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HDI BARE BOARD SPECIAL TESTING METHODS
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Although gas probes offer the benefit of little or no product marking, it is difficult to achieve one that is as fine as the best mechanical probes. Adjacent jets tend to combine, producing a short circuit just as if two mechanical flying-probe tips shorted while testing. Thus, the commercial system offered to date includes completely traditional mechanical probes as well, using these to probe closely adjacent (i.e., HDI) sites.As a flying-probe mechanism is still used, there is little speed advantage to date. Eventually some benefit may be obtained due to the elimination of any wait time for z-axis travel of the prober head, although this is already the fastest motion axis of most flying-probe systems. The cost of this technique is modestly higher than that of an ordinary flying prober.
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COMBINATIONAL TEST METHODS
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One technique that offers immediate practicality in resolving difficult testing situations for high-density product often using existing equipment is generally described as combinational testing or sequential testing. As the name implies, this is testing in one or more stages, using a combination of test techniques. Combining techniques inevitably adds complication. The simplest example is the use of a universal grid to test the majority of a product, followed by a flying probe system to test HDI features and reverify the failures reported by the grid. Software tools have simplified the process of combining test methods.
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Sandia National Laboratories, Albuquerque, New Mexico
40.1 INTRODUCTION
The electronics revolution has been sustained through the development of products having increased miniaturization, enhanced functionality, improved reliability, and reduced costs of manufacturing. In fact, innovations in electronics assembly methodologies have kept manufacturing costs low with each new generation of product.
40.1.1 Feature Density Product designers have been able to explore new packages, materials, etc., so as to increase functionality further while, at the same time, reducing the size and weight of both consumer as well as high-reliability military and space electronics.The result has been greater challenges for process engineers. For example, there is a continued trend of decreasing component sizes. Leadless ceramic chip capacitors having sizes of 0804, 0603, and 0402 are commonplace, whereas smaller 0201 devices are being introduced into product lines, particularly handheld products. Manufacturing resources are now developing the means to tool up for 01005-size components. Fine-pitch and area-array packages ball grid array (BGA), chip-scale packages (CSP), and flip-chip (FC) or direct-chip attach (DCA) provide the means to increase device functionality significantly. Input/output (I/O) interconnections that are reaching several thousands on BGA packages require stringent solder paste printing, part placement, and reflow processes controls to minimize defects. At the same time, the need for more complex, multilayer substrates has also placed limitations on the process window so as to avoid damage to circuit board microvias and fine traces that are needed to support higher assembly densities. Lastly, these challenges are made more complicated by the use of Pb-free solders. Changing equipment parameters as well as alternative surface finishes impact solderability performance and thus process yields as well.
40.1.2 Printed Circuit Board Assembly Process There are two basic steps in the assembly of a printed circuit board: (a) placement of the components (resistors, capacitors, etc.) on the substrate and (b) soldering those components into place.Although this is a fairly accurate description of a through-hole, hand-soldering operation,
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PRINTED CIRCUITS HANDBOOK
nearly all electronics assembly operations are, in fact, considerably more complex. Multiple-step assembly processes provide the versatility to incorporate different component package types and a wide variety of substrate configurations and materials as well as to accommodate frequently changing production volumes in order to meet prescribed defect levels and reliability requirements. A more accurate, albeit still relatively general, listing of assembly process steps consists of the following: 1. 2. 3. 4. 5. Preparation of the component and substrate surfaces to be soldered Application of the flux and solder Melting of the solder to complete the joint Post-process cleaning of the soldered assembly Inspection and testing
Some of these steps may be either combined together or eliminated, depending on the particular product line. 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.
Assembly Process Categories Assembly processes can be placed into the following three categories, which are described by the types of circuit board components:
Through-hole technology Surface-mount technology Mixed technology, which is a combination of through-hole and surface-mount components on the same circuit board
Within each of these assembly technologies are different levels of automation that equipment resources offer. The degree of automation will be optimized, depending on the product design, bill of materials, capital equipment expenditures, and actual manufacturing costs. It is important to remember that through-hole printed circuit boards and their assembly processes remain a critical technology in the electronics industry, albeit clearly not at the same production volumes as experienced before the advent of surface-mount technology (SMT). Through-hole technology may be used because it is the only format available for some components, particularly large devices such as transformers, filters, and high-power components, all of which require additional mechanical support that is offered by through-hole interconnections. A second reason for using through-hole technology is economics. It may simply be more cost-effective to use through-hole components, together with manual assembly (i.e., no automation) to produce an electronics assembly. Of course, through-hole technology is not limited to manual assembly. There are varying degrees of automation that can be used to assemble a through-hole circuit board.
Pb-Free Technology The introduction of Pb-free technology has not changed, per se, available electronics assembly processes (reflow, wave, hand soldering, etc.). However, it has caused manufacturing engineers to reassess the parameters used in those processes because of two factors: (1) higher process
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