exposing double sided boards

[Simon]

So I'm nearly ready to take my first step in a double sided PCB project. Now my biggest worry is exposing the two sides correctly so that they line up. I've looked around the net but most guides are for toner transfer method. So how do I do it ?

I could tape the two artworks together and then put the blank board inside but as I can only expose 1 side at a time I may move the board in turning it over. I don't want to go cutting larger than needed boards for fixing purposes or damaging my artwork by keep taping boards to it as I may want to do a number of them.

There has to be a simple solution well in the mean time off to search Utube

[Mechatrommer]

i have this both photographic and toner transfer etching. the 1st time i used photographic, i directly put it back in the drawer (maybe for the rest of its life i guess). i use toner transfer thereafter. as you mentioned it, it is an "art" work.

[Simon]

by artwork we generally mean the printout of the pcb onto tracing paper or inkjet transparency. There are projects that I'd like to make more than one of and for the price of the transparency I'm not using one copy per board.

Having looked at utube the solution was quickly revealed: yes make a pocket out of the two layers but then fix them with the board inside between two sheets of glass. So you can pick up the whole pack and flip it over, taping the two sheets of glass together will offer enough grip to stop the board sliding when it is turned over

[Zero999]

I've never done double sided before but here's how I'd do it.

Cut the board to the required size. Make it as accurate as possible, using a file and a square as needs be. Pay attention to the datum sides, make sure they're the best.

On the artwork, put a mark on each corner which you'll use to line up with the artwork. Remove the protective film from one side, expose and develop, then do the other side.

Another thing you could try is printing the artwork for both sides next to each other, flipped and one PCB thickness apart so the paper can be folded round the PCB and secured with tape.

[kaptain_zero]

I've never worked with photo sensitive pc boards before, but I've done a whole lot of work in a darkroom and these are things I used to do to 'register' multiple layers accurately.

The way I most often did it, was to use register pins. Drill two holes through the board at opposite corners (diagonally). Register the two 'negatives'  putting the same holes through them as in the board. Then you can use a couple of pins through those holes to line everything up together. Using 3 holes/pins makes it easier to line things up in full darkness if the third pin is placed in such a way that the 'neg' can only fit one way.

Another way is to use a board with a raise corner and two long side. Then cut the 'negatives' to the same exact size as the board and so they are properly registered. Then simply tuck the board into the corner with one 'neg' on top and expose, flip the board and place the second 'neg' on top and re-expose. This method is hard to do in pure darkness as things can get turned around the wrong way. My darkroom had a sodium vapor safelight that was bright enough to let me read a newspaper while still being completely safe for black/white papers I used for contact printing so I could see what I was doing. Working with color films/papers required full darkness, so pins were the only way to go. 

Using pins is generally more accurate and avoids having to carefully size each piece, and, if using 3 pins, can help avoid accidentally getting one of the layers turned around the wrong way in the dark.

I hope this makes sense....

Regards

Christian

PS: I see you found directions on how to use the board/sandwich method.....  The pins might be quicker by avoiding to have to accurately size all the parts.

[Chasm]

I've done some double sided boards during my apprenticeship.

The idea is simple: Make a sleeve of the two transparencies which is closed on two sides. Slide the PCB into it, fix the PCB at one outer edge with Tesa/Sellotape/Scotch Tape. Then expose, develop and etch.

Implementation is a bit harder.
You'll need some markers for the board outline. You have to make allowances for the board thickness. (This can be done by gluing a strip of same thickness PCB between the transparencies. This also helps in regard to the alignment.) Whatever you do, alignment won't be perfect so keep pads for THT components rather on the large side.

Pass through pins or registers as described by kaptain_zero help, but should not be necessary for simple boards. Especially since you need some way to use them with your UV Source.

[johnmx]

Put the PCB between the top and bottom transparencies with the circuit (laser printed), then put two sheets of glass, align the transparencies and hold the glass with several pincers. Because “a picture is worth a thousand words” it’s better explained in the attached pictures.
It’s very easy to get a perfect alignment.

I also made my own UV exposure box. I can select only bottom or both sides and exposure time.

[Zyvek]

I also made my own UV exposure box. I can select only bottom or both sides and exposure time.

Awesome work!

[tyblu]

As kaptain_zero mentioned, it's best to drill pilot holes. I've done 2-sided photo transfers several times, and the holes are a must for <20mil features. Transparencies stretch when printing with laser printers, so lining corners up doesn't work as well as it should. If you have ICs with a bunch of pins on your board, it's best to either arrange it such that its land pads are printed horizontally in the printer or to calibrate the printout to the degree your printer stretches transparencies. You can do this by printing a ruled border on a transparency (1mm-spacing) then measuring it; you'll get something like 1.005:1 stretch ratio, leading to ~1mm error at the top. You can get around this by printing the bottom half in the same direction, but by this point I'm usually lost, and just add a correction factor to the printout (1/1.005).

