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Content archived on 2024-05-07

Development of an adhesive chip assembly technology for dedicated electronic applications


According to the different process and product requirements of the project partners concerning the curing conditions, the thermo-mechanical performance and the interconnection mechanism, Heraeus developed three systems for flip chip bonding in the range NCA (non conductive adhesive), ACA (anisotropic conductive adhesive) and ICA (isotropic conductive adhesive): All three systems being developed had to fulfil a number of requirements concerning: -Adhesion Strength -Bubble free layers -Printability -Dispensability -Ion concentration -Curing conditions NCA are the basic systems for ACA and ICA respectively. In addition they can be used for a special flip chip technology developed at IMEC.
Both an analytical and a finite element model have been developed to calculate stress levels and deformations of adhesive bonded flip chip assemblies as a result of the bonding process and of environmental exposure. The analytical model, dedicated for thermal stresses in packages without under-fill, has shown that a high and wide connection is the best solution. Furthermore, due to the very high thermal stresses (> 50 MPa), it is concluded that under-fills are really necessary to obtain a reliable interconnection.. The FE-model includes stress development due to curing shrinkage, thermal shrinkage and an expansion due to moisture uptake. The 3D model has shown to accurately describe the phenomena and trends that are experimentally observed during flip-chip bonding on different substrates. However, the verification work showed that there is still a large deviation of calculated deformation levels from measured data. Potential model improvements have already been identified. The result largely consists of a 3D finite element model, that besides thermal and mechanical stresses is able to calculate stresses build up during curing and stresses due to moisture uptake and release. As indicated before, the model although predicting the trends in a correct way, needs further adaptation to improve quantitative agreement with measured data. The model (after this further improvement) can be used for a wide variety of applications using thermosetting polymer systems (adhesive/encapsulate or coating), also outside the primary application fields in microelectronics (besides flip chip, also e.g. optical/mechanical constructions, potting/adhesive bonding in ballasts and lamps for lighting applications). State of the art calculations on encapsulate/adhesive constructions, even in case of viso-elastic approach, generally do not take into account stresses generated during curing and due to moisture, although these stresses according to the present project can be very significant and dominant. The model used in the present project is based on well known software (MARC), which was, however, extensively modified with subroutines in order to incorporate the extra variables of curing and moisture uptake. Parts of the result are also material properties measured, e.g. shrinkage build-up with either advanced home made interferometric equipment or modified commercially available equipment.

Exploitable results

Current implementations of flip chips are mainly based on soldering. The use of conductive adhesives offers several advantages: it is applicable on non-solderable substrates. Interconnection pitch can be much smaller and implementation can be profitable in much smaller production volumes.Substantial basic experience has been built up in the different processing steps for the realization of flip-chip assemblies using different kinds of adhesives (ACA, ICA, NCA) on different kinds of substrates (glass, flex, FR-4, LTCC): design of test chips for technology development, application of the adhesives, assembly technologies, optical and electrical evaluation of the assemblies, compatibility with mixed assembly, stress modeling and reliability testing. Functional and technology demonstrators have been realized in flip chip on flex technology with ACA (LCD-drivers, hearing instrument) and flip chip on board technology with ICA (mixed assembly) and ACA (DSA for car radio). Qualifications tests have been passed including technological feasibility. Two other investigated application areas, low-cost MCM-L based on ACA / FR-4 and BGA on FR-4 with ICA are considered non-exploitable. The development of flip chip on glass technology with ACA was initiated, but did not reach the point, as initially put forward, due to technological and organizational problems at one of the partners.
This task has demonstrated the feasibility of electroless Ni/Au bumping process for very high density flip chip interconnections by using thick vertical sidewall photo-resistant process. Previously the photo-resistant materials used in the industry have not been able to stand the harsh chemical and thermal conditions common in electroless nickel bumping. The requirements and support of parties in this project encouraged and made possible for Picopak not only the development work but also the improvement of the existing production line. Since June 1998 the improved production line and new materials have been successfully in production. So far monthly volumes of one million bumped and flip-chip assembled dice have been delivered to the customers. That indicates already a good volume production potential, but further development of the photo-resistant process and production lines are continuing for improving the efficiency and consistency of the quality.

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