Multilayer board processing

There has been a continual increase in size and complexity of PCBs with a concurrent reduction in conductor and hole dimensions. Conductors can be less than 250 p.m wide some boards have conductors less than 75 pm wide. Multilayer boards greater than 2.5 mm thick having hole sizes less than 250 pm are being produced. This trend may, however, eventually cause the demise of the subtractive process. It is difficult to etch such fine lines using 35-pm copper foils, though foils as thin as 5 pm are now available. It is also difficult to electroplate holes having high aspect ratio. These factors may shift production to the semiadditive or fully additive processes. 

In 1990 the majority of U.S. PCB production resulted from subtractive or print-and-etch processing additive processes were less than 6% of the total multilayer boards accounted for 55.8%. The ratio of rigid to flexible surface areas plated is about 15 1. High performance plastics including polyimide. Teflon, and modified epoxy comprised 6% of the market ( 324 million) flexible circuits were 6.6% ( 360 million) (42). 

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 the early and mid-1960s, multilayer boards arrived on the scene, particularly for military uses, where cost was not an issue the premium was on compactness, weight, and reliability. These boards evolved with advances that occurred in metallizing and photoresist processes. Standards and specifications were prepared for conventional boards in the early 1960s, and in the latter part of that decade for multilayers. Because of migration problems, silver was permanently eliminated as a competitor for copper in metallizing circuit board holes. 

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. 

TLCPs have many outstanding properties that uniquely qualify them for these high performance multilayer boards (12). Table 7 compares the applicability of LCPs with other state-of-the-art materials for electronic packaging. They can be made into very thin, self-supporting films (<50 J.m) with a controllable CTE. By processing LCP films as described above, circuit substrates can be manufactured with a CTE around 7 ppm/°C and thermal stability over 250°C. TLCPs do not require secondary resins for fabrication into MLBs, they can be thermally bonded to themselves and to copper foil. Control of molecular orientation has been shown to result in a substrate with the desired CTE of 6 to 7 ppm/°C for matching alumina, or 16 ppm/°C for matching copper. 

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. 

Innerlayers of multilayer boards can shift during fabrication processes and create poor layer-to-layer alignment. During the artwork exposure or fabrication of the multilayer boards, the position of the copper shifts relative to the nominal stacked position. This can create breakout or missing of the copper pads when drUhng the interconnection holes. 

Deckert, C. A., Couble, E. C, and Bonetti, W. F., Improved Post-Desmear Process for Multilayer Boards, IPC Technical Review, January 1985, pp. 12-19. 

Several adhesive resins, especially acryhc-based adhesives developed for flexible circuits are not stable with the permanganate solution that is common for rigid multilayer boards, which means that a very narrow window is allowed for the desmearing of the rigid/flex process. If the materials are dipped too long in the solution, the adhesive layers swell and the rehabihty of the through holed will be damaged. 

Various new processes have been developed to build functional elements and devices on the flexible substrates. They have been generating more functions than wiring or assembling board of the discrete components. They are the similar ideas as embedded passive technologies of multilayer boards, but they generate more values with flexible substrates. Table 66.2 shows several material examples and applications. 

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. 

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. 

Boron-reinfixced aluminum is a technologically mature continuous-fiber MMC (Fig. 2). Applications for this composite include tubular truss members in the mid-fiiselage structure of the space shuttle orbiter and cold plates in electronic microchip carrier multilayer boards. Fabrication processes for boron/aluminum composites are based on hot-press diffusion bonding of alternating layers of aluminum foil and boron fiber mats (foil-fiber-foil processing) or plasma-spraying methods. 

---