surface mount parts with 0.5mm pin spacing. soldering these was... interesting.For all SMT ICs I have a slightly unconventional approach. I tin all the pads very lightly, removing solder if there is too much and testing with a multimeter to make sure there are no shorts. Place the chip and using a very lightly tinned iron with a pointed tip press the pins at two opposite corners down onto their pads. Check alignment now, while it is still easy to release the chip. If it is on straight, just proceed along the edges of the chip, pressing the legs down onto their pads. You'll find you can feel the slight give as each leg drops and allow just a half-second for the leg to fuse with the solder on the pad. Very different from using a hot air gun or an oven, but 'it works for me' - except when I do my favourite trick of forgetting to do all the pins on one side of a four-sided chip.
 
Neil
6001925/08/2011 19:37:58
24 forum posts
4 photos
Neil, thanks for the primer on developments since I last used this stuff.
 
Soldering SMT ICs - my instinct would be prepare the pads as you describe, glue the chip to the board (careful alignment of course), then hot air gun the legs. Have you tried this and found problems?
 
And is there any technology yet for plated through holes for hobby type applications?
 
Mitch
Clive Hartland25/08/2011 20:38:02
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2929 forum posts
41 photos
I remember we had a repair technique for plated through holes on PCB's. It was a kit of tubes (small) and you drilled through and inserted the tube and soldered in place.
I wonder if they are still available?
 
Clive
Robert Miller 125/08/2011 21:36:59
19 forum posts
 
A couple more pointers for SMT soldering.
 
Generous application of a flux pen containing a water soluble flux also helps, as surface tension on the solder tends to reduce the tendency to create shorts. If you do get shorts, which is usually due to melting too much solder, commercially available pre-fluxed copper braid such as Easy Braid or Dri-Wick will soak up the excess.
 
For most model engineers I've met, the grey hair would suggest the use of a 5X binocular microscope might be highly desirable if not, as it is for me, essential. I've tried 3X binocular loupes and found them inadequate.
 
Unfortunately, there are some parts available only in lead-less packages - solder paste is required for this and a hot-air rework station is desireable, There are alternatives though, the folks at SparkFun recommend using a toaster oven. They have several toutorials on their website the ones pertaining to SMT soldering are:
 
http://www.sparkfun.com/tutorials/36
http://www.sparkfun.com/tutorials/96
http://www.sparkfun.com/tutorials/97
http://www.sparkfun.com/tutorials/98
 
I have no affiliation with SparkFun except as an occasional customer
Oliver Lindley25/08/2011 23:25:33
13 forum posts
3 photos
Mega electronics sell copper rivets for diy through hole plating. Mega sell a very expensive press for fitting these, however we use the rivets at work and use a automatic centre punch with a custom made end to fit the various sizes of rivets. you do need to solder the rivets (or tin the board) as after any length of time a high resistance can develop - I've measured 33K between a pad and rivet after a weekend.
 
I've been thinking of other ways to do through hole plating, and i am considering something similar to a metallisation gun, but on a smaller scale.
 
regarding soldering surface mount ICs, the most important thing to use is flux - this can hide a multitude of sins - really untidy soldering turns into shiny neat soldering just with flux and a bit of heat.
 
you do not need to glue parts down (unless you are doing loads at a time) - i just hold a part in vaguley the right position, apply the hot air, and the surface tension of the solder pulls the part square (assuming your footprint is thermally balanced). If you get solder whiskers betweeen pins, apply flux, reheat and it will be perfect.
 
Magnification is really useful for the finer pitch stuff - definately below 0.5mm - what can look like a perfect joint to the naked eye can be a whole different story when you can actually see it.

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