Epoxy printed-circuit boards

Figure 17.81 Comparison of data from DMA, TMA and DSC of a printed circuit board epoxy resin. Source Reprinted from Perkin Elmer technical literature.

There are several industry standards covering laminate materials. One of the primary sets of standards is that established by National Electronic Mannfacturers Association (NEMA). Many laminates are clad with copper foil, with the copper later being etched to form printed circuit boards. Epoxy-glass circuit boards are among the most highly used for electronic assemblies. See also printed circuit boards electrical laminates NEMA)... 

Because the heat distortion temperature of cured epoxy resins (qv) increases with the functionality of the curing agents, pyromellitic dianhydride is used to cross-link epoxy resins for elevated temperature service. The dianhydride may be added as a dispersion of micropulverized powder in liquid epoxy resin or as a glycol adduct (158). Such epoxies may be used as an insulating layer in printed circuit boards to improve heat resistance (159). Other uses include inhibition of corrosion (160,161), hot melt traffic paints (162), azo pigments (163), adhesives (164), and photoresist compounds (165). 

Blends have also been prepared by dissolving DMPPO in a monomer and then polymerizing the monomer. An example is an epoxy—DMPPO blend prepared by curing a solution of DMPPO in Epon 828 at 85°C with an alurninum—tetramethylguariidine catalyst. Some copolymer formation is observed. The solutions can be appHed to glass cloth before curing to produce prepregs for composites in appHcations such as printed circuit boards (67). 

A.dhesiveslCements Sealants j Coatings. Excellent adhesives of high strength and high oil resistance can be prepared using nitnle mbber (25). Many references have discussed the use of nitnle mbber—phenoHc and nitnle mbber—epoxy adhesives for printed circuit boards. 

Fig. 2. Multilayer printed circuit board composite. Constmction is multiple layers of epoxy—glass and foil copper. Foil copper outermost layer and drilled through-holes are sequentially plated with electroless copper, electrolytic copper, electroless nickel, and electroless gold.

The preparation of flame-retardant epoxy resins is accompanied by inclusion of tetrabromobisphenol A [79-94-7] in the advancement process (see Flame retardants). Products containing ca 20 wt % Br are extensively employed in the printed circuit board industry. 

Electrical Laminates. A significant use for epoxy resins is in the manufacture of copper-clad epoxy-glass printed circuit boards. Systems are available that meet the National Electrical Manufacturers Association (NEMA), GIO, Gil, FR3, FR4, FR5 specifications. Currently the majority of boards are manufactured to the fiame-retardant FR4 specification. The flame retardance is achieved by the use of a soHd epoxy resin based on... 

Elastomers, plastics, fabrics, wood and metals can be joined with themselves and with each other using nitrile rubber/epoxy resin blends cured with amines and/or acidic agents. Ethylene-propylene vulcanizates can also be joined using blends of carboxylated nitrile rubber, epoxy resin and a reactive metal filler (copper, nickel, cobalt). However, one of the largest areas of use of nitrile rubber modified epoxy systems is in the printed circuit board area [12]. 

Although epoxy resins are the standard base materials for printed circuit boards, other materials with reduced flammability such as polysiloxane can be used. 

TMA expansion study of an epoxy printed circuit board showing the determination of the glass point (71). 

TBBA, a brominated flame retardant, is used in the epoxy resin laminate in printed circuit boards in most manufacturers products. In 1997, a phosphorus-based alternative to TBBA was developed by the German engineering giant, Siemens,... 

Some electrical properties of reinforcing fibers, composite resins, and the resulting composites are given in Tables 6.12, 6.13, and 6.14, respectively. These values should be taken as approximate only, especially for the composites, since fiber orientation, content, and field strengfh have an enormous impacf on fhe dielecfric properties of these materials. Some of the most widespread electrical applications for glass-fiber-reinforced epoxy systems are in printed circuit boards and electrical housing such as junction boxes. 

BISMALEIMIDE POLYMERS. These relatively new polymeric materials were developed to serve the increasing requirements for materials of high strength in high-temperature applications. Currently, a high percentage of the bismaleimides produced are used for printed circuit boards (PCBs). The materials usually are cured with aromatic amines and then compression molded into the PCBs. Future uses include aircraft structural components where bismaleimides may prove superior for high-temperature skin surface applications as compared with present epoxy composites. 

Another important factor that would be important in many of the applications where epoxy-polyimide could be used is the dielectric constant. The high dielectric values of epoxy, in the range 3.5-10, result from the many hydroxyl groups in cured epoxy resin and preclude its use in electrically demanding applications. Polyimide with its excellent physical properties of high temperature stability and low dielectric constant make it an ideal candidate to replace the glass reinforcement in epoxy printed circuit boards. 

Tetrabromobisphenol A is used in epoxy resins especially for glass fiber reinforced used in printed circuit board. Nonreactive compounds such as tetrabromophatalate ester, bis(tribromophenoxy) ethane, and decabromodiphenyl ether are also used. The use of synergists, such as antimony oxide, reduces the quantity of brominated flame retardant necessary but decreases the electrical properties required. 

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