Double-sided PCB's

[mstechca]

I have become proficient in making single-sided PCB's using the toner transfer/photographic method. (you know, where you print on a transparency, expose it to a photo-resist coated board, develop, and then etch it).

BUT now I see that some designs are too complex for a single-sided PCB. In fact, I have to be a computer to make complex single sided PCB's, even if it is for an EEPROM or FLASH programmer!

So, now I will have to go for double-sided PCB's, considering I don't want to add soldered jumpers, like a number of you suggest to me.

I checked some websites, and one site suggested that I should drill some corner holes and align my circuit with them on both sides of the board.

I made myself a cheap drill (with a toy motor, and a drill bit sticking out of the center of it, all soldered up). and with it, I have O.K. accuracy. Sometimes, when I drill, the hole isn't 100% straight through, but it is very close. If I drill holes in an unetched board, my drill goes all over the place sometimes.

What I want to do is use my drill and make a double-sided PCB using the photo-transfer method.

Before I attempt to do it, I need to know, what is the BEST way to do it?
I want to do it right the first time, and every time I do it.

and as for exposing, what is the MAXIMUM amount of time a board can wait between time the light source is moved away from the board, and the time it is inserted into the developer?
I ask this because I only have one light source, and I would think it is impossible to do both sides at the exact same time.

and remember, I do NOT have sophisticated equiptment. In fact, I DONT even have an oscilloscope.

so if anyone can suggest anything, I will take it into consideration, and I thank you for it.

 

[Optikon]

I have become proficient in making single-sided PCB's using the toner transfer/photographic method. (you know, where you print on a transparency, expose it to a photo-resist coated board, develop, and then etch it).

BUT now I see that some designs are too complex for a single-sided PCB. In fact, I have to be a computer to make complex single sided PCB's, even if it is for an EEPROM or FLASH programmer!

So, now I will have to go for double-sided PCB's, considering I don't want to add soldered jumpers, like a number of you suggest to me.

I checked some websites, and one site suggested that I should drill some corner holes and align my circuit with them on both sides of the board.

I made myself a cheap drill (with a toy motor, and a drill bit sticking out of the center of it, all soldered up). and with it, I have O.K. accuracy. Sometimes, when I drill, the hole isn't 100% straight through, but it is very close. If I drill holes in an unetched board, my drill goes all over the place sometimes.

What I want to do is use my drill and make a double-sided PCB using the photo-transfer method.

Before I attempt to do it, I need to know, what is the BEST way to do it?
I want to do it right the first time, and every time I do it.

and as for exposing, what is the MAXIMUM amount of time a board can wait between time the light source is moved away from the board, and the time it is inserted into the developer?
I ask this because I only have one light source, and I would think it is impossible to do both sides at the exact same time.

and remember, I do NOT have sophisticated equiptment. In fact, I DONT even have an oscilloscope.

so if anyone can suggest anything, I will take it into consideration, and I thank you for it.

If you intend to learn how to do it with the toner method- I dont have a suggestion.

If you intend to have a 2 sided PCB solution, my suggestion would be to stop playing with all that messy stuff and just have a cheap proto house make one for you. They can be _very_ inexpensive for 2 sided PCB in low quantities. You can make much better PCB's than you could ever dream of with the toner hacks.. IMO.

:lol:

 

[Nigel Goodwin]

So, now I will have to go for double-sided PCB's, considering I don't want to add soldered jumpers, like a number of you suggest to me.

What have you got against wire jumpers?, it's a VERY standard technique, and almost all the domestic electronics in your house will use them.

However, if you design your PCB's well you might not need many, and quite possibly (with the simple circuits you're talking about) none at all.

It certainly seems stupid to try and make double sided PCB's just to prevent a couple of wite jumpers!.

 

[RDL2004]

Keep as much of the circuitry as you can on the bottom side and use the rub-on type patterns for the few traces you need on the top.

I would guess from my past experience in photography that the latent image is good for quite a while before it's developed, as long as it is not exposed to any additional light.

- Rick

 

[mstechca]

What have you got against wire jumpers?, it's a VERY standard technique, and almost all the domestic electronics in your house will use them.

I just don't like to use them. It uses up wire, it takes more space, and the circuitry will be more complex

It certainly seems stupid to try and make double sided PCB's just to prevent a couple of wite jumpers!.

How about a couple dozen or more.

I'm dealing with eeproms and parallel ports.

 

[DirtyLude]

Okay, well you aren't using toner transfer, you are using photoresist. The 2 methods are different. The method I use for toner transfer should work even better with photo resist, since you use clear sheets.

All I do, is print both sides on sheets, then line them up under a light, so they are perfect. Tape the edges of the sheets so that the stay perfect. Insert the copper clad between the sheets and iron on the toner (in your case, expose it.)

I took pictures of the process when I did it once:

**broken link removed**

I certainly don't do this often, but I do do it every once in a while. I did it that time just because it made it easier to fit the PCB inside the project box. Certainly for regular projects it's easier to just run jumpers, even alot of jumpers.

One easy way to do double sided is make the bottom one big ground plane, and don't etch it at all. Make vias wherever you need ground. This sometime eases up routing enough and you don't even need to worry about lining up the layers.

 

[Klaus]

What have you got against wire jumpers?, it's a VERY standard technique, and almost all the domestic electronics in your house will use them.

I just don't like to use them. It uses up wire, it takes more space, and the circuitry will be more complex

It certainly seems stupid to try and make double sided PCB's just to prevent a couple of wite jumpers!.

How about a couple dozen or more.

I'm dealing with eeproms and parallel ports.

