Method for making a multilevel polyimide stencil

A method of using a laser to cut a groove or pocket of predetermined depth of less than about 0.005 inches in a stencil comprising a polyimide sheet having a thickness in the range of 0.005 to 0.012 inches including the steps of mounting the stencil on a movable work table and positioning and maintaining a laser a predefined distance from the polyimide sheet above the work table. A laser beam is directed against the polyimide sheet to cut an indentation into said polyimide sheet. The laser beam has a pulse duration and a power level to make the cut into the polyimide sheet. The method further includes the step of directing a gas against the polymide sheet where the laser beam cuts into said polyimide sheet. The gas is pressurized. The depth of the indentation cut into the polyimide sheet by the laser is determined by choosing the pressure of the gas relative to choosing the pulse duration and the power level of the laser beam. The work table is moved relative to the laser whereby the indentation cut into the polyimide sheet creates a groove or pocket as the laser beam moves across the polyimide sheet. By controlling the pressure of the gas normally used to remove dross from the cut edges of the material, the cutting process can be better controlled so that laser milling of polyimide sheet is reliably obtained.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The present invention relates to a method for using a laser to cut an 
indentation into a polyimide film. In particular is relates to a method of 
using a laser to form pockets in polyimide stencils used in the 
application of solder paste for surface mount electronic assemblies. 
2. Description Of Related Art 
Surface mount technology is used to mount electronic components on the 
surface of printed circuit boards or substrates by soldering the 
components to one or both sides of a substrate. The first step in mounting 
surface mount components to a surface board is to screen print with a 
stencil solder paste on the board where the surface mount components are 
to be positioned. 
In the manufacture of stencils, surface mount land patterns referred to as 
footprints or pads arc cut from a stencil to define the sites at which 
components are to be soldered to a printed circuit board. It should be 
understood that the design of the land patterns is critical because it not 
only determines the solder joint strength but it also influences the areas 
of solder defects cleanability, testability and repair/rework. The 
accuracy with which the land patterns are cut out from the stencils used 
in the assembly of printed circuits has a direct bearing on the quality of 
the finalized product. It is important that the solder paste align with 
the location of the solder pad and it is necessary that the aperture or 
land patterns cut out from the stencil be accurate. The accuracy in 
combination with the minute size of the components used in surface mount 
techniques results in very small tolerances for error (in the order of 
0.0005 inches). The size of the openings cut into the stencil may be in 
the order of 0.01 inches in size or less. 
Chemical etching processes are commonly used to cut out the apertures to 
form the land patterns in the stencils. While etching processes are well 
known in the art, they typically involve placing a chemical resistive 
material over the metal stencil which has openings where the platforms or 
lands are to be located. Then an etching process etches out openings where 
the lands are located. Thereafter, the protective layer of plastic on the 
metal is removed from the metal stencil. 
Newer procedures have been developed to cut out land patterns in metal 
stencils using YAG lasers. These procedures are highly accurate and 
relatively expensive when one considers that the cost of purchasing a YAG 
laser is currently in the order of $100,000 to $200,000. Further, the 
operating costs of YAG lasers are relatively expensive. The YAG lasers 
typically have a beam focal path of sufficient power to cut through 
stainless steel stencils having a thickness of 0.005 to 0.012 inches. 
Consequently, it is important that the edges cut through the metal stencil 
remain constant. YAG lasers have proven useful in this application. 
Recently, a polyimide stencil has been introduced to the market that can be 
manufactured with a more cost effective low power CO.sub.2 laser as well 
the more expensive YAG laser. This polyimide stencil sold under the trade 
mark KEPOCH is described in detail in corresponding Canadian Patent 
application Serial No. 2,181,207 filed Jul. 15, 1996 by Keith C. Carroll 
and entitled "Polyimide Stencil for use in Electronic Assemblies and 
Method of Making Same". The polyimide stencil described in this patent 
application is for a single level stencil. 
While the use of lasers is now known for cutting both polyimide film and 
stainless steel stencils, it should be understood that the lasers are 
employed to cleanly cut through the stencil and form the openings in the 
stencils. To facilitate the laser cut, it is known to direct a gas under 
high pressure at the point where the cut is to be made. The gas, commonly 
compressed air, is chosen to be at a sufficiently high pressure to blow 
away any dross formation along the edges of the stencil with the apertures 
are cut. 
