84 The PCB Design Magazine • June 2016
have printing devices. For decades, plotters and
direct imagers have used raster-scan technol-
ogy, whose speed depends solely on image area
and resolution. Image content no longer dic-
tates throughput so same-size planes and signal
layers take exactly the same time to plot.
So, negative files no longer offer benefits.
On the contrary, they introduce some serious
disadvantages:
• While positive layers have clearly defined
limits, negative layers do not, so arbitrary limits
must be imposed.
• In a mixed data set, there is no standard-
ized method by which to define which layers
are negative, so manual reverse engineering is
necessary. When all layers are positive there is
no problem.
• Most importantly, negative layers do not
contain copper pour outlines, so these must be
created. This can be done by maintaining spe-
cific clearances from the profile, for example,
but this is still guesswork and reverse engineer-
ing. In positive layers, on the other hand, cop-
per outline is clearly defined.
How to define a positive copper plane:
G04 We define the antipads
%TF.AperFunction,AntiPad*%
%AD11C....*%
....
G04 We now define the extent of the copper
pour*
LPD*
G36*
X...Y...D02*
X...Y...D01*
...
G37*
G04 And now we flash clearances
%LPC*%
D11*
X...Y...D03*
....
This gives CAM clear and unequivocal infor-
mation which is robust, numerically accurate,
and the anti-pads will register with the drill
files. And it's as compact as a negative file, and
clearly defines the extent of the copper. Perfect.
There is no longer any benefit in transfer-
ring the data in negative. On the contrary: It is a
relic from the bad old days that adds confusion,
manual work, and risk.
Always use positive copper layers.
PCBDESIGN
This column has been excerpted from the
Guide to PCB Fabrication Data: Design to Fabrica-
tion Data Transfer.
Karel Tavernier is the managing
director of Ucamco.
Figure 3: A stroked plane layer.
Figure 4: A plane layer in negative.
THE GERBER GUIDE, CHAPTERS 13 & 14
