Pen body exhibiting opposing strain to counter thermal inward strain adjacent flex circuit

An inkjet pen is formed by a pen body, flex circuit and silicon substrate. The pen body includes a first arm and a second arm upon which the substrate rests. The flex circuit is attached to the substrate and to regions of the pen body. Contact between the substrate and the arms define fulcrum points for the arms. The first arm and second arm are forced to rotate about their respective fulcrums during thermal expansion and thermal contraction. During cooling, the rotational force counters a contractive force along the pen body region contacting the flex circuit. The countering force reduces the degree of contraction by the pen body. Without such counter-force component the pen body contracts more than the flex circuit imposing a compressive tension of the flex circuit. With such counter-force, the compressive tension is reduced or eliminated. As such compressive force was the source of buckling in prior configurations, circuit, such buckling source is eliminated.

BACKGROUND OF THE INVENTION 
This invention relates generally to inkjet pen body construction, and more 
particularly, to thermal dynamics of an inkjet pen formed by a pen body, 
silicon substrate and flex circuit. 
Conventional inkjet printers include inkjet pens which eject ink drops onto 
a print media to form characters and other symbols. Ink is stored within a 
pen body and emitted through nozzles in a printhead. The printhead is 
formed by a silicon substrate and laminated flex circuit. The substrate is 
attached to the pen body and defines nozzle chambers. The flex circuit is 
attached to the substrate and the pen body. The flex circuit includes 
nozzle openings through which ink drops are ejected. 
During construction of an inkjet pen, the components undergo a wide 
variation in temperature. Typically a flex circuit is first adhered to the 
silicon substrate via a barrier film. Portions of both the flex circuit 
and substrate then are adhered to the pen body. To adhere the portions of 
the flex circuit to the pen body, the adjacent structures are heated to 
approximately 130.degree. C. The elevated temperature cures a structural 
adhesive bonding the flex circuit to the pen body. After curing, the pen 
is cooled to room temperature. 
A problem experienced during the construction of an inkjet pen is that the 
flex circuit tends to buckle and/or delaminate from the pen body as the 
pen is cooled to room temperature. Such buckling and delamination is a 
result of differences in thermal coefficients of expansion ("TCE") among 
the pen body, flex circuit and substrate. Typically the pen body is made 
from plastic, the flex circuit is made from polyimide, and the substrate 
is made from silicon--three different materials with three different TCE 
values. Specifically, as the structures are heated, the local area of the 
pen body expands more than the local area of the flex circuit. Once the 
adhesive cures, the relative positions of the pen body and flex circuit 
are fixed. During subsequent cooling, the local area of the pen body 
contracts more than the local area of the flex circuit. Thus, the flex 
circuit is left under a compressive tension. Either immediately or over a 
period of time, the tension causes the flex circuit to buckle. The 
buckling, in turn, causes delamination. Accordingly, there is need of a 
construction which avoids or counters undesirable thermally-induced 
tensions. 
SUMMARY OF THE INVENTION 
According to the invention, an inkjet pen body defines fulcrum points 
causing a rotational motion during thermal contraction which counters a 
compressive motion acting on an attached flex circuit. An inkjet pen is 
formed by the pen body, flex circuit and a silicon substrate. The 
substrate and flex circuit are attached to the pen body during 
construction. According to one method, the component temperatures are 
elevated during construction so as to cure an epoxy. Once cured the flex 
circuit and substrate are fixed relative to the pen body. Because the pen 
body has a significantly higher thermal coefficient of expansion, the pen 
body contracts more than the flex circuit and substrate during cooling. 
Because the flex circuit is flexible, buckling has occurred in 
conventional pens at the contact between the pen body and flex circuit. 
Specifically, because the flex circuit and pen body area are attached, the 
greater compression by the pen body imposes a compressive tension at the 
adjacent flex circuit. 
According to one aspect of the invention, the contraction of the pen body 
along the contact region with the flex circuit is reduced to reduce or 
eliminate the compressive force imposed on the flex circuit. 
According to another aspect of the invention, the contraction of the pen 
body along the contact region is reduced by a rotational force which has a 
force component opposing the pen body's contractive force. As a result, 
the net force imposed by the contracting pen body onto the adjacent flex 
circuit is reduced or eliminated. 