Metal stencils have been manufactured with multilevels of stencil thickness 
in addition to the through openings to accommodate selective printing 
which allows varying depths of solder paste to be deposited on the circuit 
board. Multilevel etching of a metal stencil is typically accomplished by 
chemical milling to first etch a large area, referred to in the industry 
as a "pocket", to a desired thickness for the components that require 
lower paste thickness. The pocket area is larger than the land pattern 
area of the component to prevent solder skipping and damage to the 
squeegee used in the printing process. These pockets are about 0.002 
inches deep in the stencil and are etched through chemical processes from 
the metal stencil so that the thickness of the stencil for fine pitch 
components is less than for larger components. The pocket formed in the 
mesh about the fine pitch component is an additional 0.1 inches. The 
pocket is formed first and then the rest of the stencil apertures are 
formed in a normal fashion which could include either chemical etching or 
laser cutting. Multi-level etched stencils have the advantage of allowing 
varying thicknesses of solder paste to be applied in one application. 
While etching of multi-level stainless steel stencils through chemical 
milling is known, chemical milling or etching of the polyimide material 
does not appear to be as easily attainable as chemical milling of 
stainless steel due to the manner in which the etching chemicals would 
attach the polyimide material. A discussion of chemical etching of 
polyimide film is discussed in "Accelerated Chemical Etching of 
Kapton.RTM. Polyimide Film" by J. A. Kreuz et all and presented at the IPC 
25th Annual Meeting of April 1982. This paper briefly describes that high 
energy laser beams can be used to cut precise holes. It also teaches a 
demand for this cutting in polyimide films; however, there is no teaching 
on how to use a laser beam to mill polyimide film. Accordingly, there is a 
need for a cost effective, reliable method for milling pockets in 
polyimide stencils to provide the advantages of both polyimide stencil and 
multi-level stencils. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for cutting 
indentations into a polyimide sheet that utilizes a laser. 
The present invention relates to a method for using a laser to cut an 
indentation of predetermined depth into a polyimide film. In particular it 
relates to a method of forming pockets in stencils made from a polyimide 
sheet used in the application of solder paste for surface mount electronic 
assemblies. 
The method of the present invention controls the beam of the laser by 
directing the laser beam against the surface of the polyimide sheet. The 
laser beam has its power and pulse duration determined relative to each 
other and, additionally, relative to the pressure of a gas directed at the 
polyimide sheet where the laser beam cuts into the polyimide sheet. The 
gas provides the dual functions of 1) facilitating a clean burn into the 
polyimide material by vaporizing dross formed during laser burning and, by 
controlling the gas pressure relative to controlling the laser beam power 
and pulse duration, 2) determining the depth of the indentation cut into 
the polyimide sheet. 
The polyimide stencil can be mounted on a work table that moves relative to 
the laser beam so that a groove or pattern is cut into the polyimide 
sheet. 
In accordance with one aspect of the present invention there is provided a 
method of cutting an indentation of predetermined depth in a surface of a 
polyimide film using a laser comprising the steps of: 
a) directing a laser beam from the laser against the surface of the 
polyimide film to cut into the polyimide film, the laser beam having a 
pulse duration and a power level; 
b) directing a gas against the surface of the polyimide film where the 
laser beam cuts into the polyimide film, the gas having a pressure; and, 
c) determining the depth of the indentation cut into the surface of the 
polyimide material by selecting the pressure of the gas relative to 
selecting the pulse duration and the power level of the laser beam. 
The polyimide film of the present invention has a thickness in the range of 
about 0.005 to 0.012 inches. The preferred polyimide films are KAPTON.RTM. 
and CIRLEX.RTM. (polyimides of DuPont). The polyimide materials suitable 
for use in the present invention should lend themselves to being cut by a 
low power laser. 
It is envisaged that the depth of the indentation cut into the polyimide 
film is less than about 0.005 inches. The maximum depth that the laser may 
cut into the material in accordance with the teachings of the present 
invention will vary depending upon the values of laser beam pulse 
duration, laser beam power, and the pressure of the gas and type of gas 
used. Consequently, if the depth of the indentation cut into the polyimide 
film is too shallow, more than one pass or beam pulse at that location may 
be required to increase the depth of the indentation cut. 
