Homebrew PCBs

Hello Mark,
                  I am an ex printer technician and I have been toying with 
the concept of direct printing with a laser for the last couple of days.

What concerned me is that the conductivity of the copper on the PCB was 
likely to cause the image on the PCB to be blurred or poorly focused along 
the Y axis.

In normal operation with paper the image is transferred from the drum to the 
page by a single line (wire) across the X axis. Manufacturers go to some 
trouble to make this wire as thin as possible and ground two parallel lines. 
This improves Y focus and I expected this would become a problem due the 
conductive copper coating.

I am very interested to hear what your experiments are revealing.

If you are using the image drum and you have the above problem then you need 
to provide shielding above and below the point on the image drum that is 
closest to the PCB.

Ideally the shielding voltage should be further away from the from PCB 
voltage than the image drum itself. The easiest way to achieve this is 
probably have a divider on the HT side and upping the actual HT.

I am a bit rusty on the theory so please let me know if this is the problem 
and I will do my revision and get back to you with a mode detailed 
information.

If this is simply a fusing problem, ie the image is fine on the PCB then 
there could be a number of causes.

Firstly there are two types of fuser today and the laminator may be the more 
modern type.

The older type used a heated roller and often this roller had a soft 
coating, this type is ideal for what you want as it will provide an even 
pressure to the toner. It will also work better at a lower temperature 
(desirable) as it will have a greater surface area of physical contact.

The older type that had a solid roller is less suitable as the two hard 
surfaces together will cause uneven pressure and the reduced surface contact 
will require more temperature which in turn increases the probability of 
blurring.

Fuser temperature needs to be accurate. All fusers have a temperature sensor 
and are closely regulated.

The older type where good but had two problems for normal printer use.
1) Heating a whole roller took a lot more energy.
2) The complete roller has far more thermal mass and it takes time to get 
the higher thermal mass to an accurate temperature. This caused long delays 
before the printer could print its first page after power on or standby and 
this delay is an important spec to manufacturers.

The newer type works completely differently. The roller is somewhat 
flattened on the side that contacts the paper and does not revolve. Around 
this roller is a film of thermally conductive material that can revolve 
around the roller to allow the paper free movement. There is thin strip 
heating element below the thermally conductive film that can change in 
temperature very rapidly doe to it's low thermal mass.

I doubt this newer type is any good for what you want for two reasons -
1) It is not very pliable and would probably cause uneven pressures.
2) It is not very robust and I would expect the edges of the PCB to wear 
into it very quickly.

The older type with the hard roller is probably not much good either due to 
uneven pressure and higher required temperatures.

The older type with the soft roller would be ideal.

I don't know what is in the laminator you have. Or even if that have any 
form of temperature regulation. Very crude regulation could be done with a 
light dimmer but two warnings on this. Don't even try it if you are not 
experienced with electrical things or you have no safety breakers on you 
power supply (RCD, CBR) etc. And The power requirements for the laminator is 
likely to be several hundred watts and most light dimmers do not go up that 
far.

Some other thoughts I have been thinking are to do away with the image 
transfer all together. Perhaps the image unit could still be used to provide 
an even coating of toner to the complete surface of the PCB. The new scanner 
would be installed with a high powered laser and an optical attenuator 
placed in the path for the syntonisation sensor to prevent damage to it by 
the higher power.

It idea is that the scanners laser provide the power to fuse the toner to 
the board and rest of the toner is just removed by blowing it a way.

I haven't the math for it yet so I don't know what power laser would be 
required.

The power to the toner at any point is the product of the laser power and 
the time. The faster the scanning the lower the effective power. However on 
the other hand is the scanners need for speed. The scanner accuracy is 
dependent on its inertia which is the product of rotational mass by the 
rotational speed squared. So obviously there are limits that will dictate 
the minium laser power required.

Thanks.

----- Original Message ----- 
From: "Mark Lerman"

Well, I'm not doing toner transfer, actually, I'm using a modified
laser printer to print directly on the board - the toner is still in
powder form, so it is relatively easy for it to smudge. I suspect
I'll have to use a hot plate and directly compress the board between
two plates using a teflon sheet between the toner and the top plate.
The laminator caused only a little smudging, but it was my first
attempt and I used the carrier. Perhaps just a teflon sheet will
allow use of the laminator. We'll see.

Mark