According to another aspect of the invention, the pen body defines an 
opening into which the substrate is placed during assembly. The substrate 
rests on a first arm and a second arm portion of the pen body. A contact 
location between the substrate and first arm defines a fulcrum for the 
first arm. A contact location between the substrate and the second arm 
defines a fulcrum for the second arm. The substrate is a substantially 
rigid body which expands and contracts less than the pen body. As a 
result, the arms are forced to rotate about the fulcrum during heating and 
cooling stages of the pen construction process. Specifically, the first 
arm and second arm rotate during the cooling stage in a manner imposing a 
component force in the region adjacent to the flex circuit which partially 
counters the pen body's contractive force. 
According to another aspect of the invention, the reduced contractive force 
approximates the contractive force of the cooling flex circuit. Thus, the 
pen body and flex circuit once attached contract to a similar degree in 
their contact region during cooling. As a result, the compressive tension 
acting on the flex circuit in prior pens is reduced or eliminated. 
One advantage of the invention is that the reduced or eliminated 
compressive tension is insufficient to cause buckling. By eliminating 
buckling, a significant source of flex circuit delamination is eliminated. 
A meritorious effect is that fewer pens are found to be defective during 
construction and later use. Another meritorious effect is that flex 
circuit openings remain aligned with the underlying nozzles during an 
extended life of the pen. 
These and other aspects and advantages of the invention will be better 
understood by reference to the following detailed description taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
Flex Circuit Buckling in Conventional Inkjet Pens 
FIG. 1 shows the printhead region of a conventional inkjet edge-feed pen 
10. The term edge-feed refers to feeding ink to an edge of the printhead. 
The pen 10 includes a pen body 12, silicon substrate 14, and flex circuit 
16. During operation, ink flows from into channel 18 and through to 
printhead nozzles 20. The nozzles are defined by the silicon substrate 14 
and flex circuit 16. When a nozzle is fired, ink is ejected through a 
nozzle opening 22 to a print media sheet. 
During construction, the flex circuit 16 is attached to the silicon 
substrate 14. The flex circuit 16 and substrate 14 then are attached to 
the pen body 12. As described in the background section, the pen body 12, 
substrate 14 and flex circuit 16 are heated to cure an adhesive (e.g., an 
epoxy). The thermal dynamics occurring during cooling generate compressive 
forces along the plane of the flex circuit 16 in the locally attached 
areas 24-26. During cooling or at some later time the compressive forces 
cause buckling in the flex circuit 16. In such instances, the compressive 
forces also are referred to herein as "buckling forces". The ensuing 
buckling typically leads to delamination of the flex circuit 16. FIG. 1 
shows buckling at reference number 28 and delamination at reference number 
30. The problem is particularly significant in instances in which the 
buckling 28 and/or delamination 30 does not occur immediately, but occurs 
later during packaging, distribution or use of the pen 10. 
Inkjet Pen Structure for Countering Buckling Forces 
FIG. 2 shows an exploded view of an inkjet edge-feed pen 40 according to 
one embodiment of this invention. The pen 40 includes a pen body 42, 
silicon substrate 44 and flex circuit 46. The pen body 42 is formed of a 
rigid plastic material, (e.g., polysulfone). Preferably, the pen body 42 
is monolithically defined by plastic mold injection. Alternative 
processes, shapes and constructions also are used. The silicon substrate 
44 is a rigid body fabricated using integrated circuit technologies to 
define printhead nozzles, firing resistors and conductive leads. The 
printhead nozzles define nozzle chambers 41. The flex circuit 46 is a 
flexible member, which in one embodiment is formed from a base material 
made of polyimide or other flexible polymer material (e.g., polyester, 
poly-methylmethacrylate), along with conductive paths made of copper, gold 
or other conductive material. The flex circuit 46 with the base material, 
conductive paths and bonding slots is available from the 3M Company of 
Minneapolis, Minn. The flex circuit 46 includes a nozzle openings 47. 
The pen body 42 includes a planar surface 50 defining an opening 52. A 
first arm 54 and a second arm 56 define walls within the opening 52. Each 
arm 54, 56 has openings for allowing ink to pass. To maintain rigidity the 
arms 54, 56 are ribbed in some embodiments. The first arm 54 and second 
arm 56 are fixed relative to each other at a distal region 57 (See FIG. 