It should be clearly understood that the any one or more of the laser beam 
pulse duration, laser beam power, or gas pressure can be chosen such that 
a clean burn is obtained through the polyimide film. However, the present 
invention is not concerned with cutting cleanly through the polyimide 
stencil with a laser beam but with the ability of cutting pockets into the 
polyimide stencil so as to allow for a multi-level polyimide stencil to be 
manufactured entirely by laser processing. Typically, after the pockets 
have been formed in the polyimide stencil, the next step in the 
manufacture of the stencil is to cut out the land patterns by burning 
apertures through the polyimide stencil. 
In the preferred embodiment of the present invention the gas used is air 
under pressures in the range of about 0.5 to 3.0 Bar. Alternatively, the 
gas used may be nitrogen or oxygen. The pulse duration of the laser beam 
lies in the range of about 1 to 50 micro-seconds. The laser beam current 
is chosen to be in the range of about 50 to 150 milli-amps where the laser 
beam current is a function of beam power. Each of these three variables is 
chosen relative to the other to control the depth of the indentation cut 
into the polyimide film. 
It is envisaged that both YAG and CO.sub.2 lasers can be employed to 
perform the indentation cutting of the present invention in the polyimide 
film. In the preferred embodiment a CO.sub.2 laser is utilized because it 
is less expensive to use and the power requirements to cut into the 
polyimide film are low. 
In accordance with another aspect of the present invention there is 
provided a method of using a laser to cut a groove of predetermined depth 
of less than about 0.005 inches in a stencil comprising a polyimide sheet 
having a thickness in the range of 0.005 to 0.012 inches. The method 
comprises the steps of: 
a) mounting the stencil on a movable work table; 
b) positioning and maintaining a laser a predefined distance from the 
polyimide sheet above the work table and directing a laser beam against 
the polyimide sheet to cut an indentation into the polyimide sheet, the 
laser beam having a pulse duration and a power level corresponding to an 
average power level according to a beam irradiated area; 
c) directing a gas against the polyimide sheet where the laser beam cuts 
into the polyimide sheet, the gas having a pressure; 
d) determining the depth of the indentation by choosing the pressure of the 
gas relative to choosing the pulse duration and the power level of the 
laser beam; and, 
e) moving the work table relative to the laser whereby the indentation cut 
into the polyimide sheet creates a groove as the laser beam moves across 
the polyimide sheet.

DETAILED DESCRIPTION OF EMBODIMENTS 
Referring to the drawings the preferred embodiment of the present invention 
are described. Throughout the description of the preferred embodiment, 
reference will be made to cutting, or milling (cutting into), of the 
polyimide stencil material by the use of a CO.sub.2 laser. It should be 
understood that a YAG laser may be used however because the stencil of the 
present invention lends itself to being readily cut by low powered lasers, 
it is more economical to use a CO.sub.2 laser which cost is in the order 
of magnitude less expensive than a YAG laser. That is $100,000.00 for a 
YAG laser versus approximately $10,000.00 for a CO.sub.2 laser. 
Further, the preferred method of manufacturing the apertures in the 
multi-level polyimide stencil of the present invention is similar to the 
method described in the above-identified Canadian patent application of 
Keith Carroll, however, the method of the present invention differs in 
that method provides for pockets to be initially cut into the polyimide 
stencil prior to the apertures being cut into the stencil. It is the with 
the formation of pockets in the polyimide stencil to which the preferred 
embodiment of the present invention is directed. 
Referring now to FIG. 1 there is shown a forming apparatus or table 10 used 
to pre-fabricate a stencil ready for cutting by laser. The table 10 
includes a series of clamps 12. Any type of clamp may be utilized such as 
manual or automated clamps. The illustrated clamping arrangement 12 
comprises a stationary member 14 secured to frame 16 of table 10. Passing 
through the stationary member 12 is an adjustable threaded stem 18 which 
is secured at one end to clamp 20. Clamp 20 comprises a pair of opposed 
plates which are drawn towards each other by screws or bolts 22. The clamp 
12 is also provided with a nut 24 for securing the position of the plates 
20 relative to the stationary member 12. For purposes of clarity only one 
of the clamping members 12 has been labelled in FIGS. 1 and 2 of the 
drawings. 
Initially a piece of polyimide material is a cut as shown as 30 in the 
FIGS. The edges and the corners of the polyimide material are placed 
within clamps 12 at edge portions of the polyimide material. The clamps 12 
are then adjusted through rotation of securing nut 24 and stem 18 to draw 
the polyimide material 30 tight. The polyimide material used in this 
process is that sold on the market by DuPont under the trade-marks 
KAPTON.RTM. and CIRLEX.RTM.. KAPTON is used for thicknesses below 0.007 
inches and CIRLEX is used for thicknesses above 0.007 inches. 
