# Patent

## Title
Laminate warpage control

## Abstract

A laminate includes a core, a buildup layer having a top and a bottom, the bottom contacting the core and a solder mask contacting the top, the solder mask including at least one warpage control region formed on a top surface of the solder mask.

## Background
BACKGROUND
The present invention relates to microelectronic devices and, and more specifically, to controlling warpage of the laminate on which other microelectronic devices may reside.
Once formation of semiconductor devices and interconnects on a semiconductor wafer is completed, the semiconductor wafer is diced into semiconductor chips, or “dies.” Functional semiconductor chips are then packaged to facilitate mounting to a larger device.
The packaging generally provides mechanical protection and electrical connections to an external element. One typical packaging technology is Controlled Collapse Chip Connection (C4) packaging, which employs C4 balls each of which contacts a C4 pad on the semiconductor chip and another C4 pad on a packaging substrate. The packaging substrate may then be assembled on a larger circuit board. The combined chip and laminate may be referred to as a module in some cases. Of course, a module may include other elements such as a lid.
Mounting electrical components such as discrete resistors, discrete capacitors, transistors, digital circuits, etc. on laminate is well known. It is common for such laminate to contain many layers. Typically, most of the components are mounted on the surface. Some of the conductors used to interconnect the components may also be printed on the surface. The inner layers are primarily used to interconnect the components through other conductors printed on these inner layers and conductive vias passing through the outer and inner layers. For complex circuits, the surface area must be carefully allocated to fit the many requisite components. Also, in the case of capacitor components, it is desirable to position some of the capacitors near other, associated components to minimize path length and thereby minimize parasitic inductance.
As will be understood, greater the number of interconnects in a particular area, the more likely it is that differences in a coefficient of thermal expansion (CTE) between the interconnect (e.g., wires) and the surrounding laminate material may result in deformation of the laminate in such a region. The deformation can lead to buckling or other deformation of laminate. This may, in turn, cause connections on the surface to break. One such possible break may occur between the capacitors and the die. Of course, buckling or other deformation could cause other defects or malfunctions to occur as well.
Further, in operation, the die may include some areas that run hotter than others. This can, again, lead to stresses experience by connections (e.g., the C4) between the die and the laminate. Such stresses can, in severe cases, lead to a broken connection which can reduce the effectiveness of a particular die.
SUMMARY
According to an embodiment of the present invention, a method of forming a laminate is disclosed. The method also includes: receiving a first laminate; placing one or more electronic elements on a upper surface of the first laminate; exposing the first laminate to heat; determining locations of warpage on the upper surface; receiving a second laminate; and forming second laminate warpage control regions at locations on an upper surface of the second laminate that correspond to the location of warpage.
According to an embodiment of the present invention, a laminate that includes a core and a buildup layer having a top and a bottom is disclosed. The bottom contacts the core. The laminate also includes a solder mask contacting the top, the solder mask including at least one warpage control region formed on a top surface of the solder mask.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a side view of an example of laminate having a die connected thereto;
FIG. 2 shows a top-view of an example of a laminate including a source of warpage;
FIG. 3 shows a top-view of an example of a laminate including a source of warpage with a warpage control element disposed to flatten out warpage;
FIG. 4 is a flow chart showing one method to flatten warpage according to one embodiment;
FIG. 5 shows a top view of a laminate with protection features disposed/formed thereon;
FIG. 6 shows an example of a laminate with a die secured thereto including a layer of underfill;
FIGS. 7A and 7B show, respectively, a die placed on an upper surface of a laminate and surrounded by trench type protection features form in the upper surface and a close up view of one of the trenches;
FIG. 8 shows a die placed on an upper surface of a laminate and surrounded by trench type protection features form in the upper surface after underfill has been added; and
FIGS. 9A and 9B show a laminate in top view and cross-section that includes depressions for placement of elements on the upper surface of the laminate.



