21st Century Manufacturing Episode 9 pdf

234 Computer Manufacturing Chap. 6 Flame 6.1 Computer packaging levels (from Computer Organizanon and Design, 2nd ed. by David Patterson and Joho Hennessy,@ 1996. Morga.u KaUfmllWl Publisilers.). From the tup left from the previous chapter, packaged integrated circuits (ICs) (e.g., the main processor and memory chips) are first assembled onto PCBs (the motherboard and the eight main memory boards, assembled vertically on the motherboard). System level packaging on the lower left also shows lhe secondary memory (floppy and hard drive). The schematic shows the main functional abstractions of the physical devices. Packaged chip Components Level 1 Lc\'t'!1 I Input Output Interface' Compiler 6.2 Printed Circuit Board Manufacturing 236 TABLE 6.1 Key Functional Abstractions of a Computer System Component Function I. Processor (CPU) data path 2. Processor (CPU) control 5. Output Performs arithmetic operations Sends signals that determine the operation and sequencing of data paths, memory, and use of the input/output devices (8) Primary (main) memory; volatile memory of programs or data being used by the processor (b) Secondary (floppy or hard drive) memory; nonvolatile memory or storage of programs Includes keyboard, mouse, voice activation, digital camera, incoming e-mail, fax, and so forth Includes screen, printer, outgoing e-mad.fex, and the like 3. Memory 4. Input 6.2 PRINTED CIRCUIT BOARD MANUFACTURING 6.2.1 Introduction Printed circuit boards (PCBs) provide the foundation for the interconnections among subcomponents. The interconnections are provided by copper tracks that are applied to the circuitboard in aseries of additive or subtractive processingsteps similarto those used in the fabrication of integrated circuits (ICs). Copper lands are also applied for the con- nections to individual ICs and components. Colloquially speaking,the PCB is a "subway map" of circuit tracks that connect the ICs and other devices located at the stations. The board itself also provides the rigid structure that holds chips and other fragile system components in place and allows for the physical connections to the outside world of input/output devices such as the hard drive, monitor, keyboard, and mouse. The earliest circuit-laying methods used screen printing techniques, from which the term printed circuit board or printed wiring board developed. Photolitho- graphy is now the preferred method for circuitizing boards. There are three general types of PCBs, depicted in Figure 6.2: • Single-sided boards have copper tracks on only one side of an insulating sub- strate. •Double-sided boards consist of copper tracks on both sidesof the insulating layer. •Multilayer boards are constructed from alternating copper and insulating layers. 6.2.2 ·Startlng Board" Construction Starting boards are so-called because the circuit patterns have not yet been applied. A double-sided PCB isa flat laminated "sandwich." A thin substrate (0.25to 3 mm thick) of insulating material is sandwiched between thin copper foil (0.02 to 0.04 mm thick) on both sides.Epoxy resin is the most commonly used insulating polymer for the inner substrate, usually reinforced with an epoxy/glass fiber called e-glasaThe insulating sub- strate is formed bytaking multiple sheets ofthin glass fiber impregnated with partially 238 Computer Manufacturing Chap. 6 I f eoppe,rOil C -~copperfOil Insulation substrate ___ ~ Insulation ~ substrate ======1 Copper foil (b) Insulation layers (0) 'lame 6.1 Three types of printed circuit board structures: (a) single-sided, (b) double-sided, and (c) multilayer showing vias and pathways between layers (courtesy of Groover, 1996) cured epoxy. The boards are then pressed together between hot plates or rolls. The heat and pressure cure and harden the laminates, creating a strong and rigid board that is heat and warp resistant. 6.2.3 B08rd Preparation The starting board must be prepared for further processing through a variety of shaping procedures. First, shearing operations are carried out to obtain the desired Insulating substrate Lands Via hole Insertion hole Plated through-hole Buried via hole 7 -Partially buried via hole Copper 6.2 Printed Circuit Board Manufacturing 237 size for the final computerfelectronic equipment. Second, tooling or alignment holes, typically 3 mm in diameter, are drilled or punched into the corners of the board. The tooling holes are used to precisely align the boards as they move from one machine to another in the sequence of fabrication steps. The board may be bar coded at this point to facilitate identification. The final step in this preparation phase is to care- fully clean and degrease the surfaces. While board making does not require the strin- gent cleanliness standards of chip making, a fairly high level of cleanliness is essential to minimize defects. 