Testing substrate-attach adhesives

Table 6.6 NASA MSFC SPEC-592 requirements and test methods for die- and substrate-attach adhesives... 

Another method for measuring either die- or substrate-attach adhesion strength involves attaching a contact tool to the top of the die or substrate with an adhesive that is stronger than that of the die-attachment adhesive. The tool applies a vertical force until the die detaches from the substrate or until a minimum specified strength is achieved. The test method and failure criteria are specified in Method 2027 of MIL-STD-883F. i... 

Table 6.4. NASA MSFC SPEC-592, Requirements and Test Methods for Die- and Substrate-Attach Adhesives... 

Materials are qualihed by passing a series of tests as severe or more severe than those of the intended application. Generally, accelerated environmental and mechanical tests are specihed and performed on actual or simulated parts representative of the hnal product. Since the parts are subjected to accelerated tests, some of which are destructive, qualihcation parts are seldom deliverable items. Once an adhesive and process have been qualihed, no changes in composition or processes are allowed without requalihcation. Testing at levels above the acceptance tests for an extended duration or number of cycles is necessary. The qualihcation tests for some applications, for example, for die and substrate attachment, have been specihed in NASA and MIL specihcations while those for other applications are being developed by industry and industry associations. 

The first specifications for adhesives were generated by the staff of NASA and the DoD who were prompted by the high reliability that was required of microcircuits used in aerospace programs. These specifications covered primarily die and substrate attachments for hermetically sealed integrated circuits, hybrid microcircuits, and multichip modules. Subsequently, with the increased use of surface-mount adhesives in the assembly of commercial printed-wiring boards and underfills for flip-chip devices, industry associations took the lead in generating the requirements and test methods. 

The test methods covered in this chapter are those most closely related to adhesives for die and substrate attachment, surface-mounting of components, underfill, and optoelectronic assembly. Some tests, such as bleedout and electrical stability, for which there are no standard test methods, are not covered but information for these procedures was already addressed in other portions of this book, for example, in Sections 2.1 and 6.1. 

Electrical-stability testing is essential for conductive adhesives used for electrical connections. Electrical conductivity can degrade at elevated temperatures, on aging with or without power, and on exposiue to humidity and temperature. The specific test method used depends on the application. One test used for die-attach adhesives specified in NASA MSFC-SPEC-592 (now inactive) involves a series of gold-plated Kovar tabs attached with conductive epoxy to metal pads on an interconnect substrate. In the test vehicle, a bias of 5 V and cmrent density of 139 3.9 A7cm (900 A/in ) are applied to a series of wire-connected tabs, and the resistance change is measured after exposure to 150 °C periodically up to 1,000 horns. The maximum allowable resistance change is 5%. 

Adhesion. Adhesion measurements provide a figure of merit for the degree of conductor bonding to the substrate. The various mechanisms for bonding to the substrate are composition-dependent and were discussed in Sec. 8.2.3 under Inorganic Binders. This property is one of the most difficult to measure in terms of reproducibility and correlation. This is a result of the many factors that affect adhesion test results. These include, but are not limited to, the following specific conductor formulation, substrate, test pattern, test preparation, wire attachment method, solder type, and adhesion test method. 

Plastic parts with fabric attached to the surface must pass a series of durability tests including the cross-cut adhesion test. The cross-cut test is important in order to assess the adhesion strength of the fabric to the snbstrate plastic. This adhesion test provides a rating for the adhesion between all layers from the fabric surface down to the substrate plastic. The original ASTM standard D3 3 59-02 was developed to measme adhesion between thin paint films and flat, metal snbstrates (4). The cross-... 

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