Some Background on Drill Files

Modern drill machines and CAM software rely on industry standard commands that control the format, units and tool selection/drill diameters. The default settings for drill tape from APD/Allegro does not include a machine readable header or advanced control comands. We ran the Allegro NC Tape with default settings for a IC package and here is what we got:

COMMAND                                   COMMENT

;LEADER: 12                                holdover from paper tape!
;HEADER:                                   holdover from paper tape
;CODE  : ASCII                             other formats are now obsolete
;FILE  : ncdrill1 for TOP and BOTTOM       useful as check
;Holesize 1.  =  12.0 PLATED MILS          comment for the drill operator
;Holesize 2.  =  35.0 PLATED MILS          this info should be in the tool select
;Holesize 3.  =  37.0 PLATED MILS          command
;Holesize 15. =  52.0 NON_PLATED MILS      non plated through holes should be
;Holesize 16. =  63.0 NON_PLATED MILS      in a separate drill file!
;Holesize 17. =  95.0 NON_PLATED MILS
;Holesize 18. = 128.0 NON_PLATED MILS

G90                                         this is the first machine readable command
                                            it indicates that the coordinate data is
                                            absolute; G91 would indicate incremental

X02015Y01090                                drill coordinates
X03625Y01479                                it is up to the operator to insert the
X03704Y01794                                the proper drill (i.e. one of diameter 12.0 mils)
X01620Y00935
X02595Y01285
X02572Y01360
.
.
X01054Y00640
X00927Y01197
M00                                         M00 halts the machine. This is only needed on a 
                                            single spindle machine where the operator then must
                                            put a new drill into the machine. (When were PCB's
                                            last drilled like this?)

X00675Y01805                                more drill coordinates; let's hope the right drill
X00775Y01755                                tool was loaded by the operator.
X00675Y01705
X00775Y01655
M00

and so on

M30                                          program end

The default APD settings always output a drill file in INCHES even if the design is in metric.

Specific Missing Data

So what should the file include that is currently missing?

Data Units - it is important to have a machine readable command that specifies whether the data is in units of mm or inch. The command is very simple:

INCH
METRIC

It is also possible to use M71 and M72 which are recognized as commands for METRIC and INCH respectively.





Data Format - like Gerber data, drill data has a "format." Since the decimal points in drill data are routinely eliminated one must know where to re-insert the decimal point in order to properly read the file. Note that the data format for the drill file does not always match the format used for the Gerber file (even though the data from the two different formats should line up very precisely.) Unlike Gerber data, there is no data format command. Excellon indicates in their operating manual that for inch data, the format is 2.4 and that for MM units there are three possible formats:

INCH
2.4 - 00.0000

MM
3.2 - 000.00
4.2 - 0000.00
3.3 - 000.000

However their document does not specify how this should be entered into the drill file header.





Zero Suppression - it is common to either delete (i.e. suppress) the leading zeros or the trailing zeros in both drill and Gerber files. Since the decimal point is missing, you need to know both the data format (see above) and which side to count from when re-inserting the decimal point. What is unusual here is that for drill files the command LZ means leading zero's are included and TZ means trailing zeros are included. In Gerber the L indicates that leading zeros are removed. The LZ and TZ are generally put on the same line as the UNITS command.

INCH, LZ
INCH, TZ





Tool Select - this is by far the most important command that is missing. Instead of putting the drill hole diameters as a comment and then stopping the program after each set of coordinates the output should use a tool select command.

The tool select command, T includes a tool number and should include the diameter defintion. The "C" in the tool command defines the diameter of the drill.

T01C0.050

indicates that this is tool 01 and it has an 0.050 diameter drill. Tool index can range from 1 to 99. Note that tool diameters are specified in inches (if the units=INCH or in MICRONS if the units=METRIC. If in inches you must use a decimal point.

A Proper Drill File

How should a proper drill file look?

COMMAND                                   COMMENT

;FILE: ncdrill1 for TOP and BOTTOM          useful as check
M48                                         indicates start of header information
INCH,LZ                                     indicates the units in inches with Leading Zero Included
                                            suppressed 


%                                           indicates start of drill data
G05                                         drill mode (as opposed to route mode)
G90                                         absolute data
T01C0.012                                   selects tool #1 and diameter = 0.012 inch
X02015Y01090                                drill coordinates
X03625Y01479                               
X03704Y01794                               
X01620Y00935
X02595Y01285
X02572Y01360
.
.
X01054Y00640
X00927Y01197

T02C0.035                                   selects tool #2 of diameter 0.035 inch
X00675Y01805                                more drill coordinates
X00775Y01755                               
X00675Y01705
X00775Y01655
T03C0.037                                   selects tool #3 of diameter 0.037 inch

and so on

M30                                         program end w rewind ... 

BTW - never mix plated through holes and non plated through holes in the same drill file. The two different types of holes are drilled at different times during the manufacturing process.