Its quite a BIG step from single sided boards to double sided PCB boards.
You not only have to get perfect registration of both sides, you also have to take into account that you cannot solder some components on BOTH sides of the board.
The reason one goes to double side boards is because of increased complexity of the circuit. This means that the usual jumper links are replaced by through links or 'vias'. A clever way to limit these is to use component legs, soldered on both sides but, as I mentioned above, not all components let you do that. So, keep that in mind when you design double sided boards.
Commercial boards use plated through hole technology to get around that but this is way beyond the home board brewer's capabilities.

The reason you need very accurate registration is very evident when you use IC pin patterns with small lands, maybe even tracks passing between the 'legs' and when using connectors. Single components allow larger pads which in turn can tolerate coarser aignment but that is not the case with SIL and DIL components.

You would do well to take up the suggestion above and let a commercial outfit do the etching and also the drilling unless you are willing to get a much better drilling set up than that which you described.
If you design your board with the software compatible to your nearest PCB manufacturing shop you can do the lot via e- mail.

Klaus

 

[andy257]

Hi m8,

I make double sided pcbs all the time and once you have the technique nailed its easy as pie. I use easypcb to do the job but the principles are the same no matter what software you use.

Design your circuit and get your artwork (side a, side b)

The trick i use is to develop one side (drill alignment holes) then develop the other. Etch both sides togther. Use the protective film to recover your exposed side so it doesnt become over developed in the tank while the other side is being developed.

Develop the first side, then remove and drill corner holes to align the two sides. If you have designed your art work correctly both artworks (side a side b) should align using the corner holes perfectly. To get the drill to locate bang on centre make the inner hole of the circle smaller than your drill size. This way the drill has something to use to locate the centre. You should do this with all holes on your pcb to ensure the hole is bang in the middle of the pad. I use like a .4mm or .5mm hole instead of your normal .8 or 1.0 hole in the artwork.

I dont bother with any form of sticky tape to secure the artwork, my light box seems good enough to hold it in place pretty well. You may wish to try some, its up to you.

To make the connections from one side to the other there are pins you can get which you plop into your connections and solder on both sides. They are really good and perfect size. They are almost flush with the pcb.

I am british so theres no point posting the part numbers i use, sorry.

https://uk.farnell.com/productimages/farnell/standard/42260597.jpg

Hope it helps

Andy

 

[inaPICle]

All I do, is print both sides on sheets, then line them up under a light, so they are perfect. Tape the edges of the sheets so that the stay perfect. Insert the copper clad between the sheets and iron on the toner (in your case, expose it.)

I like that idea and I may well try it.

The method I use at the monent is to add a dummy pad at each corner of the PCB layout. Then I apply the toner transfer to the top side of the board and drill the dummy pad out with a fine bit.

Then I stick the PCB on a light-table and use the pinpricks of light to line up the bottom side transfer paper which I tape down with laser printer labels. Then I run it through the laminator.

Since I exclusively use SMT devices on double layer board the bottom layer is usually fairly sparse and the layers are connected by vias. Since I can't through plate I stuff a piece of 24 guage wire in the via and solder top and bottom. I place the vias where I have room to fix the alignment errors on the bottom if something slips a little.

This allows for a lot more mis-alingment between the layers than using through hole components where all the pads on a DIP what to be spot on, top and bottom, or you can get shorts. For home made double sided boards the aligment problems are far larger with PTH than soldering issues with SMT parts.

And having said that, it has lined up perfectly every time so far. But one of these days I know I'll be grateful for the room to move.

inaPICle

 

[philba]

I, too, make double sided TT boards all the time. It took a couple of tries to get good at it but it's not hard at all. I did have touble aligning the two artwork pieces until I built a light box. It's just a box that hold two 40W light bulbs with a piece of frosted glass on top. Now aligning the art is a snap.

by the way, be careful about registration. With the pcb blank between the sheets, make sure that there isn't any movement of one sheet. I use a laminator and I have to work to feed the assembly evenly bewteen the rollers. If it's not in the "tangential" plane of the contact point of the rollers (too high or low), one sheet will be pulled before the other and cause mis-registration. It's usually not a big problem and most boards can still be etched and drilled but I'm trying to perfect my skill here.

On vias and non-plated through holes. yeah, it's the way it is. I use wires for vias and design my boards so that I make connections between layers at component leads that can be soldered on both sides of the board. I call the sort of thing TT Design Rules.

 

[mstechca]

I'm going to go give my first board a shot, and hope and pray to god my drilling is accurate.

It seems that a large number of you use holes for alignement, and that is what I will do too.

I will try to drill the alignment holes BEFORE any exposing or etching begins, (yes, this means I will drill through my transparency) and then I will align the circuit to the holes.

Let's hope and pray to god it works.

EDIT:

The first attempt was abandoned because of too many scratches in the artwork. (caused by me trying to fit the entire PCB in a small container)

The second attempt worked alright. The only thing that went wrong (minor) was that the alignment of the two sides was about 1mm off, and the holes didn't line up too well. But I am lucky, because I reserve space on my board for situations like these.

But then what can you expect from making a toy motor the ONLY source of a drill? :shock: :lol:
I probably should show a picture of it :lol:

Fortunately I managed to fit the IC's in, but not 100% straight. I had to bend some leads, but that is not a big problem.


Now I got another question.

Normally when I do single sided boards, I usually insert the components first and then solder them all in place.

What is the best (or the most recommended) way to solder components to double-sided boards, even if some tracks are on the component side of the board?

I'm afraid that if I do it wrong, I could end up with an open circuit somewhere, and I could risk breaking tracks.