DETAILED DESCRIPTION
Provided herein are methods of forming a laminate that may reduce in a local area the effects of warpage.
With reference now to FIG. 1 a simple block diagram of a portion of a microelectronic device 100 is shown. The device 100 includes a chip 102. The chip can be an integrated circuit or other type of device and is sometimes referred to as die. A decoupling capacitor or other supplementary element 116 (e.g., a resistor, inductor, voltage regulator) may also be supported. The purpose of such elements is known and further, as disclosed herein, it may be beneficial that such elements be as close to the chip 102 as possible. While the following discussion may refer to a capacitor, it shall be understood that element 116 may be any of the elements discussed above or any other element.
The chip 102 and element 116 are supported by and carried on a laminate 104. The laminate 104 may be formed (but need not be) such that it includes a core 106. The core 106 may include metal or other connection elements that connect certain elements to other elements and be formed of a glass-cloth polymer. For simplicity sake, only three connection elements (connections 120, 122 and 137) are shown and are discussed further below.
The laminate 104 also includes a top buildup layer 108 and a bottom buildup layer 110. The top and bottom buildup layers are typically formed of an organic composite polymer. The bottom buildup layer 110 may include means for connecting a ball grid array 130 so that power, ground and data can be delivered from an external to the chip 102 or vice versa. Of course, the ball grid array 130 may be replaced with a so-called land grid array in certain instances. Either allows the module (combination of chip 102 and laminate 104) to be connected, for example, to a printed circuit board.
A solder mask 110 may be formed on top of the top buildup layer 108. The solder mask 110 allows for connections from the chip 102 and element 116 to the laminate 104. As shown, solder balls 160 are used to for such connections. Of course, other connection mechanisms may be utilized. To make the connections, a reflow process is performed that may give rise to the stresses on the laminate described above. In some cases, a solder mask 111 is also formed on the bottom of the laminate 104.
For example, as shown, a switching signal may be received from a power element 132 of the array 130. The signal traverses connections 120 to a capacitor 116. While shown as a direct line, the connection 120 may pass through multiple vias in buildup layers 108, 110 to traverse different levels therein. While the buildup layers 108, 110 are shown as a single layer, it shall be understood that such layers may be formed of multiple layers. As such, connections 120 are illustrative only. The signal received by capacitor 116 through connection 120 is also provided to a power terminal of the die 102 through connection 122. Of course, the connection 122 may pass through multiple vias in buildup layers 108, 110 to traverse different levels therein The capacitor 116 may be referred to as “de-coupling capacitor” in some cases. The purpose of the capacitor 116, in such a case, is to provide a “near” power supply to thereby reduce inductive losses in the switching signal. Thus, the closer the capacitor 116 is to the die, the better. Both the capacitor 116 and the die 102 are connected to a common ground or negative terminal 135 in the illustrated embodiment along connection 137.
It has been discovered, however, that the connections of the capacitor to the combination of the laminate 104 and solder mask 110 (generally referred to as the “board” 150 herein) may be susceptible to breaking when a stress such as board warpage occurs. Also, it has been discovered that in some cases, warpage may cause connections between the die 102 near its edges or corners to break.
FIG. 2 shows a top view of a laminate 104 according to one embodiment. The laminate includes die 102 and capacitor 116 disposed on an upper surface 202 thereof. The upper surface 202 corresponds to the solder mask 110 discussed above. The laminate also includes a region where a stress on the board may be created that is designated as stress region 160. The stress region 160 may be a region where elements with different CTE's are closely located (e.g. a regions with heavy wiring) or any other region where laminate warpage may occur. The stress region 160 may be discovered by exposing the laminate 104 to thermal cycling with after, for example, the die 102, capacitor 116 or other elements have been attached to it. The thermal cycling my lead to warpage shown by warpage lines 162 in FIG. 2.
According to one embodiment, an upper surface 202 of the board (e.g., solder mask 110) may be etched or other otherwise thinned or locally thickened to compensate for global warpage. With reference now to FIGS. 1 and 3, a warpage control region 302 is formed in an area that overlies the warpage lines 162 (e.g., the location in the board where warpage has previously been discovered to exist. In one embodiment the warpage control region 302 may be formed by thinning the solder mask layer 110 in those regions. In another embodiment, dielectric film may be placed in certain regions on top of the mask layer 110 to thicken that area. In yet another embodiment, a laminate may include warpage control regions that are thinning regions and others that are thickening regions.
The warpage control region 162 may be formed, for example, by forming cuts or other depressions in the solder mask layer 162. Such a region may be referred to as a thinning region herein. Such cuts/depressions may be formed by placing a mask and chemical etching or by using a laser or a combination of both. In such a case, the region may be a 5 μm×5 μm grid having one or more different levels form by such laser cutting, etching or a combination of both.
Alternatively, the warpage control region 162 may be formed by adding a dielectric film that is the same or similar to the material forming the solder mask. Such a warpage control region may be referred to as a thickening region herein. The sizing of such regions may be the same or similar as a thinning region in one embodiment.
The location and type of the region 162 may be determined as described in relation to FIG. 4 in one embodiment.
FIG. 4 is a flow charts describing one manner in which the location of one or more warpage control regions 302 may be determined. At block 401 a warpage model is used to make a first approximation of where elements are to be added/removed to control warpage. Such a model may be formed by understanding of the layout of the laminate and where stresses may occur.
At block 402 a laminate is provided. The board may include some or all of the interconnects and vias needed to form a complete system when appropriate elements (e.g., dies/capacitors/resistors/etc.) are placed on it. The laminate may also include connection location having solder paste filling holes in the solder mask.