It should be understood that the polyimide material used in the present 
invention has very little memory associated with it and the stretching or 
the tightening procedure described with respect to FIG. 1 is done for the 
purposes of ensuring that the polyimide material is flat with no ripples 
in the material. The thickness of this material is typically in the order 
of 0.005 to 0.012 inches. It is a translucent material. 
The next step is to place an aluminum frame 32 on the tightened sheet of 
polyimide material 30. 
Referring now to FIG. 2, the aluminum frame 32 has its opposing sides drawn 
in by clamp mechanisms 34 and 36. These mechanisms comprise two threaded 
rods 38 passing across and above the aluminum frame and being secured to 
blocks 40 by means of nuts (not shown). The rods are adjusted relative to 
the blocks 36 so as to compress the sides or draw in the sides of the 
aluminum frame relative to each other. Typically the sides of the frames 
may be drawn in as much as 0.080 inches. 
The aluminum frame 32 is secured to the polyimide sheet by means of an 
epoxy resin which may be applied to the edges between the inside of the 
aluminum frame 32 and the polyimide film 30 as shown for a portion at 42. 
It should also be understood that the epoxy may be applied to the aluminum 
frame surfaces that come into contact with the polyimide film. 
Typically the epoxy resin may take anywhere from a few hours to a few days 
to cure. During this curing time the table 10 maintains the relationship 
between the polyimide film and the aluminum frame. 
After the adhesive has cured, the next step is to remove the clamps 34 and 
36 and the clamps 12. This will cause the aluminum frame to return to its 
original shape and thereby ensure that the polyimide film 30 is secured 
tautly to the aluminum frame 32. It should be understood at this time that 
any excess materials for the polyimide film 30 extending beyond the frame 
32 may be trimmed by a simple cutting knife. 
At this stage in the process, a pre-fabricated stencil of polyimide film 30 
surrounded by aluminum frame 32 is formed (FIG. 3). The size of the 
aluminum frames can be anywhere from 8 inches to 29 inches and can be made 
depending on the various types of boards that will require printing using 
the stencil. By pre-fabricating the stencils to that state shown in FIG. 
3, the turn around time to complete an order is solely dependent on the 
cutting speed of the laser. 
The next step in the processing of the multi-level stencil of the present 
invention is to use a laser to cut out a depressed pocket in the polyimide 
film. The co-ordinates and areas of the pocket patterns to be cut into the 
polyimide stencil are fed to computer 50 (FIG. 4) which controls the 
operation of CO.sub.2 laser 52 and the positioning of computer numerically 
controlled (CNC) table 51 shown in FIG. 4 mounted relative to platform 56. 
It should be understood that CNC tables are known in the industry and the 
schematic representation of the table should be all that is necessary to 
understand the method of making the stencil of the present invention. The 
CO.sub.2 laser 52 is mounted upon a table or platform 56 and is provided 
with a helium tank 58, nitrogen tank 59 and CO.sub.2 tank 60. The CO.sub.2 
laser 52 generates from its laser head 62 a focal beam 64 which cuts into 
but not through the polyimide film 30 of the stencil in this step. The 
computer 50 controls the laser beam pulse duration, laser beam power, 
laser pulsing rate, and gas pressure of gas emitted against the surface of 
the polyimide film where the laser cuts into the surface. The CNC table 
typically moves at a speed considerably slower than the pulse rate of the 
laser beam and hence the speed of movement of the CNC table relative to 
the pulse rate is not a significant factor in determining the depth of cut 
into the polyimide stencil. The distance 56 between the laser head 62 and 
the stencil polyimide film 30 is maintained constant by a mechanical and 
optical sensing system (not shown) working in conjunction with the 
computer 50. This maintains the beam power level at an average power level 
to the area of the polyimide sheet irradiated by the beam as the beam 
moves across the sheet. 
