Multilayer board processing Lamination

For the production of base materials for circuit boards with higher performance (e.g., glass/epoxy or graphite/cyanate ester combinations) and of multilayer boards, the laminating resin acts as adhesive or special bonding prepregs must be used. The requirements for the resins, which act as adhesive, depend on both the processing conditions and the desired properties of the final circuit board and are similar to those described above. 

The preparation of a multilayer board involves laminating together in precise registration individual two-sided boards. Each plane of circuitry is separated by layers of prepreg prior to the lamination process. 

Ordered polymer films made from poly benzthiazole (PBZT) and poly benzoxazole (PBO) can be used as substrates for multilayer printed circuit boards and advanced interconnects to fill the current need for high speed, high density packaging. Foster-Miller, Inc. has made thin substrates (0.002 in.) using biaxially oriented liquid crystal polymer films processed from nematic solutions. PBZT films were processed and laminated to make a substrate with dielectric constant of 2.8 at 1 MHz, and a controllable CTE of 3 to 7 ppm/°C. The films were evaluated for use in multilayer boards (MLBs) which require thin interconnect substrates with uniform controllable coefficient of thermal expansion (CTE), excellent dielectric properties, low moisture absorption, high temperature capability, and simple reliable processing methods. We found that ordered polymer films surpass the limitations of fiber reinforced resins and meet the requirements of future chip-to-chip interconnection. 

In Europe the term multilayer board includes cross-laminated timber elements used for construction parts in buildings and shuttering boards. Both these types of boards are mainly produced with MUF adhesives in gluing processes with relatively long pressing times or in processes with high temperatures. Polyurethane (PUR) adhesives are already in use as an alternative to MUF adhesives for production of building elements whereas EPI adhesives are in the introduction phase. 

The purpose of copper pads surrounding the drilled holes in PWBs is to accommodate any potential layer-to-layer or pattern-to-hole misregistrations and thus prevent any hole breakout outside the copper area of the pads. This misregistration is caused mainly by the instability and movement of the base laminate during its processing through the PWB or multilayer board (MLB) manufacturing steps. 

Laminates based on Rhone Poulenc s Keramid 601 polyimide resin are fabricated in a conventional laminating press, and processed in a manner similar to that used for epoxies but with an extended cure cycle or post-cure. The room-temperature mechanical and electrical properties are similar to epoxy laminates, as shown in Table 9.4. At elevated temperatures, the polyimides exhibit exceptional stability. In particular, the thermal coefficient of expansion in the Z axis does not change significantly up to approximately 240°C, as shown in Fig. 9.11. Exhaustive tests have shown that polyimide-based multilayer boards can withstand repetitive thermal cycling at elevated temperatures (>150°C) without cracking of plated through holes. Similar excellent results were also obtained in solder shock tests (10 s at 288°C in molten solder). The thermal stability of these materials is retained at temperatures of approximately 200°C for continuous exposure in air, which has qualified them for military applications. 

A process has been described for laminating polyimide films to each other and to conductive metal foils in the production of flexible electrical circuitry. This will find use in aircraft, spacecraft, and missiles as well as industrial applications. It is expected that the market for such constructions will grow as the market share of multilayered circuit boards increases. 

To facilitate the many choices of laminates and their associated properties, industry standards groups such as the IPC have defined minimum performance specifications and have issued several specifications to inform the selection process. Some of the most commonly used material specifications are those that deal with laminate,prepreg, and copper foil. IPC lOl, Specification for Base Materials for Rigid and Multilayer Printed Boards, and IPC-4652, Metal Foil for Printed Wiring Applications, are the primary specifications for clad laminates, prepregs, and foils. Another specification, IPC- 4104, Specification for High Density Interconnect (HDI) and Microvia Materials, deals with many of the new materials for HDI, such as epoxy-coated microfoils, as discussed in this chapter. 

An adhesive film is laminated on a rigid board with a release sheet first. The boards are then routed by punching or by NC router. It is a very simple process if the adhesive material is pressure-sensitive adhesives (PSAs). Each piece of stiffener is placed on the flex circuits with appropriate pressure, mostly by hand.The process is more compUcated when a thermo-set type adhesive material is required. A temperature higher than 160°C with a pressure higher than 20 kg/cm is required for more than 30 min. A similar heat press used for multilayer circuit boards or film coverlay is necessary. Dummy boards should be prepared to make the pressure uniform. 

SEQUENTIAL BUILD-UP A process for making multilayer PWBs in which already finished multilayers are laminated together to form a higher-layer-count final board, or in which additional outerlayers are added to finished multilayer PWBs. 

Substrates made by the multilayer process from tape cast alumina have received considerable attention in recent years for multichip module (MCM) applications. An MCM consists of an array of closely packed chips on an interconnect board several inches on a side. Cofired ceramic is attractive relative to organic laminates because its thermal conductivity is almost 2 orders of magnitude higher, an important consideration in high-density circuitry. In addition both alumina and aluminum nitride ceramics are more closely matched to silicon in CTE than are organic boards. For similar reasons, alumina and AIN are attractive for ball grid array (EGA) mounting of chips. ... 

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