May 1998
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Removable Chip Modules Provide Temporary Interconnections
ICs are grouped or mounted inside the RCM,™ enabling a defective device to be easily removed and replaced.
By James Rathburn, Gryphics Inc., Minneapolis, Minn.
The need to decrease the cost of manufacturing electronic products, while increasing their signal performance, has resulted in the development of a group of patent-pending modules known as replaceable or removable chip modules (RCMs), shown in Figures 1 and 2.
Figure 1.The RCM offers an easy way to connect ICs without solder reflow.
The RCM concept was driven by advances in electronic components and system needs for both military and commercial markets and an attendant demand for techniques to produce high performance, low cost and reliable interconnect products. Moreover, the increasing popularity of chip-scale-packaged devices has given rise to the necessity for supporting the connection of more devices in a smaller area and has focused more attention on the parasitic effects of the interconnects.
Consider, for example, that each individual connection point adds an opportunity for failure, cost and signal degradation. As the number of connections increases, the statistical potential for failure increases along with the potential that some components may have to be reworked or discarded.
RCMs can take many forms and can be put to many uses. The RCM accepts the IC devices where each terminal on the device is contacted and connected, which may be a PC board, substrate, flexible circuit or an intermediate circuit that is contained directly within the RCM. A loaded RCM acts as an individual IC throughout the test cycle.
Typical manufacturing methods currently utilize lead bonding, flip-chip solder deposits or conductive adhesives to attach the individual devices. Each of these methods has its own associated reliability and process problems, as well as the likelihood for rework or repair.
Today, due to the proximity of devices, knowing whether each device will meet its performance specifications is of increased importance. Many times guardbanding or simulation must be employed to
Figure 2. The RCM transports ICs through the test and burn-in cycles.
Table 1. Design Requirements
- Products must be low in cost and conductive to high volume manufacture while still allowing for low cost in small volume.
- Products must provide high signal integrity and low inductance interconects.
- Products must be capable of accepting fine pitch semiconducter devices with the flexibility for varied I/O counts.
- Devices must be interconnected in a removable manner, with out the use of moten solder, wirebonding or underfill.
- Products must be low profile, with high circuit density and a small form factor.
- Products must incorporate thermal management options for high power or heat-sensitive devices.
determine the acceptance or failure of the product due to parasitic effects throughout the interconnections and within the test process. At the system or end-use level, the way the module or package is connected to PC boards can introduce additional opportunities for failure.
These issues, combined with the significant increases in performance required for future devices and systems, demand cost effective, high-performance connecting products. The methods used to produce these products must include provisions for recognizing such inherent yield factors as low inductance, fine pitch connections between devices and system components, thermal management and functional test. They must also be conducive to low cost manufacturing.
RCM Development
A variety of modules are being developed to meet a wide range of application and performance parameters. The design requirements in Table 1 establish the product base.
Product Definition
The modules that were conceived based on these design requirements have evolved into a group of products that might be considered a mixture of package features and comlector or socket features rolled into one. The group consists of a few basic principles, with the common element being a method of achieving low inductance interconnects that can be arranged at fine pitch, with a large compliance tolerance range to accommodate the variations in devices. A multimode compliance system is employed to provide a mechanical means of connecting to each terminal as required.
The theory of operation revolves around the basic shape of the contact member and its interaction with its housing, compliant encapsulant and opposing terminals on the device and circuit board. One contact of choice is a compliant twisted ribbon of a copper alloy material, which is 1.0 mm or 1.5 mm in height when compressed. This contact is made of a flexible, tempered material and provides several modes of compliance. The part consists of a slightly forked (pronged) end which transitions into a slight bend. The forked tips provide a redundant contact function and facilitate a slight wiping action on touching a terminal pad or solder deposit.
The transition to a bent point, after encountering the tip, allows for a slight lead-in to prevent a pure, vertical encounter with the forked tip. This feature, combined with the slight relief the forked tip provides, prevents excessive damage to either the terminal or the contact tip and provides a flexure point for additional initial compliance. The part then transitions to a straight section that assists with contact tip position and location. Further along the part is a twisted portion of the contact, where the base material is formed approximately 90 degrees along its vertical axis resulting in a helix. This feature provides a structural form in the part, preventing flexural damage. The physical twist of the contact member also provides a fluted shape for encapsulant bonding.
