Die- and substrate-attach adhesives

Table 5.2 Properties of some die- and substrate-attach adhesives... 

Table 5.2. Properties of Some 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. 

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... 

During die attach, an adhesive creates a mechanical interface between the die and substrate in a semiconductor assembly. A typical dispense pattern has intersecting lines with strategically placed dots so that after die placement, the fluid is evenly distributed (Figure 10-12). Most die attach adhesives also provide thermal and/or electrical conductivity between the die and substrate. 

Some adhesive materials and processes are used across many applications. For example, adhesives used to attach bare die and substrates in hybrid or multichip modules may be used for a wide variety of ground-based military... 

The stress analysis model developed by Suhir includes the normal stresses acting in the die as well as the interfacial stresses responsible for the cohesive and the adhesive strengths of the bonding layer [168]. The effects of the die size, attachment compliance, and non-linear behaviour of the attachment material are discussed and analyzed to assess the relationship between the compliancy of the attachment, the die fracture, and the fatigue life of the assembly. Starting from a complete tri-material assembly, the author elaborates a set of 90 equations which can be substantially simplified in the case where the adhesive layer has a very small thickness or Young s modulus in comparison to the die and substrate values. The axial X and interfacial k compliances of a tri-material assembly, reduced to a die-substrate assembly in the case of the axial compliance, are, respectively. 

Aromatic polyimides have found extensive use in electronic packaging due to their high thermal stability, low dielectric constant, and high electrical resistivity. Polyimides have been used as passivation coatings, (1) interlayer dielectrics, (2) die attach adhesives, (3) flexible circuitry substrates, (4) and more recently as the interlevel dielectric in high speed IC interconnections. (5) High speed applications require materials with a combination of low dielectric constant, flat dielectric response versus frequency and low water absorption. 

Adhesives that reduce and dissipate stresses consist of low-modulus elastomeric types. Low-stress adhesives are required in attaching large die, large substrates, and flex circuitry. Low-modulus elastomeric adhesives are also used to attach and fillet large components to dampen vibration and shock. 

Bare die and other chip devices are attached with electrically conductive or nonconductive adhesives to ceramic substrates having defined circuit patterns produced by thin-film vapor deposition and photoetching of metals or by screen-printing and firing of thick-film pastes. With recent advancements in fine-line printed-circuit boards, adhesives are also finding use in attaching bare die to PWBs, a technology known as chip-on-board (COB). 

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