[mikeselectricstuff]

The trick is to line up the 2 sides, then staple it on 3 sides to form an envelope. The key is getting the distance from the edge of the PCB to the staple right - long enough that it doesn't curl away from the PCB surface at the PCB edge, and short enough to minimise slippage. Typically about 5mm. Use small staples - the stapled thickness should be less than the PCB thickness.
You can use tape, but stapling is easier to fix without affecting the alignment.

Protect the top side form light (old style rubber mousemats are good), Turn over carefully - it helps a lot of your exposure box has a flush top to you can slide the board off instead of lifting off.

[johnmx]

Transparencies stretch when printing with laser printers...

No they don't stretch. You probably use inject transparencies in a laser printer. There are transparencies for every type of printer.

For good results you can use a home laser printer.

For the best results go to a copy shop and ask to print from pdf to transparency in a color photocopying machine, but printed only in black and white. These are the best machines. Remember to print at home in paper and take that sheet with you to the copy shop just to check if the photocopying machine didn’t change the size of the drawing. For that simply put the transparency on top of the paper sheet.

Remember that the photocopying machines print on top of the paper, so you need to mirror the top layer and the bottom layer is not mirrored. In this way you will have the toner of both layers on the side of the PCB.

I don’t understand why you guys talk about drill pilot holes, tape or staples. Nothing of that is needed. I make double-sided PCBs with traces down to 12 mils and small vias in 10 minutes including the preparation, exposure, development and etching. I've never had a misaligned PCB.

[Chasm]

One cost cutting advice regarding transparencies and laser printers:

If you are using a laser printer make sure that the transparencies are actually compatible to the printer!
Using simple inkjet - or even the wrong laser transparencies will most likely result in molten transparency and broken printer.

The same is true for monochrome transparencies in color laser printers, color lasers use higher temperatures in their fuser unit.
But it is also true for the modern monochrome and color multi function office copiers/printers/fax machines, they use much higher temps than the home office stuff.

[Mechatrommer]

good work johnmx! even with the censored pcb.

[Psi]

i've done a few double layer boards using the laser printer transfer method and press'n'peel blue pcb paper.

First I picked two through hole pads on opposite corners of the design and poked a hole in the centre of each with a pin.
I did this to the same holes on both the top and bottom press'n'peel blue sheets.

I then tape one sheet onto the blank PCB and drilled out the two holes using a 0.2mm drill bit. (The holes you poked into the sheets allow you to see where to drill even though the press'n'peel is taped with tracks facing the board.)

Then I use two of the actual 0.2mm drill bits to poke through the holes and put the other press'n'peel sheet on and let the drill bits align it down onto the pcb

Now there are two drill bits sitting halfway into the pcb holding a sheet of press'n'peel on both sides.

NOTE: You need to use the type of drill bits where the the drill bit shaft is the same diameter as the hole size it cuts. Not the type that has a 3mm shaft and a 0.2mm drill attached to one end.

Once you have that assembled you can be pretty sure your board is aligned, since you have two drills through the same hole holding both sheets onto the pcb.  You can then apply some tape to the other side to hold that piece of press'n'peel on and remove the drills carefully.

Then i put one sheet of normal paper over both sides to stop the heat from melting the tape to the bottom of the iron and making a mess.

Then iron it, and you're done.
It's still tricky, but if you do it right it normally comes out ok.
You'll never get the holes between top and bottom throughhole pads to be exactly in the centre on both sides when you drill them but it's normally close enough.

[Simon]

One cost cutting advice regarding transparencies and laser printers:

If you are using a laser printer make sure that the transparencies are actually compatible to the printer!
Using simple inkjet - or even the wrong laser transparencies will most likely result in molten transparency and broken printer.

The same is true for monochrome transparencies in color laser printers, color lasers use higher temperatures in their fuser unit.
But it is also true for the modern monochrome and color multi function office copiers/printers/fax machines, they use much higher temps than the home office stuff.

Yes I'm using inkjet transparency on a gel printer and it does not come out very dense so I have to use 2, I'm thinking of getting an inkjet just for PCB's

[Zero999]

If you want a second printer I'd recommend a decent monochrome laser which should produce enough coverage for PCBs and will be very cheap to run so good for all of you monochrome printing jobs.

The Brother HL 5240 is good and very cheap to run, something like 2.2p per pages including paper and that's with the official toner.

Dough I paid £100 for a refurbished mode (the cheapest at the time)l a few months ago and now you can pick up a brand new one for £76.
http://www.google.co.uk/products/catalog?q=Brother+HL-5240&oe=utf-8&client=firefox-a&cid=16951771083253417501&ei=6_EaTbHnNJe5-Qbd0ty2Cw&sa=title&ved=0CA4Q8wIwATgA#p

[Simon]

but what are transparency prices for laser ? I can probably pick up a free inkjet from someone on freegle/freecycle particularly after xmas

[Simon]

The trick is to line up the 2 sides, then staple it on 3 sides to form an envelope. The key is getting the distance from the edge of the PCB to the staple right - long enough that it doesn't curl away from the PCB surface at the PCB edge, and short enough to minimise slippage. Typically about 5mm. Use small staples - the stapled thickness should be less than the PCB thickness.
You can use tape, but stapling is easier to fix without affecting the alignment.