At block 404 one or more components are placed on the laminate. The process of placing the components can include depositing a die with associated C4 balls or other connection mechanisms (e.g., wires) on the corresponding holes in the solder mask. Other elements, e.g., capacitor 116, may also be placed in the same manner.
At block 406, the board including the components is exposed to a reflow process. The reflow process is a process in which the pre-solder is melted to form a permanent bond between the component and the board. As is known, the temperature of such a process is high. It has been discovered, and as described above, that such high temperatures can lead to board warpage due to the differing CTE's of elements in the board.
At block 408, after the reflow process, the location where warpage occurs is determined. The may be several different areas where warpage occurs.
At block 410 one or more locations to form warpage control regions 162 are determined. In one embodiment, a warpage control region will be located at one or more locations where an upper surface of the board is buckled upwards. In addition, the location where troughs occur (due to the buckling) can also be determined.
At block 412, the depth of the warpage control regions at the locations determined in block 410 is determined for cases where a thinning region is to be applied. Block 412 may also or alternatively, determine the height of the film to added if thickening region is to be placed.
At block 414, a new laminate is provided. This laminate may have the warpage control regions formed in the locations and of the type determined at block 412 in block 416. The warpage control regions can be formed before or after the die is connected to the laminate but is performed before a reflow process occurs. The warpage control regions will help to reduce or eliminate warpage during such a reflow process.
The above description may be thought of as a global correction to warpage. In one embodiment it may provide the warpage control regions such that after a reflow, the surface of the laminate is flat or substantially flat. In other cases, certain warpage control regions can be added to protect various components placed on laminate. Consider an example where a laminate is integrated into a portable device. In such a case, if the device is dropped, a force may be transferred across the laminate towards an edge of the die. In such case, it may be beneficial to have thickening or thinning regions near edges and/or corners of the die to prevent a pressure wave or the like from hitting the edge of the die and stressing connections between the die (or other component) and the substrate. Thus, there may be times where, near a component, the solder layer has its thickness varied. As illustrated in FIG. 5, example thickened feature locations are shown. These features may also help reduce possible damage to the connections between the die and the laminate due to uncontrolled warpage in one embodiment.
In more detail, in FIG. 5, a top surface of a laminate 502 is shown. A die 504 is connected to that top surface 502 as is component 506 such as a capacitor. One or more protecting features 508 may be provided to protect edges or sides of the die or component from stresses emanating from other locations on the die. The features 508 may be straight (508a), from obtuse angles (508b), form right angles 508c or form acute angles 508d. Such protecting features may be formed as either troughs or built up elements or a combination of both. The protecting features, thus, may be formed in manners similar to the thinning and thickening regions described above. That is, in one embodiment, the protecting features may be trenches formed in the solder mask and in another they may be formed by adding a dielectric material to the top surface of the solder mask. The protecting features may server to reduce or direct stresses from the regions of the less sensitive regions of the top surface.
Further, during operation, the die itself can cause pressure to exist in the laminate. With reference now to FIG. 6, a laminate 600 is shown with a die 602 connected thereto. During operation, the die 602 may develop local hot spots (e.g., under a core) that are shown by regions 604, 606. It has been discovered that these hot spots may lead to similar CTE issues as described above. In some cases the effects are such that the electrical connections between the laminate 600 and the die 602 at the edges of the die 602 may be stressed and even break. In FIG. 6, an example of such connections are shown by connectors 610.
Also illustrated in an underfill layer 608. Underfill is generally dispensed on a corner or in a line along the edge of the die with capillary action to transport the underfill under the die 602. After dispense, the underfill is typically heated in order to cure it. The underfill can during dispense move outward as well and the exact edge may not be known. Note that connectors 610 extend through the underfill 608. That is, the underfill 608 may surround the connections in one embodiment.
According to one embodiment, and with reference now to FIGS. 6-8 at or near edges of the die, one or more depressions 620 may be formed. These depressions may serve to direct the stresses from the edges die 602 to regions of the laminate 600 that is less sensitive to CTE or other forms of stresses. The depressions may include side walls 622 that extend downwardly at an angle α. Values for a may vary from any value including and between 45 to 85 degrees. Further, and as best illustrated in FIG. 8, the depressions 620 may also serve to limit the outwardly flow of underfill 608. Such control may be beneficial in that by controlling outward flow, the bounds of the flow may be known and allow for elements to be placed closer together as less room may be needed to account for overflow.
In some instances, multiple elements (e.g., capacitors or any other supporting element) may need to be located close to a particular die. With reference to FIGS. 9A and 9B, consider the example where multiple capacitors 900a-900n (collectively, 900) are to be used in supporting operation of die 902. In such a case, the capacitors will all have to be placed (and there could be over 100 in some cases) before a reflow process that locks them into position occurs. However, even slight air or physical perturbations can cause the capacitors to move. Further, the same may also be true of the die 900 but only the capacitors 900 are discussed for simplicity.
According to one embodiment, the solder mask 920 of the laminate 904 includes depressions 910 that correspond, in this example, to a particular capacitor 900. That is, depression 910a corresponds to capacitor 900a. The depressions extend only part way through the solder mask 920. In one embodiment, contacts 950 may be formed on an upper surface of the bottom 922 of the depression 910 (see, e.g., depression 910c). The walls of the depressions can have the same angle α shown above.
The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