Figure 3.Tips are rotated 90 degrees from each other, which promotes a tendency to balance the loading of the contact and reduces the risk of excessive flexure in any direction.
In addition, the tips are rotated 90 degrees from each other when viewed from an end, which promotes a tendency to balance the loading of the contact and reduces the risk of excessive flexure in one direction (Figure 3). The same geometry is repeated on the other end of the contact member so that vertical orientation is symmetrical, allowing for high speed assembly. The assembly method results in a pattern of randomly-oriented contact elements which provide balanced compliance across the device in contact.
Figures 4 and 5 each show one of several variations possible. The lower portion of the RCM consists of a formed section conducive to compliance against a circuit board pad. The upper contact portions are formed to mate with a solder deposit or solder ball-formed lead or land pad. The contact members contain features for precise location within the housing. The initial movement of the contact members upon connecting with a given terminal is enabled by the encapsulant without appreciably deforming the contact member. At a certain point in the movement, the base copper alloy begins to deflect and engages without flexing the material excessively.
Figure 4. ICs may be characterized tested and burned-in employing an as used" configuration.
Module Benefits
The fundamental benefits intended to be gained from all modules are: low parasitic connections, fine terminal pitch, low cost, a high level of integration and the ability to remove and replace failed devices. The internal arrangement of devices can achieve high density, with devices located in close proximity to each other. With the issues of rework and solder
flow heat effects eliminated, devices can be placed almost edge to edge. By allowing for demating of devices, failed or faulty units can be removed. Additionally, the ICs can be characterized, tested and burned-in employing an "as used" configuration, if applicable (Figure 4.)
The ability to integrate devices and several connector styles into one module is another benefit of the modular design. Depending on the application, several devices can be installed in a removable manner with external connections to the system or outside environment.
Vertical, lateral, and 90 degree connections can be made in the same connector containing multiple semiconductors. These RCM features eliminate the requirement for bends or transitions when used in flexible circuit applications. Additional connectors at the system or product level are also eliminated.
Low Cost, High Performance
Many applications where components are plugged into sockets or connectors may benefit from eliminating the
Table 2. RCM Applications
Figure 5. I his illustration shows one of several variations possible with the RCM Here the module is compliant with a circuit-board pad.
connector cost totally at the system level. With an RCM, the end user or consumer can install ICs directly on the system board without soldering, eliminating the cost of the unpopulated connectors included in many products. In addition, some traditional connector applications, which use a rmated pair of connectors, can be contained in the RCM without the extra cost. Moreover, when interfacing to area arrays with solder balls or solder deposits, a cost saving option would be to eliminate the solder entirely. Depending on the application, the RCM connections can be made directly to the bond sites where the solder balls are attached, essentially in a land grid array format.
Multimode Compliance Test and Burn-In
When the same principles used in high volume production applications are applied to lower volume characterization, test, emulation and burn-in applications, costs are also low. In the event the RCM does not act as the end system or product-level interface, using a test product with the RCMs can at the very least provide a low parasitic interface that benefits from high volume production of some of the components. Conventional multi-package or multichip modules can be simulated with relatively low cost, great flexibility and high signal integrity.
RCM Applications
In general, any product that employs semiconductor devices, connectors and interconnecting circuits may be a viable application. Table 2 illustrates applications with device lead pitch as small as 0.19 mm that are under development.
Conclusion
RCMs are intended for applications where low parasitic, finepitch interconnects are desired at low cost. While many variables affect unit cost„such as configuration, contact choice, intended use or life and production volume„most RCM applications should cost far less than conventional methods.
While there are many appropriate uses for RCMs, achievable device pitch and alignment must be considered. Testing and qualification programs are underway to document the mechanical and electrical performance of select applications.
Mr. Rathburn is the President of Gryphics Inc., a startup technology company which began in March 1997. The company specializes in the design, development and manufacture of products for the computer, semiconductor and electronics industries. Gryphics'proprietaryproducts include a wide variety of interconnect and mixed technology modules for use in electronics. Mr. Rathburn is the author of several U.S. and foreign patents related to interconnect andpackaging technology. He can be reached at 612.479.4044, fax 612.479.121 0.