Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2001-01-11
2003-04-15
Moore, Margaret G. (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S413000, C428S447000, C428S448000, C427S386000, C427S387000, C427S096400, C156S329000, C156S330000, C523S400000, C524S439000, C528S012000, C528S014000, C528S015000, C528S033000, C528S040000, C525S107000
Reexamination Certificate
active
06548175
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to improved conductive adhesives for solder-free interconnections in microelectronic assembly processes for attachment of electronic components to a substrate and, in particular, for chip carrier-to-substrate interconnections. These adhesives are characterized by low tensile modulus, low resistivity, high adhesion strength, and durability of these properties during reliability stress conditions of thermal shock, thermal aging, and temperature/humidity (85° C./85%RH) exposure of the assembled devices.
2. Description of Related Art
Improved conductive adhesives are required for Pb solder-free interconnection solution as lead/tin (Pb/Sn) alternatives in the fabrication of electronic packaging structures as for single chip and multi-chip electronic modules, typically used in high speed computers, automotive electronics, medical and telecommunication devices, cellular phones, and other consumer products. For reliable product performance with conductive adhesives as interconnecting materials for attaching a ceramic chip carrier or surface mount components as electrical resistor or capacitor, to flexible or rigid printed circuit boards (PCB), it is important for a conductive adhesive as the bonding material to have the requisite properties to withstand and absorb the thermal coefficient of expansion (TCE) mismatch induced material stresses between dissimilar contacting materials. For example, in the case of ball grid array packages (BGA) with ceramic chip carrier-to-board connections where the organic board which is typically the epoxy-glass organic board, FR-4, and the bismaleimide triazine (BT) resin based material has a TCE about 20-35 ppm/°, and that of a ceramic chip carrier 3-7 ppm/°, the metal filled organic conductive adhesive as the interconnecting material must be capable of maintaining bond integrity to assure performance reliability of the electronic module during reliability assessment stress exposures involving thermal shock and 85° C./85%RH temperature-humidity excursions and in long term operation. Some of the desirable properties of the organic based conductive adhesives for Pb-free interconnections to provide the benefit of Pb elimination in electronic devices include: low stress, low tensile modulus, low resistivity, stable joint resistance with temperature and T/H exposure, high bond strength, void-free bonding, and bond integrity during environmental stress conditions.
The commonly available thermosetting conductive epoxies which are typically derived from glycidoxy ethers of bis-phenol A and bis-phenol F epoxy resins are generally high modulus materials and thus present a concern about the long term functional reliability of the product when these adhesives are used for bonding dissimilar materials with significant TCE differential, for example, ceramic chip carrier to organic printed circuit board (PCB). Also, most of the commercially available formulations of isotropic conductive adhesives are Ag-filled where the issue of silver migration during temperature-humidity exposure is one of the concerns in their use as interconnection materials in place of Pb/Sn solder. The available flexible epoxies are not satisfactory due to the low thermal stability, performance variability, and in some cases have too short a pot life for practical use in a manufacturing environment. Available electrically conductive thermoplastic polymer based compositions are generally solvent-based which have the problem of dispersion stability, resin bleed, voids in the bond line, and performance variability. Electrically conductive silicone elastomeric compositions are yet another class of conductive adhesives for a variety of applications including Pb-free interconnections, elastomeric connectors in electronic module assembly, EMI shielding and heat sink attachment. Although these materials have the compliance and stress absorbing property to qualify for interconnections between materials with significant difference in TCE, these have been found to have limitations in terms of adhesion to Au and other relevant surfaces, problem of bond failure and resistance increase when the assembled structures are exposed to T/H environment.
U.S. Pat. No. 5,699,228 (Alkov et al.), is concerned with a method of interconnecting a leadless multichip module to a printed wiring board using solder-free interconnects to form the electronic module. U.S. Pat. No. 5,528,466 (Lim et al.) is concerned with mounting and cooling a plurality of integrated circuits using elastomeric connectors and a plurality of electrical components having terminals onto a PCB surface. U.S. Pat. No. 5,182,623 (Hynecek) is concerned with an interconnect structure and heat sink.
U.S. Pat. No. 5,700,581, assigned to the assignee of the present invention, is concerned with solvent-free epoxy-siloxane based adhesives for attachment of silicon device chip to ceramic substrate in the fabrication of single chip (SCM) and multichip (MCM) electronic modules. The conductive adhesives described in this patent comprise a siloxane precursor in conjunction with anhydride or hydroxy benzophenones as curing agents, cure catalyst/cure accelerator, and silver flake as the conductive filler. These adhesives are described as having high die shear strength which is maintained when the adhesive bonded chip-substrate assembly is exposed to thermal shock involving −65° C. to about +150° C. thermal excursions, and temperature-humidity cycling (85° C./85%RH). In the case of silicon chip (TCE about 3 ppm) attachment to ceramic substrate (TCE about 3-7 ppm) application, the conductive adhesives are not exposed to thermal expansion mismatch stresses which would occur with the bonded materials that have significantly different TCE. For example, in the case of the bonding of a ceramic chip carrier (TCE 3-7 ppm) to an organic board (TCE 20-35 ppm), there is significant mismatch in the thermal expansion of the two materials and therefore, there is a need for improved conductive adhesives that would have the necessary performance characteristics for interconnection reliability in applications involving TCE mismatched bonded materials. Also, based on the concerns about silver migration with silver filled conductive adhesives commonly described in the prior art and the ones described in U.S. Pat. No. 5,700,581, there exists a need for improvement in the electrically conductive adhesives as alternative interconnections which do not present the problem of metal migration when the assembled device is subjected to reliability stress conditions and to temperature-humidity environment during normal operation.
The present invention addresses the need for improvements in conductive adhesives, especially the necessary requirement for compliant interconnections for absorption of stresses generated when there is significant TCE mismatch between the bonded materials, and the issue of silver migration with silver filled adhesives. The invention provides isotropically conductive adhesives which have low modulus and thus are capable of absorbing thermal stresses with TCE mismatched bonded materials, which have low resistivity and undergo no significant change in resistivity with thermal cycling and temperature-humidity excursions, which maintain high adhesion strength with Au and ceramic, and which employ Au or Pd surface coated Ag as the preferred filler instead of the commonly used Ag flake, and which provide stable performance during reliability stress exposure and assures performance durability in long term operation of assembled devices.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a conductive adhesive which may be used as interconnecting materials in the manufacture of electronic components, and, to attach electronic components to a substrate, and, in particular, to attach a ceramic chip carrier to a substrate or an organic board such as a printed circuit board (PCB).
It is another object of the present inven
Jackson Raymond A.
Ostrander Amy B.
Perry Charles H.
Sachdev Krishna G.
Shannon Megan J.
Cioffi James J.
DeLio & Peterson LLC
Feely Michael J
Moore Margaret G.
Tomaszewski John J.
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