US 20070205501 A1
A method of packaging includes placing a restrainer on a package during processing. The method includes clipping the restrainer in place and then exposing the package to high temperatures. After processing the restrainer is removed. An alternative process attaches a component die to a substrate having a cavity in a first surface. The process may then include dispensing and curing an underfill material in the cavity, and attaching a lid to the first surface of the substrate.
1. A method of packaging, comprising:
attaching a component die to a substrate having a cavity in a first surface; and
attaching a lid to the first surface of the substrate.
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This application is a division of, and claims priority to, U.S. Ser. No. 10/856,371 filed May 27, 2004.
Component die packages, such as those used in packaging semiconductor devices, have become larger and more complicated than those previously used. This has led to outgoing package warpage after the package assembly process. Package warp may create many problems for downstream users. For example, pin grid array (PGA) packages, warpage contributed to poor pin tip true position that may lead to pin rework. For land grid array (LGA) packages, the warpage may lead to high resistance or open contacts between the package and the socket. For ball grid array (BGA) packages, excessive warpage may lead to surface mount problems. In addition, BGA packages have JEDEC (Joint Electron Device Engineering Council) standards for warpage. With the larger and more complicated packages, these standards may be difficult to achieve.
In addition, current 90 nanometer (nm) wafer technologies may use low-k dielectric layers, such as porous cured dielectrics, in their build up layers. This requires that the package impose almost no stress on the die, as die stress can lead to cracks and bumps. These defects create multiple reliability issues for packages, including open failures, short failures, reliability stress failures and may result in component dysfunction failures.
In most cases, the package warpage is caused by the underfill epoxy cure process, generally performed at high temperatures. Current approaches may add a mechanical reinforcement to the package, increasing costs, or accepting the additional warpage, leading to wasted packages that do not meet the relevant standards. It is also possible to lower the cure temperature, but that largely depends upon the material properties and may compromise the quality and reliability of the components.
The invention may be best understood by reading the disclosure with reference to the drawings, wherein:
During the assembly of electronic component packages, the die containing the electronic components may be packaged between a package substrate and a lid. The package may go through multiple processes that involve elevated temperatures. Examples of such processes include chip attach reflow, deflux, epoxy underfill prebake and cure, integrated heat spreader (lid) cure, and ball attach reflow. The exposure to these levels of heat may affect the package by causing it to warp excessively.
Excessive warpage may be defined with regard to a relevant metric, such as the JEDEC coplanarity standard. Generally, coplanarity is a definition of how much a package is bent based upon assigned measurement points. The measurement points may differ depending upon the connections used between the package and the mounting surface, such as ball grid array or land mounting. The mounting surface is the surface to which the package is being connected, such as a printed circuit board. The current JEDEC standard is that the packages have a coplanarity of less than 8 mils.
Some approaches to dealing with excessive warpage include modifying assembly processes through temperature profile optimization to control warpage. Reducing the temperature of the various processes or slowing the cooling rate can help avoid cracks in low-k layers. Some devices are being manufactured with a low-k dielectric layer as the interlayer dielectric (ILD). These layers may be more delicate and crack more easily than previously used layers. Lengthening the cooling or heating processes lengthens the process required to package the devices, lowering the efficiency of the line, as well as possibly increasing defects due to partial cures occurring at the lowered temperatures.
Other approaches include adapting mechanical reinforcements to assist in holding the substrate planar, such as some current implementations of integrated heat spreader lids. Some manufacturers may just accept the loss caused by die cracks, high coplanarity, and highly tilted lids due to coplanarity. Others may just accept their inability to meet JEDEC standards for packaging, or may try to change the standards for an entire industry. Others may include high numbers of pin reworks in their processing specifications, but this increases costs.
In some cases, the coplanarity increase may be in the range of 4-12 mils at the end of the assembly and test process. This may depend upon the combination of different die and package dimensions. Data indicates that the warpage induced in the package after chip, or die, attach is of a convex profile as shown in
It is possible to pre-concavely warp the package during some of the high temperature processes such that the end result is a package having very little coplanarity. For example,
It is possible that the pre-concavity may result from any of the higher temperature process, to offset the induced convexity in later processes. The discussion below uses the process of the underfill epoxy cure, for ease of discussion only.
The die 12 is mounted to the packaging substrate 10 through die bumps 14. The packaging substrate may be several different types of substrates, such as ceramic, metal, semiconductor, etc. Die bumps 14 provide connection between the routing layers of the substrate 10 and the active components on the die. The use of die bumps is just an example of such a connector. The die may be packaged into a package comprised of the package substrate 10, the lid 22 and a sealant 24. The lid 22 in this example is an integrated heat spreader. A thermal interface material 18 may reside between the lid 22 and the die 10 to promote heat exchange. The underfill 20 assists in securing the die to the packaging substrate, as well as providing mechanical robustness to the connections. Package connectors, such as solder ball 26, may connect the package to a mounting surface, such as a printed circuit board (PCB). Again, the specific implementations of these various components are only intended as examples and for ease of discussion.
During the processing of these various parts of the package, the package is exposed to high temperatures, which promotes warpage. During most of these processes, the package substrate resides on a carrier that transports the substrate and the various components through the processes. It is possible to modify the carrier and adjust the processes to induce concavity into the package substrate.
