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Intelligent Multi-Chip-Module (MCM) Package Enhancement

IP.com Disclosure Number: IPCOM000035293D
Original Publication Date: 2005-Jan-20
Included in the Prior Art Database: 2005-Jan-20
Document File: 4 page(s) / 98K

Publishing Venue

IBM

Abstract

In recent years, the attachment of Multi-Chip Modules (MCM) to system boards using Land Grid Array (LGA) connectors has grown in popularity. To facilitate this attachment, high-force compression mounting systems have been designed which unfortunately have induced some substrate cracking failures. At present, the amount of force used to compress the MCM to the system board has been based on a series of calculations. This amount of force was then limited by either utilizing a preset torque (via a torque wrench) or a preset actuation distance (via a mechanical down-stop). In both cases, methods have proved acceptable, provided that nothing changed in the design of the board, MCM, LGA, or any other component used in the base calculation. However, if any one of the components have been changed by design, cost reduction reasons or product quality escapes, problems may occur. This article describes an approach that can be implemented that will provide "intelligence" to the assembly to prevent excessive mechanical loading from being imparted to the MCM.

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Intelligent Multi-Chip-Module (MCM) Package Enhancement

At present, no method is currently used to verify whether the forces used to mount the hardware to the system board exceed a predetermined set point. As an example of the strains imparted, please note the Strain vs. Load data provided below. Fig. 1 represents a typical "dry-capping" strain gage response of a mechanical good module's top-surface. In this case, no thermal interface material was applied and the strains are relatively benign.

Fig. 2 represents similar data, but in this case the module contains the thermal interface material and reflects a marked increase in strains by the ceramic substrate (due to hydrostatic pressure). Although these data are imperative during module and LGA assembly development, no means currently exists to monitor this critical attribute once module & assembly design are released to production.

uStrain vs Instron Compression Force KN - Dry Run

Fig. 1

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uStrain

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Safe Build ATC 4.5 20 mil paste

Fig. 2

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Strain 0 Strain 5 Strain 1

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The approach described here employs the marriage of strain gage & flex-film technologies to create a device that can be micro-BGA attached to a substrate to in situ monitor the strain on the top surface of an encapsulated module. Specifically, incorporated in the design of the system is the use of a rosette-style (or a combination of other uni-directional-style) strain gage that would be mounted on the surface of the MCM in a predetermined location for maximum coverage (see Fig 3a). The gage's design utilizes flex-technology whereby containing the sensor and a micro-BGA pad mounting area (see Fig. 3b). The gage portion would be bonded to the MCM's substrate at the desired monitoring location, while the opposing end would be bonded via a micro-BGA structure wired through the substrate so the sensor signals can be passed onto the System board (and therefore the system control network) via the LGA connector. Note. the attachment of the device is completed during the normal module assembly phase (i.e., chip & capacitor attach) using flex-circuit-based attach micro-BGA technology being developed for mounting other devices via flex onto a module (ref. Colgan et al). Once mounted, this gage can then be used for monitoring as follows; (1) during the encapsulation process to ensure damaging strain levels are not being imparted to the substrate during module assembly, (2) during the module/LGA assembly process to insure that the forces used to compress the MCM to the board do not exceed the preset limit and/or (3) used dynamically in an operation system to monitor the MCM for failure and be a part...