3). In one embodiment the fixed region 57 embodies a "living hinge" which 
allows the two arms 54, 56 to rotate relative to each other. In another 
embodiment the two arms 54, 56 are separate but held together to define 
the region 57 about which the arms 54, 56 can rotate. Each arm 54, 56 
extends from the region 57 to or in the proximity of a contact region with 
the flex circuit 46. 
Referring to FIG. 3, a ledge 58 and barrier 60 are defined along the first 
arm 54, and a ledge 62 and barrier 64 are defined along the second arm 56. 
The silicon substrate 44 rests on the ledges 58, 62 and is bound by the 
barriers 60, 64. During assembly the silicon substrate is attached to the 
ledges 58, 62 and barriers 60, 64. Preferably, the substrate 44 fits 
snugly to the ledge/barrier seats. In one embodiment, the barriers are 
spaced slightly closer than the length of the substrate 44. Thus, during 
assembly, arms 54, 56 are spread to fit the substrate 44 in place. In a 
preferred embodiment, the substrate 44 extends from the ledges 58, 62 
approximately flush to the opening 52 at the planar surface 50. The flex 
circuit 46, thus, is able to define a plane adjacent to the planar surface 
50 and the outer edge 68 of the substrate 44. 
Respective chambers 70, 72 are defined at each side of the silicon 
substrate 44. Chamber 70 is bound by the flex circuit 46 at one boundary, 
by a wall portion 74 at an opposing second boundary, by a surface 76 of 
substrate 44 at an adjacent third boundary, and by a wall portion 78 at an 
adjacent fourth boundary. Similarly, Chamber 72 is bound by the flex 
circuit 46 at an one boundary, by a wall portion 80 at an opposing second 
boundary, by a surface 82 of substrate 44 at an adjacent third boundary, 
and by a wall portion 84 at an adjacent fourth boundary. 
A manifold chamber 88 is defined by the arms 54, 56 and the undersurface 86 
of the substrate 44. Ink flows from a storage reservoir (not shown) in the 
pen body 42 through openings 53, 55 in arms 54, 56 into the manifold 88. 
Ink flows from the manifold 88 through channels into chambers 70, 72. The 
ink then flows from the chambers 70, 72 into respective nozzles. Sidewalls 
89 (See FIG. 2) of the pen body 42 are corrugated in one embodiment 
inducing little constraint on the arms 54, 56. Alternative, designs and 
shapes are used in other embodiments to minimize side-wall or other 
constraint on the arms 54, 56. 
The mating of the substrate 44 to the respective ledge/barrier combinations 
along the first arm 54 and second arm 56 generates a pair of fulcrums. 
More specifically, a fulcrum line 90 occurs where ledge 58 and barrier 60 
meet and a fulcrum line 92 occurs where ledge 62 and barrier 64 meet. 
These lines 90, 92 also correspond to the lines where substrate surfaces 
76 and 86 meet and where substrate surfaces 82 and 86 meet. Arm 54 levers 
about the fulcrum 90. Arm 56 levers about the fulcrum 92. 
Each arm 54, 56 has a thick portion adjacent to the fulcrum 90, 92 which 
thins along the arm toward the fixed portion 57. Each arm 54, 56 also 
thins from the fulcrum 90, 92 to the contact region between the pen body 
42 and flex circuit 46. Because the substrate is a rigid body, the fulcrum 
lines 90, 92 tend to limit the movement of the first arm 54 and second arm 
56. As a result, thermal forces occurring in the arms 54, 56 cause arm 
rotation about the fulcrums 90, 92. The fulcrums 90, 92 also offset 
slightly due to thermal forces in the substrate 44. It is the rotation of 
arms 54, 56 which counter the compressive forces acting upon the flex 
circuit 46. 
Assembly and Thermal Dynamics 
Using known assembly methods, the substrate 44 and flex circuit 46 are 
attached to the pen body 42 with an adhesive epoxy. In one embodiment the 
epoxy is cured at an elevated temperature of approximately 130.degree. C. 