The wider base area provides more surface area contact between the package substrate 10 and the lid. The width of the base will depend upon the die and the package, but generally as much surface area between the base and the substrate as is possible without contacting any active components on the die 12 is desirable.
The deeper cavity prevents undue pressure upon the die during processing. As discussed above, some components use a low-k dielectric layer that may crack more easily than other materials. The cavity may need to be deep enough to avoid contact with the die while fastened into place with the restrainer lid.
It is possible that current IHS lids may be used as the restrainer lids, more than likely with the addition of a spring. A possible difference is that IHS lid will not be bonded to the surface of the substrate, as typically occurs when the component die is packaged. The IHS lid should be removable after a portion of the process during which its function as a stiffener has ended, such that other processes may be performed, such as thermal interface material dispense and cure. The absence of a sealant and the thermal interface material may provide enough of a gap between the lid and the die to prevent cracking of the die and its various layers. The base may be wide enough to provide sufficient surface area as described above, much depends upon the size of the die and the package.
A packaging system is shown in
A spring 32 on the underside of the clip 30 may prevent undue pressure on the restrainer lid that could cause pressure on the die. The spring allows for some adjustability, but is optional as the clip could be configured so as to ensure that there would not be any pressure on the lid that may cause pressure to be put on the die.
The attachment of this clip to the carrier and the substrate restrains the package during the high temperature processes that would otherwise cause it to warp convexly. It may have the effect of causing the package to warp concavely. The concavity then may be mitigated by further high temperature processing that would cause a counteractive convex warpage. However, it is possible that the restrainer lid and clip may keep the substrate planar without inducing any concavity. This may be enough to overcome subsequent convex warpage and the resulting substrate could be within the relevant specifications for coplanarity.
Moving from left to right, it can be seen that the package is convexly warped after the chip attach process at 40 for the restrainer flow and at 50 for the control flow. The post epoxy cure process resulted in a concave profile for the restrainer flow at 42. The convexity increased for the control flow at 52. After the ball attach reflow at 44, the restrainer leg has a coplanarity range of approximately 4 to 9 mils convex. The control flow has a coplanarity range of 6 to 11 mils convex at 54. As a result of the restrainer lid, more of the packages in the restrainer process flow will meet the JEDEC coplanarity standards of coplanarity of less than 8 mils.
In addition to modifying the process flow to include the packaging apparatus discussed above, it is possible to modify the packaging substrate and lid configuration to decrease coplanarity. An example of such a modification is shown in
The component die 12 is packaged such that the component die resides in the cavity. The die bumps or other connectors are provided as before, but are now located in the cavity. The underfill 20 and the die bumps 14 may also be used as before, but now used to fill the cavity 60, at least partially. The thermal interface material 18 lies between the component die 12 and the stiffener or lid 66. Sealant 24 lies between the portions of the substrate away from the cavity and the stiffener or lid 66.
The lid 66 is no longer a typical IHS lid as was shown in
While not shown in
On the top, or first surface, of the substrate, it may be desirable that the component die be planar with the top surface of the substrate. This would be after mounting and underfill. This allows the thermal interface material and sealant to be dispensed in uniform layers and the flat lid 66 to be bonded uniformly across the surfaces of both the die and the substrate, contributing to the mechanical robustness of the package. It is possible that the die may lie below the plane of the substrate surface 62, or slight above, and planarity could be attained by management of the thickness of the thermal interface material and/or the sealant. These adjustments are possible during the packaging process. The resulting package is a planar package, meaning one that is mostly flat with low coplanarity as compared to any relevant standards.
An embodiment of a process flow is shown in
The addition of the flat stiffener lid may serve two purposes. Initially, it may straighten out any warpage induced by processes occurring prior to the attachment of the stiffener. Secondly, it may provide enough mechanical robustness to the package such that warpage is controlled in subsequent processes.
Another consideration in the use of the stiffener lid is the selection of the sealant used to attach it with regard to low coplanarity. In experiments conducted using the stiffener lid, an aluminum alloy lid was attached using Sumi G3 Epoxy and Dow 3-6265 sealant. Characterizations of the two sealant/lid combinations are shown in
On the right side of this graph, the coplanarity results for the Sumi G3 Epoxy is shown. The coplanarity after the underfill epoxy cure is shown at 84, and after the lid attach and ball reflow at 86. As can be seen here, the coplanarity is very much reduced when compared two the Dow sealant, having a range of 5-6 mils, all of which are compliant with the JEDEC specification. Expanded views of the coplanarity results at the post epoxy underfill point for each sealant are shown in the upper and lower portions of the far right of the figure.
In general, regardless of the sealant used, it can be seen that the use of the stiffener lid results in most of the packages being JEDEC compliant with regard to coplanarity. Similarly, the use of the restrainer lid during processing resulted in a majority of the packages being JEDEC compliant. In this manner, the warpage of the package is controlled and the efficiency and yield of the packaging lines are increased.
Thus, although there has been described to this point a particular embodiment for a method and apparatus for warpage control in packaging, it is not intended that such specific references be considered as limitations upon the scope of this invention except in-so-far as set forth in the following claims.