Board assembly process

It is important that the manufacturing engineer and operator understand the critical steps in the printed circuit board assembly process to ensure the manufacture of a cost-competitive, reliable product. That understanding includes both the general function of the equipment as well as the activity taking place inside the machines. The following sections of this chapter describe in detail the printed circuit board assembly processes. 

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. 

Table 45 lists the Weibull parameters for the 80 lead UTQFP component. Notice that only a few components were tested in some cases. This was a result of large fallout during the board assembly process resulting in misplaced components as well as solder joint bridging. A relative comparison based on alloy A1 is shown in Figure 33. Weibull analysis was not performed for alloy All as there were only two failures for this alloy. Comparing the mean life, all alloys performed better than alloy A1 for this component. Although the mean life for All was not calculated, it would have exhibited a longer mean life compared to alloy A1 based on the first failure data given in Table 45. The same trend can be observed by using a first failure criterion, except for A14, which had lower first failure life than Al.

The simple analysis presented above confirms that new formulations are required to produce stable, reliable products for field use. Practical system requirements, as defined by Mil Spec conformity and the use of standard fabrication and assembly processes, definitely require that a electro-optic polymer system with better thermal properties than thermoplastic acrylates be developed. That this is true for optical interconnection boards and modules is not surprising because of their complexity. It is perhaps remarkable that it remains true for even simple devices, such as a packaged, pigtailed traveling-wave modulator. The ultimate success of electro-optic polymers will be their use in cost-effective products that are used by systems designers. 

MIDs create new demands on assembly because of their complex geometric shape. The new requirements on assembly processes caused by different MID types in order to develop qualified production systems for MID assembly must be considered (Figure 43). The new task of mounting SMDs onto 3D circuit boards calls for new capabilities in dispensing and mounting systems. Both processes work sequentially and are realized with Cartesian handling systems for PCB production. 

For example, a highly filled epoxy resin, for potting wires into a board connector, represents about one or two cents of the total component cost. The cost associated with mixing, heat curing, handling, and cooling hot pats adds about five cents to the assembly process cost. Finally, cost associated with downstream quality control and rejected parts adds, conservatively, another two or three cents. 

Optimization is an activity carried out in almost every aspect of our life, from planning the best route in our way back home from woik to more sophisticated approximations at the stock market, or the parameter optimization for a wave solder process used in a printed circuit board assembly manufacturer optimization theory has gained importance over the last decades. From science to applied engineering (to name a few), there is always something to optimize and of course, more than one way to do it. 

Fig. 7. Schematic depicting a flip chip assembly processing utilizing ABC adhesive, (a) A chip is aligned to the ABC preform which is on a carrier film, (b) The ABC preform is tacked to the die at 100°C and imder a pressure of 150 g for 30 s, and then the carrier film is removed after the chip is cooled down, (c) The chip with ABC preform is aligned to a mating chip carrier (FR4 board), (d) The chip/ABC preform is attached to the chip carrier at 100°C and under a pressure of 150 g for 30 s, and then the package is cooled down.

The recommended lead-free solder formulation is Sn-Ag-Cu for board assembly but there are other formulations such as Nickel-Palladium (NiPd), or Nickel-Palladium with Gold flash (NiPdAu). Passive components, to be compatible with a lower temperature Lead process (which is 215°C for 50/50 Tin/Lead formulations and 230°C for 40/60 formulations) and the higher lead-free process of up to 260°C, use pure matte Tin for their contacts. The use of lead in solder is partially based on several potential reliability issues. Pure Tin component leads have been shown to result in inter-metaUic migration in the termination of the electronic component and the growth of tin whiskers which could cause short circuits (which is why there is a exemption for military use (only) components). 

In the surface mount assembly process, type 11 and type III boards will always require adhesive to mount the SMDs for passage through the solder wave. This is apparent when one envisions components on the bottom side of the substrate with no through hole leads to hold them in place. Adhesives will stay in place after the soldering process and throughout the fife of the substrate and the product, since there is no convenient means for adhesive removal once the solder process is complete. Additionally, adhesives can be used to enhance both thermal and electrical conduction between device features and board features. This means use of an adhesive must consider a number of both physical and chemical characteristics ... 

The need to use lead-free solder in surface mount technology applications where soldering temperatures of between 220 °C and 240 °C are likely to be encountered has necessitated the use of such high temperamre polyamides as DuPont s Zytel HTN LX resins for board assembly. It is interesting to note that some Japanese companies have been using lead-free solders in their flow soldering processes since 1997. 

Size is often the primary driver for MCM-based systems. The typical multicomponent discrete assembly provides a sihcon-to-board efficiency of <10 f>ercent (actual total die area versus the total printed circuit board area). MCM technology can often increase the sdicon-to-board efficiency to 35 or 40 percent with chip and wire assembly processes, and to 50 percent or higher with some of the higher-density processes. Thus, with reduced size and weight, MCMs offer a practical approach to reducing overall system size while providing enhanced performance due to a reduction in the interconnect distance between chips. Multichip modules typically use three to five times less board area than their equivalent discrete solution. ... 

PCB performance In addition to the preceding rehability challenges posed by lead-free assembly processes, it also has to be guaranteed that all other performance characteristics of the printed circuit board stay the same. This includes dielectrical properties such as dielectric constant Dc (which influences impedance), dissipation factor Df, and thermomechanical properties such as copper peel strength, glass transition temperature, or coefficient of thermal expansion (CTE). These properties should not be affected by the assembly processes applying higher temperatures. 

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. 

Assembly processes can be placed into the following three categories, which are described by the types of circuit board components ... 

The need for higher processing temperatures limits the assembly process window of Pb-free solders. A higher nominal temperature is needed to accommodate the temperature variation at components across a circuit board to ensure melting of the solder and adequate wetting and spreading at each interconnection. On the other hand, the maximum temperature must be limited to prevent thermal damage to heat-sensitive devices and the circuit board. 

The following are important design considerations regarding the assembly process for a through-hole printed circuit board products ... 

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