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15.2.2009

Researching Toner Transfer method of PCB making (part II).

So, I have tried to do a bigger 10x15 cm PCB using the paper I was excited about in the PART I - HP Premium Photo Paper (glossy) - see here .

Unfortunately, when I tried to print such large board the laser printer drum transferred part of the picture from the upper part of the paper to the bottom part. Also, the toner does not stick good enough to the paper and spread all over the paper.

I have tried feeding the paper through the printer without printing anything on it in order to just fry it a little bit. It helped, the pcb picture printed well, but when I transferred the picture to the copper all filled areas were pitted! The funny thing is that the toner is fully transferred from the paper but the result is still full of pits. That means that photolayer is damaged and does not bond with toner anymore during printing.

So, if I preheat the paper I cannot do transfer correctly, if don't preheat I cannot print well. It is clear now, that the paper is no good for the TT. So, let's forget about HP Premium Photopaper.

I went to a shop and bought Lomond 120g/m2 glossy with improved coating (code 0102053) http://www.lomond.com

The first thing I tried is to do what I have done with HP paper - transfer the picture to copper and quickly peel away under cold water. I tried it several times but it does not work this way with this paper. In some places the toner comes off the board when I peel the paper off. I have left the iron temperature the same as for HP Premium Photopaper.

I transferred the picture and put the pcb into a warm water (50C). After 10 minutes it easily came off. However, the photolayer has stuck in all small gaps (0.2mm and smaller). I decided to ignore this problem just for now and went ahead with size measurements.

So, this paper looks very promising. However, another question came up: linear distortions during printing. To measure linear printer distortion I decided to use a scanner. It gives better resolution and almost no linear distortions of its own. To make sure the scanner does not introduce its own distortions I scanned and measured PCB one way then I tuned it 90 degrees and scanned and measured again.

The PCB is 100x150mm: photo

As you see, there is a circle in the middle of the board. The inner diameter is 15mm. Eventually I have measured horizontal diameter 15mm and vertical diameter 14.9mm

Sizes (the frame on the picture was made with padding, I have measured the frame):

Horizontal line: -0.6 mm, must be 79.6mm, on one side I got 79.0, on the other 79.0), error: 0.8%
Vertical lines: minus 1-1.4mm, must be 129.1, one side is 128.1, other side is 128.5, error 0,8%.

As you see the distortion is very small. Depending on your goal you can leave it alone as it is or scale the printed layout horizontally and vertically to compensate the error. Of course, you printer most likely has different error numbers, so you have you find it out for yourself.

The problem with photolayer in small gaps was solved pretty easily. I have put the pcb into a warm water once again (60C) and after 10 minutes I rubbed it heavily with a toothbrush. Almost all 0.2mm gaps fully cleared. Some of them still were clogged, but there were only a few of them, so it is easy and quick to clean them completely with a needle.

Cleaning of 0.1mm gaps is impossible. Nothing can get it from such small places without damaging toner around.

Now the main part. Track 0.09mm turned out to be 0.2mm, as in the previous time.

Overall result turned out to be same: min track is 0.2, min gap is 0.2 with some additional cleaning and 0.3 with any extra work.

Two sided pcb also went well. I ironed the first side for 5 minutes, turned the pcb and ironed the other side for 3 minutes. Everything turned out very good. All holes must be marked and drilling considering the distortion error.

I have done 5 PCBs 100x150 mm using this paper and got no problems or broken tracks 5 out of 5 PCBs.

Next step: print on preheated paper.

Step 1: print on the paper

I have fed the paper 2 (two) times through the printer. It curled up a bit, but that's okay. Print setup: 600x600dpi, transparent medium print mode (max toner density).

I have created a new test layout 100 x 150 mm, tracks from 0.05mm to 0.4mm, gaps from 0.1 to 0.4mm, frame to measure the distortions.

Tracks and gaps are put horizontally, vertically and at 45 degrees in different places of the layout.

To measure everything I scanned the pcb using 3200 dpi resolution, which take 7 minutes and tons of RAM.

