Surface-mount adhesives

Brominated epoxy resins are the reaction product of epichlorohydrin and brominated bisphenol A. They are primarily used in applications where ignition resistance is a requirement, such as printed-circuit boards and other products that need to be flame-retardant. Tetrabromobisphenol A is the largest flame retardant in terms of commercial use at present. It is used in an estimated 95 percent of all flame-retardant printed wiring boards and is used in many flame-retardant surface-mounted adhesives.2 It is manufactured by several producers and is priced as a commodity product. 

Single part epoxy presents another option. Some materials, such as surface mount adhesives, have the curing catalyst locked in a phase change material that is released with heat. Cure temperatures range from 100 to 150°C. The material supplier controls the mixing process, and these materials have excellent shelf life. 

Surface-mount adhesives 35-50% epoxy resin, 1-10% amine or polyamine curing agent, and 10-30% mineral filler and thixotrope Mondur MC/castor oil and castor oil polyols (>60%), filler 5%) Eccobond 125F (Henkel), Epo-Tek 70E-4 (Epoxy Technology), Epi-bond 7275 (Cookson Electronics) Flexobond 442 (Bacon Industries)... 

Furthermore, surface-mount adhesives (SMAs) used in pin transfer must have relatively low viscosities to facilitate pickup by the pins and transfer to the PCB. Because viscosity is so critical, the temperature of the adhesive reservoir must be controlled and preferably maintained between 25 °C and 30 Some SMAs that may be applied by pin transfer are listed in Table 4.17. 

Surface-mount adhesive, solder paste, solder mask, and RTV dispensing Thermal grease, solder paste, and surface-mount adhesives... 

Surface-mount adhesives, silver epoxy, solder paste, and comer and edge bonding... 

Table 4.23 Viscosity changes vs pot life for a surface-mount adhesive ...

Epibond 7275-Series Surface Mount Adhesives Technical Data Sheet, Cookson Electronics (Revised Mar.29,2007). 

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. 

With the advent and wide use of surface-mount adhesives, several industry and industry-association documents have been generated. Examples include the following ... 

IPC 3406, Guidelines for Electrically Conductive Surface-Mount Adhesives... 

Table 6.8 compares the requirements and tests for IPC CA-821 with those of Siemens SN 59651, both of which are applicable to surface-mount adhesives. 

Table 6.8 Requirements and test methods for surface-mount adhesives (IPC CA-821 and Siemens SN 59651)... 

The spread and slump for surface-mount adhesives may be measured according to IPC-SM-817, Method 4.5.5. According to this method, sets of adhesive dots (minimum three per set), 0.65 cm in diameter and 0.25-mm thick, are deposited (for example, by screen or stencil printing) on each of two frosted glass microscope slides. One sample is stored 50-70 minutes at 25 5°C and 50 5% relative humidity and then measured for spreading of the uncured adhesive. The second sample is cured and measured for the increase in pattern from the original dimensions. The acceptance criterion is an increase of less than 10% of the original diameter of the dot or pattern. 

For example, solvent or chemical resistance of adhesives used in surface-mount assembly may be tested according to Method 4.5.6 of ANSI/ lPC-SM-817, General Requirements for Dielectric Surface-Mounting Adhesives Although this method specihes specific solvents including isopropanol 1,1,1-trichlorotrifluoroethane an azeotrope of 6% methyl alcohol and 94% trichlorotrifluoroethane and a 10% alkaline... 

General Requirements for Dielectric Surface Mounting Adhesives, ANSI/ IPC-SM-817, Method 4.5.5 Nov. 1989. 

Guidelines for Electrically Conductive Surface Mount Adhesives, Institute for Interconnecting and Packaging Electronic Circuits, IPC 3406. Jul. 1996. 

---