## Claims

1. A method of forming a laminate, the method comprising:
providing a first laminate;
placing one or more electronic elements on a upper surface of the first laminate;
exposing the first laminate to heat;
determining locations of warpage on the upper surface;
receiving a second laminate; and
forming second laminate warpage control regions at locations on an upper surface of the second laminate that correspond to the location of warpage by removing portions of the upper surface.

2. The method of claim 1, wherein the second laminate upper surface is a solder mask layer.

3. The method of claim 2, wherein the second laminate warpage control regions are formed by removing portions of the solder mask layer.

4. The method of claim 2, wherein the second laminate warpage control regions are formed by adding dielectric material to the solder mask layer.

5. The method of claim 2, wherein the second laminate warpage control regions is a 5 μm×5 μm square.

6. The method of claim 2, wherein the second laminate warpage control regions includes an outer boundary and are formed by removing portions of the solder mask layer within the outer boundary and adding dielectric material to the solder mask layer within the outer boundary.

7. The method of claim 1, further comprising;
adding one or more electronic elements to the second laminate upper surface.

8. The method of claim 7, further comprising:
exposing the second laminate to heat.

9. The method of claim 8, wherein exposing the second laminate to heat includes a reflow process.

10. A laminate comprising:
a core;
a buildup layer having a top and a bottom, the bottom contacting the core; and
a solder mask contacting the top, the solder mask including at least one warpage control region formed on a top surface of the solder mask, wherein the at least one warpage control region is a depression formed in the solder mask.

11. The laminate of claim 10, wherein at least one of the at least one warpage control regions is a 5 μm×5 μm square.

12. The laminate of claim 10, wherein at least one of the at least one warpage control region includes an outer boundary and is formed by removing portions of the solder mask layer within the outer boundary and adding dielectric material to the solder mask layer within the outer boundary.

13. The laminate of claim 10, further comprising;
one or more electronic elements disposed on the top surface.

14. The laminate of claim 13, wherein the electronics elements include a die and a capacitor.