Printed wiring assemblies

The most important application of parylenes is as a conformal coating for printed wiring assemblies. These coatings provide excellent chemical resistance, and resistance to fungal attack, hi addition, they exhibit stable dielectric properties over a wide range of temperatures. 

Figure 1.4 Printed-wiring assembly (4x8 in.) with surface-mounted components. Courtesy The Boeing Co.

Lonox L5020/ Kyzen Environmentally friendly, easy to use, highly alkaline aqueous amine blend Batch, inline, immersion heated systems for flux removal from printed wiring assemblies N/A N/A N/A None to boiling 0.31... 

Fungus-resistance tests are used to determine the reaction of adhesives to several species of fungi under high humidity and a warm environment in the presence of inorganic salts. Testing for fungus resistance is more appropriate for exposed adhesives as in printed-wiring assemblies than for hermetically sealed devices or modules. One test method specified as Method 4.5.14 in IPC-SM-817 references Method 2.6.1 of IPC-TM-650, Test Methods Manual... 

Coefficient of thermal expansion Tee.—Surface-mount assembly process subjects the printed wiring assembly to more numerons temperature shocks than typical through-hole processes. At the same time, the increase in lead density has caused the designer to nse more and more layers, making the board more susceptible to problems concerned with the base material s coefficient of thermal expansion Tce- This can be a particular problem with regard to the z-axis expansion of the material, as this induces stresses in the copper-plated hole, and becomes a rehabihty concern. Figure 13.12 shows typical z-axis expansion for a variety of printed circuit base laminate materials. 

Repair and rework procedures differ between through-hole-mount and surface-mount interconnections. That difference is due to the size and geometries of the interconnections, the munber of I/Os on the packages, the thermal sensitivity of the components, and the overall density of the printed wiring assemblies. The following subsections describe these factors for the two technologies. 

The details of these materials are discussed in other chapters. However, it is important to understand their role in the overall circuit board assembly process. First of all, the functions of these three materials require that they have good adhesion to all surfaces. Therefore, the printed wiring assembly must be cleaned of any flux residues as well as those residues left behind from the cleaning procedure. 

Early ovens utilized focused and unfocused IR lamps, mounted in the reflow oven tunnel. These bathe the solder paste-coated circuit boards and associated components with a broad spectrum of photonic energy heavily weighted to the IR end of the electromagnetic spectrum. The radiant energy absorbed by the printed wiring assembly (PWA) and related materials brings about the thermal increase needed to reflow solder. As the board travels beyond the last reflow heaters at the exit end of the oven, the absorbed heat is lost to the environment or the board is actively cooled by fans. The cooling results in resolidification of the molten solder and solder-joint formation. 

Turbini, L. X, Bent, W. R., and Ready, W. X, Impact of Higher Melting Lead-Free Solders on Reliability of Printed Wiring Assemblies, Journal of Surface Mount Technology, Vol. 13, No. 4,2000, pp. 10-14. 

IPC-7912 Calculation of DPMO and Manufacturing Indices for Printed Wiring Assemblies... 

MIXED ASSEMBLY A printed wiring assembly that combines through-hole components and surface mounted components on the same board. 

PRINTED WIRING ASSEMBLY DRAWING A document that shows the printed wiring base, the separately manufactured components which are to be added to the base, and any other information necessary to describe the joining of the parts to perform a specific function. PRINTED WIRING LAYOUT A sketch that depicts the printed wiring substrate, the physical size and location of electronic and mechanical components, and the routing of conductors that interconnect the electronic parts in sufficient detail to allow for the preparation of documentation and artwork. 

J.K. Bonner, L. Del Castillio and A. Mehta, Hi-Rel Lead-Free Printed Wiring Assembly, The Proceedings of Surface Mount International, (Chicago), Sept 22-26, 2002, p 453... 

In addition to computer monitors (which account for almost 90% by weight of the lead (Pb) in electronic waste found in landfills) printed wiring assemblies are the other major source of Pb in electronic products. The tin-lead solder alloys used in components and on solderable terminations, and as a preservative coating on cards and boards, are the applications for lead-free solder compositions. 

Turbini, L.J. Bent, W.R. Ready, W.J. Impact of higher melting lead-free solders on reliability of printed wiring assemblies. Journal of Surface Mount Technology 2000, 13, 10 14. 

FIG. 1 A typical surface mount soldering technology printed wiring assembly. (Courtesy of Sandia National Laboratories.)... 

The third process that was developed for Pb-Sn eutectic and near-eutectic solders is referred to as wave soldering. Wave soldering was developed as a large-volume process for through-hole printed wiring assemblies it continues to be used for mixed assemblies that have both through-... 

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