Protect the top side form light (old style rubber mousemats are good), Turn over carefully - it helps a lot of your exposure box has a flush top to you can slide the board off instead of lifting off.

surely with staples the two layers will "rock" from side to side ?

[mikeselectricstuff]

but what are transparency prices for laser ? I can probably pick up a free inkjet from someone on freegle/freecycle particularly after xmas

Use tracing paper in lasers - cheap and good toner  adhesion - get at least 90gsm as the thin stuff crinkles in the fuser.

[mikeselectricstuff]

The trick is to line up the 2 sides, then staple it on 3 sides to form an envelope. The key is getting the distance from the edge of the PCB to the staple right - long enough that it doesn't curl away from the PCB surface at the PCB edge, and short enough to minimise slippage. Typically about 5mm. Use small staples - the stapled thickness should be less than the PCB thickness.
You can use tape, but stapling is easier to fix without affecting the alignment.

Protect the top side form light (old style rubber mousemats are good), Turn over carefully - it helps a lot of your exposure box has a flush top to you can slide the board off instead of lifting off.

surely with staples the two layers will "rock" from side to side ?

Not quite sure what you mean, but here is no lateral shift - depending on PCB size I typically use 1 or 2 staples per side on 3 sides. It is important to use a small stapler to keep the profile low.
 

[Zero999]

but what are transparency prices for laser ? I can probably pick up a free inkjet from someone on freegle/freecycle particularly after xmas

Tracing paper will definitely work with a laser. I've never tried plastic film but I don't see why it wouldn't work.

[mikeselectricstuff]

but what are transparency prices for laser ? I can probably pick up a free inkjet from someone on freegle/freecycle particularly after xmas

Tracing paper will definitely work with a laser. I've never tried plastic film but I don't see why it wouldn't work.

Plastic film tends to soften through the fuser, leaving crinkles when it cools. Toner adhesion and coverage is also often poor. The rough surface of tracing paper gives good adhesion and minimal toner drift, so line density is consistent across each line - I've often seen toner 'bunch up' at the edges of lines printing on plastic.

[DJPhil]

The trick is to line up the 2 sides, then staple it on 3 sides to form an envelope. The key is getting the distance from the edge of the PCB to the staple right - long enough that it doesn't curl away from the PCB surface at the PCB edge, and short enough to minimise slippage. Typically about 5mm. Use small staples - the stapled thickness should be less than the PCB thickness.
You can use tape, but stapling is easier to fix without affecting the alignment.

Protect the top side form light (old style rubber mousemats are good), Turn over carefully - it helps a lot of your exposure box has a flush top to you can slide the board off instead of lifting off.

surely with staples the two layers will "rock" from side to side ?

Not quite sure what you mean, but here is no lateral shift - depending on PCB size I typically use 1 or 2 staples per side on 3 sides. It is important to use a small stapler to keep the profile low.

I believe I understand what Simon is worried about. If you staple two pieces of paper together and attempt to slide them against each other perpendicular to the staple they'll often have a millimeter or more of play due to the torsion on the staple. This could lead to registration problems.
I think it could be mitigated by stapling in a zigzag pattern or at right angles to each other. Clever mixture of the staples so they're not all parallel should do the trick.

[mikeselectricstuff]

The trick is to line up the 2 sides, then staple it on 3 sides to form an envelope. The key is getting the distance from the edge of the PCB to the staple right - long enough that it doesn't curl away from the PCB surface at the PCB edge, and short enough to minimise slippage. Typically about 5mm. Use small staples - the stapled thickness should be less than the PCB thickness.
You can use tape, but stapling is easier to fix without affecting the alignment.

Protect the top side form light (old style rubber mousemats are good), Turn over carefully - it helps a lot of your exposure box has a flush top to you can slide the board off instead of lifting off.

surely with staples the two layers will "rock" from side to side ?

Not quite sure what you mean, but here is no lateral shift - depending on PCB size I typically use 1 or 2 staples per side on 3 sides. It is important to use a small stapler to keep the profile low.

I believe I understand what Simon is worried about. If you staple two pieces of paper together and attempt to slide them against each other perpendicular to the staple they'll often have a millimeter or more of play due to the torsion on the staple. This could lead to registration problems.
I think it could be mitigated by stapling in a zigzag pattern or at right angles to each other. Clever mixture of the staples so they're not all parallel should do the trick.

It doesn't really happen - first the tracing paper has fairly high surface friction, and also the tail ends of the staple pressing on the surface, combined with the rectangular cross-section of the staple wire mean it has a natural preference to spring to the nominal position it was at when stapled. With a single staple there is some potential rotation due to enlarging of the holes, but once you have 2 staples in pretty much any relative orientation, you just don't get any noticeable shift.