Rule of thumb. If you would you would not be happy with this kind of material on the surface of whatever you are coating, remove your finger prints.

• So a very thin coat of paint is 0.02mm, but often thicker.  So wash your hands, then it’s probably OK to touch. But don’t “Pizza and Paint”.
• Thermal paste on your CPU is 0.07mm  to 0.12mm thick, So Pizza hands are a definite problem. A single normal finger print may well produce a small but noticeable effect to cooling. A well handled CPU with a few prints is probably not good, you would not leave scraps of plastic wrap on the CPU after all.
• Some glues may dilute your finger print and not care (super glue). While other (silicone adhesives) will probably bond poorly.
• Solder – The NASA soldering standards (NASA_STD_8739 .3) state: “When handling metal surfaces that are to be soldered is unavoidable, clean, lint-free gloves or finger cots shall be used. “. Many commercial soldering standards also follow this advise for handling of both solder and parts.
  • Whats interesting is that some manufactures are particularly concerned about what is on your hands (moisturisers and hand lotions cited as being particularly problematic to solder).
  • The finger print contamination of solder joints is often resolved by just keeping the joint hot until the solder takes. This extra heating burns of enough contaminate to allow solder to flow, but can damage components amongst other problems.
  • UPDATE: The new IPC standards (IPC-J-STD-001ES), which NASA, and many other companies, have adopted, don’t mention fingers or gloves specifically. They only broadly reference that handling mechanisms stall not contaminate the board or parts.

I spent a lot of time calculating track widths to make sure my circuit board could handle the current I was using.
Then I had to do more calculations for font ratios. so that that all lines were wide enough to work with the silk screening process.

Sick of this; I quickly used matlab to create a few charts so I could look up the answers quickly.
I thought I would share these charts and the matlab scripts, hope they are useful.

Font Ratio Chart

To my experience, In Eagle Cad (and others) font ratios for silk screens work best using vector fonts. The cam processor, other tools, and factory will often use vector fonts regardless; – so using vector fonts tends to keep things compatible. In the text properties dialogue you must set the ratio correctly so that (Size * Ratio) > “Factory Minimum Line Width”.  To make it worse, fonts are best wrangled using Mils (1/1000th of an inch [what’s an inch]).

Typically in Eagle Cad, I would set up text for silk-screening as follows.

Anyway, here is the chart

To use it:

• Find the colour that corresponds to your manufactures minimum silk screen resolution.
• Find your font height in the x-axis
• The corresponding ratio is given in the y-axis.

PCB Track Width Chart

Track width is related to a lot of factors (acceptable temperatures. how insulated the tracks are from the air, current, acceptable power loss, etc). For anything complicated (internal PCB layers, high currents/voltages, installation in a vehicle/ heater / oven) go do proper calculations!

Looking for a good track width calculator, I found this and like it a lot:  http://www.4pcb.com/trace-width-calculator.html

99% of the time, as a hobbyist, you just want an external PCB track that does not raise in temperature by more than 10ºC. You would be using either 0.5, 1 or 2 oz copper tracks (eek, more imperial units). So I implemented the formula presented on the calculator mentioned above in matlab.  Then I plotted 3 lines, one for each common copper thickness, creating charts that seem correct for normal hobbyist type situations.

Four charts follow metric and imperial versions of high and low current situations. Find the graph that suits you and keep it handy.

Disclaimer: Your mileage may vary. A) I may be wrong, and accept no liability for that. B) Silkscreens, lacquer, protective coatings, hot electrical components may invalidate these figures.  If your doing anything medical / military /  safety critical / mass produced / potentially dangerous; this page is not an appropriate source of information, go find an engineering book, or something peer reviewed.

Metric – High Amps

Metric – Low Amps

Mils (Imperial)- High Amps

Mils (Imperial)- Low Amps

Source Code

Fonts

%----------------------------------------------------------------------------------%
%                                    BUSYDUCKS.COM                                 %
%                            Making you pro-duck-tive                              %
%                                                                                  %
%  Author: Duckman   Date: 10/3/15   Ver: 1.0   Licence: Creative Commons (by-sa)  %
%                                                                                  %
%  Calculates silckscreen font ratios.                                             %
%  Compile with Matlab / Possibly Octave                                           %
%                                                                                  %
%  Permision given to freely copy/paste "code snippets" into your own code. For    %
%  other uses (e.g. derivative works) the Creative Commons Attribution Share-      %
%  alike license applies (cite busyducks.com). This means commercial use is ok.    %
%----------------------------------------------------------------------------------%
s = [10 : 5 : 100];
r = zeros(7, max(size(s)))

for x = 2:8
    r(x-1,:) = ((x*100) ./ s)';
    plot(s, r);
end

plot(s, r);
title('PCB Silk Screen Font Ratios')
xlabel('Font Height (Mils)')
ylabel('Font Ratio (%)')
legend('2 Mil', '3 Mil', '4 Mil', '5 Mil', '6 Mil', '7 Mil', '8 Mil');
grid on

Track Width

%----------------------------------------------------------------------------------%
%                                    BUSYDUCKS.COM                                 %
%                            Making you pro-duck-tive                              %
%                                                                                  %
%  Author: Duckman   Date: 10/3/15   Ver: 1.0   Licence: Creative Commons (by-sa)  %
%                                                                                  %
%  Calculates Track widths.                                                        %
%  Compile with Matlab / Possibly Octave                                           %
%                                                                                  %
%  Permision given to freely copy/paste "code snippets" into your own code. For    %
%  other uses (e.g. derivative works) the Creative Commons Attribution Share-      %
%  alike license applies (cite busyducks.com). This means commercial use is ok.    %
%----------------------------------------------------------------------------------%
function plotTraceWidth (metric, small)
%Formula from: http://www.4pcb.com/trace-width-calculator.html
%sample usage: plotTraceWidth(true, false);  
%will save a .png to the current working directory;  

amps = [0.1 : 0.1 : 10];
thickness = [0.5, 1, 2];

if(small)
    amps = amps * 0.2;
end

%external layers, IPC-2221
 k = 0.048;
 b = 0.44;
 c = 0.725;
 tempRise = 10; %deg C

factor = (k*tempRise^b);

for it = 1:3
    t = thickness(it);
    area = amps./factor;
    area = area.^(1/c);
    width(it,:) = (area./(t*1.378))';
end

if (metric)
    %convert to metric
    mmPerMil = 0.0254;
    width = width .* mmPerMil;
end

plot(amps, width);
title('PCB Track Width')
xlabel('Amps')
legend('0.5 oz', '1 oz', '2 oz', 2);

name = 'track_width';
if (metric)
    ylabel('Track Width (mm)')
    ticks = [1:15];
    name = [name '_metric'];
    if(small)
        ticks  = [0.2:0.2:2];
        name = [name '_small'];
    end
else
    ylabel('Track Width (Mils)')
    name = [name '_mils'];
    ticks = [50:50:2000];
    if(small)
         ticks = [10:10:200];
        name = [name '_small'];
    end
end
set(gca,'Ytick', ticks)
grid on
saveas(gcf, [name '.png']);