Frame sizes in the layout:

Width: 79.61 mm
Height: 129.10 mm

Real sizes:

Width: 79.5 mm on one side, 79.7 mm on the other side (error 0.1%).
Height: 128.8 mm on both sides (error 0.2%).

Tracks:

nominal - horizontal - vertical - diagonal
0.05 - 0.09 0.09 0.08
0.10 - 0.13 0.15 0.13
0.15 - 0.17 0.24 0.21
0.20 - 0.23 0.28 0.24
0.25 - 0.27 0.31 0.25
0.30 - 0.31 0.34 0.32

Strangely the diagonal track of 0.25 came out just excellent.

Gaps:

nominal- horizontal - vertical - diagonal
0.1 - (0.08-0.14)* (0.15-0.10)** 0.16
0.2 - (0.18-0.23)* (0.20-0.23)** 0.23
0.3 - 0.33 (0.29-0.31)** 0.32
0.4 - (0.42-0.38)* (0.39-0.42)** 0.41
*upper gap and lower gaps
**left and right gaps 

I have turned the board 45 degrees and remeasured diagonals in horizontal position. All gaps matched (except 0.1)! So, if you need stable gap size - layout you tracks diagonally.

Step 2: Toner transfer

PCB preparation: polishing, cleaning with dish washing cleaner, scratching with usual household soda until water does not roll of the board. It is almost the same preparation as for photoresist method.

Transfer itself is done as described above. Then 10 minutes in warm water, peeling and toothbrush cleaning.

The first problem: low quality transfer. It did not happen with non-heat-preshrunk paper. With preshrunk paper the toner does transfer well to pcb. It stays on the paper in some places. However, it is not very bad. Not as bad as with HP paper, just one corner turned out bad. It could be that I have selected too low temperature or did not iron well.

Frame

Width: 79.05-79.20 error 0.5-0.7%
Height: 128.65 error 0.3% 

As you see all sizes became less because the paper have shrunk even more after ironing.

Tracks:

nominal - horizontal - vertical - diagonal
0.05 - broken- broken - broken
0.10 - 0.15 0.15 0.20
0.15 - 0.18 0.26 0.28
0.20 - 0.24 0.28 0.30
0.25 - 0.29 0.34 0.31
0.30 - 0.33 0.41 0.35

Gaps:

nominal - horizontal - vertical - diagonal
0.10 - (0.13- 0.16) (0.18-0.14) (0.17-0.18)
0.20 - (0.19-0.23) (0.21-0.25) (0.22-0.23)
0.30 - (0.32-0.34) (0.33-0.34) 0.32
0.40 - (0.36-0.42) (0.40-0.43) 0.43

Overall track quality became worse.

Results:

  • As I have said before, track of 0.1mm is impossible with TT. 0.15mm seems to be the absolute minimum and such tracks may not survive etching.
  • Heat-preshrinking paper helps to reduce size distortion but lowers the quality of tracks and gaps.

After that I was almost ready to give up TT and went ahead with photoresist. While I was digging photo resist method I figured out how to make TT better!

Here is the result:

These numbers are after toner transfer and before etching (measured by scanning at 3200dpi).

The paper is the same, the iron is the same, a little bit different chemicals and different hand moves.

Tracks:

0.1 came out as 0.12 , fully working, WOW! 
0.15 - 0.21 - this is my printer problem, it just does not like this size. 
0.2 - 0.25 - just a bit off, very good

Gaps

0.1 - still unreal
0.2 - 0.19 !!! fully clean! 
0.3 - 0.29 !!!
0.4 - 0.37 !!!

WOW! WOW! WOW! Das ist fantastisch!

I did not believe when I did it first time. I did another pcb - the result was the same! The main thing I figured is how to make the track edges very smooth, like in photoresist method. The result even looks pretty much like photoresist. I seems like it cannot be any better!

Here is the scan of the resulting PCB (3200 dpi): scan (1MB, 126 pixels per 1mm, count for yourself).

The resulting method is described here: Making PCB with toner transfer

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