The final assembly method for attaching substrate 44 and circuit 46 to pen 
body 42 includes a heating stage and cooling stage. According to the 
invention, the compressive forces occurring during the cooling stage are 
countered so as to avoid buckling forces at the flex circuit 46. 
FIG. 4 shows the printhead region 91 of the pen 40 during the heating 
stage. The elevated temperature causes internal expansion forces 102 in 
the flex circuit 46, internal expansion forces 104 in the substrate 44 and 
internal expansion forces 106 in arms 54, 56. A typical thermal 
coefficient of expansion (TCE) for the polyimide flex circuit 46 is 
approximately 15-20 ppm/.degree.C. A typical thermal coefficient of 
expansion (TCE) for the silicon substrate 44 is approximately 5 
ppm/.degree.C. A typical thermal coefficient of expansion (TCE) for the 
plastic arms 54, 56 is 100 ppm/.degree.C., but ranges between 50 
ppm/.degree.C. and 200 ppm/.degree.C. Using fillers, the TCE for the 
plastic is reduced in alternative embodiments to approximately 35 
ppm/.degree.C. Because the TCE values differ significantly, the pen 
components expand different amounts. The plastic pen body has the highest 
TCE and thus expands the most. 
Within region 110 of FIG. 4, the flex circuit 46 contacts the pen body 42 
via an epoxy adhesive (not shown). Because the pen body has a higher TCE 
value, the pen body 42 portion in region 110 expands more than the flex 
circuit 46 portion during the heating stage. Before the epoxy cures, the 
flex circuit 46 is able to move relative to the pen body 42. Thus, the 
flex circuit 46 offsets along the planar surface 50 during the heating 
stage. Similar thermal dynamics occur at other areas in which the flex 
circuit 46 and pen body 42 are attached, (e.g., region 111). Because the 
arms 54, 56 have a higher TCE value than the silicon substrate 44, the 
expansion of arms 54, 56 produces a net force causing rotation of the arms 
54, 56 about their respective fulcrum lines 90, 92. A clockwise force 112 
acts upon arm 54, while a counter-clockwise force 114 acts upon arm 56. 
Once the epoxy cures, the pen body 42, substrate 44 and flex circuit 46 are 
fixed relative to each other at the epoxy adhering points. Thus, the flex 
circuit 46 portions in regions 110, 111 are fixed relative to the pen body 
42 portions in the same regions 110, 111. 
After curing, the cooling stage commences to bring the pen 40 to room 
temperature. FIG. 5 shows the printhead region of the pen 40 during the 
cooling stage. The decreasing temperature causes internal contraction 
forces 122 in the flex circuit 46, internal contraction forces 124 in the 
substrate 44 and internal contraction forces 126 in the pen body 42 and 
arms 54, 56. Because the TCE values differ significantly, the pen 
components contract by different amounts. The plastic pen body has the 
highest TCE and thus contracts the most. 
Within region 110 of FIG. 4, the flex circuit 46 is now adhered to the pen 
body 42 via the epoxy adhesive. Because the pen body 42 has a higher TCE 
value, the pen body 42 portion in region 110 contracts more than the flex 
circuit 46 portion. Similar thermal dynamics occur at other areas in which 
the flex circuit 46 and pen body 42 are attached, (e.g., region 111). It 
is because of these differences in contraction lengths that buckling 
forces occur in prior art pens 10. 
According to the invention, however, the arms 54, 56 cause countering 
forces to reduce the compressive forces acting on the flex circuit 46 in 
regions 110, 111. Because the arms 54, 56 have a higher TCE value than the 
silicon substrate 44, the contraction of arms 54, 56 produce net forces 
causing rotation of the arms 54, 56 bout their respective fulcrums 90, 92. 
