Printed circuit boards reliability

Pricing as well as reliability considerations have led to an almost exclusive use of Si-based (i.e. Si and SOI) micro machined devices. Packaging and assembly has focused on ceramics (A1203, AIN, Low Temperature Co-fired Ceramics LTCC), Printed Circuit Board (PCB-) and Surface Mount Device (SMD-) technology and multichip modules (MCM s). 

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. 

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically connect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 x 10 " to 3 x 10 " ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 x 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable connections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

Microfluidic Boards Microfluidic board interconnection technologies seek to mimic the electronic printed circuit board for ease of use, reliability, and versatility. In microfluidic board technologies, the substrate contains the passive microfluidic channels and reservoirs, with active components (e.g., sensors, valves) mounted on top (e.g.. Ref. [2]) (see Fig. 1) similarly to electronic components mounted on a printed circuit board (PCB) or multichip modules (MCM) in surface-mount technologies (SMT). Electronic coimections between components are provided on top of the board, while fluidic interconnection is usually provided in the board material itself. Microfluidic circuit boards can be made from a variety of materials, including silicon, glass, and polymers, e.g., polydimethylsiloxane (PDMS), epoxy, or polymethyl methacrylate (PMMA). Microfluidic boards can be as simple... 

Conductive Columns. Nitto Denko Corp. developed an ACF for fine pitch flip chip applications (27). The features of this ACF were (1) connectability between bumpless chips and fine pitch PCB (2) high electrical conductivity (3) repairabil-ity (easy to peal off chips from a printed circuit board at elevated temperatures) (4) high reliability and (5) potential storage at room temperature. There are other notable features too (7) ACF is usable at pitches down to 25 fim, (2) the conductive elements are micrometallic columns as opposed to random-shaped particles, and (3) this adhesive matrix consists of a thermoplastic polymer resin, conductive columns coated with an insulator, and a high Tg polymer, which completely separates the columns from the adhesive (Fig. 6). 

St. Cyr, Valerie A., New Laminates for High Reliability Printed Circuit Boards. Proceedings of IPC Technical Conference, February, 2006... 

One important consideration when designing electronics is to ensure that the electrical components operate at temperatures that will maintain long life and be reliable. Current carrying capacity of the printed circuit board traces is apart of managing the board tenperature, which directly impacts the components. (A trace is a copper conductor in a printed circuit board.The terms conductor and trace are used interchangeably for a printed circuit throughout this chapter. Track is another common term for trace or conductor.) Properly sizing the traces for current is necessary to achieve the desired temperature rise at the board level. 

The quality of a hole drilled through a printed circuit board is measured by its ability to interface with the following processes plating, soldering, and forming a highly reliable, nondegrading electrical and mechanical connection. 

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