In accordance with the teachings of the present invention, the depth of the 
indentation, groove, or pocket 63 cut into the polyimide stencil is 
determined by choosing the pressure of the gas relative to choosing the 
laser beam pulse duration and the laser beam power. The power is related 
to the laser beam current and the value of laser beam current can be 
controlled relative to beam pulse duration and gas pressure. It has be 
determined that by varying the gas pressure the cut into the polyimide 
film can be controlled. In particular, the depth of the indentation can be 
cut up to about 0.005 inches in a polyimide film having a thickness in the 
range of 0.005 to 0.012 inches. The depth of the cut can be controlled by 
choosing the values of laser beam pulse duration, laser beam power and gas 
pressure in accordance with the following relationship: 
##EQU1## 
where "d.sub..delta. " represents the depth of the indentation, 
"It" represents laser current, is proportional to laser power and is in the 
range of 50 to 150 milli-amps, 
".delta." represents pulse duration of the laser beam and is in the range 
of 1 to 50 micro-seconds, 
"P" represents air gas pressure and is in the range of 0.5 to 3 Bars, 
".delta.1" and ".delta.2" are constants associated with pulse duration and 
are respectively 40 and 30, 
"k" represents a constant associated with the laser being used and is 
1.909.times.10.sub.3 .pi., and 
"c" represents a constant associated with the polyimide material and is 
6.67.times.10.sup.-4. 
It should be understood that the constants will change for differences in 
the characteristics of the polyimide films used and the type of lasers 
used. 
FIG. 6 illustrates graphs of the relationship of the depth of the 
indentation cut into the polyimide film vs. changes in laser beam pulse 
duration for selected values of laser beam current power and gas pressure. 
The values of the graphed curve C1 to C4, D1 to D4 and E1, the pulse 
duration can be adjusted to vary the depth of the cut. Further, the effect 
of varying gas pressure on depth of penetration can be seen in the 
relationships between curves C1 to C4 and D1 to D4. These curves show that 
as the gas pressure increases the curves move up. The effect on the 
changes of beam current (power) on the depth of the indentation cut is 
shown between curves C1-D1-E1, C2-D2, C3-D3, and C4-D4. The relationship 
between these curves shows that as the current/power increases the curves 
move to the left effecting pulse duration. Clearly the curves show the 
major effect that controlling the gas pressure of air has on controlling 
the cutting depth into the polyimide material. 
After the laser milling or indentation cutting step as shown in FIG. 4, the 
resultant stencil is shown in FIG. 5 and is labelled 70. For the purpose 
of illustration only, an indentation is shown at 270, a groove is shown at 
272, a pattern of contiguous lines/grooves with the beam drawn over the 
lines at 274. The use of the polyimide stencil lends itself to cutting by 
low powered lasers such as CO.sub.2 laser. 
After the pockets have been cut into the polyimide stencil, the next step 
is to cut the land opening patterns through the stencil. The coordinates 
for the land openings pattern on the stencil are fed to computer 50 (FIG. 
7) which controls the operation of CO.sub.2 laser 52 and the positioning 
of computer numerically controlled (CNC) table 51 shown in FIG. 7 mounted 
relative to platform 56. The CO.sub.2 laser 52 generates from its laser 
head 62 a focal beam or cutting beam 64 which cuts through the polyimide 
film 30 of the stencil. The apertures cut into the stencil are shown at 
66. These apertures correspond to land patterns for surface mount 
technology used in the manufacture of printed circuit boards. 
After the laser cutting step shown in FIG. 7, the resultant stencil is 
shown in FIG. 8 and is labelled 70. The use of the polyimide stencil lends 
itself to cutting by low powered lasers such as CO.sub.2 laser. The 
polyimide stencil is also to be cut relatively quickly compared to the 
cutting of stainless steel stencils. As a result, the process lends itself 
to pre-fabrication and follow up by laser cutting which means there can be 
a substantially quick turn around time associated with the manufacture of 
stencil 70 once an order is placed. 
The next step in the process is to clean the dross from the stencil which 
has occurred from the cutting process. It should be understood that the 
polyimide material does not have much dross since the scum built up by the 
cut evaporates for the most part leaving a relatively clean burn. Any 
dross formation left is easily removed as shown in FIG. 9 by the 
application of a solution 72 to the stencil. The solution 72 simply 
comprises a water based solution including a mild detergent. Once the 
detergent is applied to the surface of the stencil it is rubbed off with 
the use of a hand cloth. The cleaning step of the polyimide stencil is 
very quick and easy to accomplish compared to the much harder step of 
electro-polishing using acids for stainless steel stencils. 
It should be understood that various alternative embodiments may be readily 
apparent to a man skilled in the art in view of the teachings as set out 
here and above.