Thus, a counter-clockwise force 132 acts upon arm 54 during cooling, while 
a clockwise force 134 acts upon arm 56. The significance of the rotational 
forces 132, 134 becomes apparent in FIG. 6. FIG. 6 is force diagram 138 
showing the contractive force 122 acting upon a point 140 located on a 
portion at flex circuit 46 within region 110. Force diagram 138 also shows 
the contractive force 126 and rotational force 132 acting upon a nearby 
point 142 located on a portion of arm 54 within region 110. Reducing the 
forces 126 and 132 acting on the arm 54 to components orthogonal to the 
flex circuit 46 contractive force 122 results in an equivalent force 
diagram 160. Force diagram 160 shows a force 150 acting on point 142 which 
is parallel to the force 122 acting on point 140. Force diagram 160 also 
shows a force 152 acting on point 142 which is perpendicular to the force 
122 acting on point 140. Of significance is the parallel component force 
150. The parallel component force 150 is the contractive force 126 less 
the opposing component of the rotational force 132. Thus, the rotational 
force 132 counters some of the contractive force 126 at the arm 54 portion 
within region 110. As a result, the parallel force component 150 is 
reduced by the arm rotation causing the arm 54 to contract less than it 
would without the rotational force. Ideally, the rotational force 132 
counters enough of the contractive force 126 so that force 150 is 
approximately equal to force 122. In such case, the length of contraction 
of pen body 42 within the contact regions 110, 111 will approximate the 
length of contraction of flex circuit 46 within the contact regions 110, 
111. In a preferred embodiment, the rotational force 132 counters at least 
enough of the contractive force 126 that any resulting compressive force 
acting on the flex circuit 46 is insufficient to cause buckling. 
Specifically the resulting compressive force, if any, does not cause 
buckling when the pen 42 cools or thereafter, during the desired useful 
lifetime of the pen 40. 
To achieve a sufficient rotational countering force, the arms 54, 56 are 
elongated (from a cross-sectional view such as shown in FIGS. 3-5), and 
have a thicker portion in the vicinity of the fulcrum lines 90, 92 than at 
the portions along the arm away from the fulcrum. In one embodiment the 
arms 54, 56 taper from the respective fulcrum to a hinge portion 57 at 
which the arms merge. Also, the arms 54, 56 become thinner (from the 
cross-sectional view such as shown in FIGS. 3-5) from the fulcrums toward 
the regions 110, 111, respectively. In addition, each arm 54 extends at 
one angle from the fulcrum 90, 92 to the hinge area 57 and at another 
angle from the fulcrum 90, 92 to the contact region 110, 111. By having a 
"bent" arm 54, 56, thermal contraction yields a greater countering 
component to the contractive force of the pen body in contact regions 110, 
111. According to exemplary embodiments, the angle of the bend in each arm 
54, 56 is between 135.degree. and 180.degree., inclusive, and preferably 
between 150.degree. and 180.degree., inclusive. The actual bend is 
prescribed based upon the thermal coefficients of expansion (e.g., of body 
42, substrate 44, and flex circuit 46), substrate 44 width and channel 70, 
72 widths. 
By knowing (i) the TCE values for the flex circuit 46 and arms 54, 56, (ii) 
the elevated temperature at the time of curing and (iii) an approximate 
room temperature, one can experimentally derive arm 54, 56 lengths and 
shapes which achieve the desired countering force. 
In a specific embodiment, the substrate 44 has a length of 5.0 mm and a 
thickness of 1.0 mm. Each chamber 70, 72 adjacent the substrate 44 has a 
width of 1.0 mm. For a change in temperature of 120.degree. C. and thermal 
coefficient of TCE=56, TCE=20, TCE=4.7, the following results were 
obtained: 
chamber 70, 72 thermal strain=0.00396 mm/mm 
arm 54, 56 thermal strain 0.005643 mm/mm 
arm height (height component of arm length)=3.56 mm 
arm rotation=0.00396 radians=0.23 degrees. 
The arm height is independent of temperature as expected. The temperature 
change induces arm rotation. In this example, the 0.23 degrees of rotation 
suffices to eliminate any buckling potential imposed on the flex circuit 
46. 
Meritorious and Advantageous Effects 
One advantage of the invention is that the reduced or eliminated 
compressive tension is insufficient to cause buckling. By eliminating 
buckling, a significant source of flex circuit delamination is eliminated. 
A meritorious effect is that fewer pens are found to be defective during 
construction and later use. Another meritorious effect is that flex 
circuit openings remain aligned with the underlying nozzles during an 
extended life of the pen. 
Although a preferred embodiment of the invention has been illustrated and 
described, various alternatives, modifications and equivalents may be 
used. Therefore, the foregoing description should not be taken as limiting 
the scope of the inventions which are defined by the appended claims.