8.2.4 Hole Drilling, Punehing, and Plating Additional holes are then created in the board. Automatic hole punchers or CNC drilling machines are used. For large batch sizes punching is the most efficient, but CNC drills can also drill a stack of several panels, thereby increasing productivity. The holes allow conducting paths, called vias, between the two sides of a double- sided board. Other insertion holes are for any pin-in-hole (PIH) components. Addi- tional holes provide anchoring locations for heat sinks and connectors. These holes, or vias, drilled through any insulative layers are nonconducting. Therefore, conductive pathways must be created between the sides of a double-sided board. These pathways are typically formed by electroless plating. This process is tai- lored to the deposition of copper onto the epoxy/glass fiber surface of the through- holes. Regular electroplating will not work because the surfaces are nonconducting. Electroless plating takes place chemically in an aqueous solution containing copper ions, but without any anode/cathode action. Specific details of these reactions are given by Nakahara (1996) and Duffek (1996). 6.2.5 Circuit Lithography In this important step, a circuit pattern is transferred to the board's copper sur- face(s) using selective photolithography and etching. PCB industries may use a sub- tractive method. In Figure 6.3,the starting board's surface is already the thin copper foil. It is first coated with a polymer resist, sprayed on in liquid form or rolled out over the board from a spool of dry film (see Clark, 1985, p.175). Ultraviolet (UV) lithography then exposes the resist in areas that are not wanted for circuits. The exposed resist is then strip-washed away; next, the now-unprotected copper areas are chemically etched with any of the following solutions: ammonium persulphate, ammonium hydroxide, cupric chloride, or ferric chloride. The remaining, unattacked copper areas constitute the board's circuitry or the lands. Alternatively, an additive process of PCB circuitizing begins with an unclad board, namely, the insulating material without a copper foil. Photoresist is spread on the board and then exposed in the pattern of the desired tracks. This exposed photoresist is then strip-washed away. Thus, at this stage, the board exhibits the exact pattern of the desired tracks except that no copper has been laid down yet into these "valleys." During electro- plating, the board isshielded under the "hills" of remaining photoresist. Meanwhile, the copper is added-that is-c-electroplated, into the exposed "valleys," creating the desired circuits and lands (Figure 6.4). 238 I leop",foil ( ~Substrate ~ Starting board 5'h,"' (2) Computer Manufacturing Chap. 6 /Resillt / (1) ~~,m'ini"gooPP" r===J'",ml,"d) (3) F1gure 6.3 Subtractive method of circuitizing (courtesy of Groover, 1996). In the subtractive process, the copper foil is protected where the circuits and lands are needed. Lithography exposes the resist in areas that are not ultimately required. Once the exposed resist has been removed, that part of the copper isetched away. The final sketch shows the desired layout. Insulating substrate co."" II I ) ) : ) ~Eltttrol'~ ~I.ti"g (3) (1) (2) ~oopper ~O<l'",,) (4) Flpre 6.4 Additive method of circuit manufacture (courtesy of Groover, 1996). Photoresist is spread on an unclad board and tben exposed in the pattern of the desired tracks. This exposed photoresist is then strip-washed away. During electroplating, copper is electroplated into these exposed -veueys,'' creating the desired circuits aodlands. 6.2.6 Muttllayer Board Fabrication In multilayer board fabrication, the circuit designs are first applied to individual boards. Once the layers have been integrated, a multilayer board resembles a double- sided board from the outside, but it will have several integrated middle units with Butlerooat Starting board 6.3 Printed Circuit Board Assembly 23. F1pre 6.5 Surfacelaminar circuitscreated forblind andburied viabolesin multilayerooards. copper patterns on both sides. Precise alignment between each layer is obviously essential and is achieved by the alignment pins that fit tightly into the tooling holes. It may have already occurred to the reader that creating vias and connec- tions among the inner boards involves a special manufacturing challenge. In par- ticular, the creation of buried and blind vias deserves special attention. Surface laminar circuits (SLCs) are created using intermediate photolithography methods on the inner boards (Figure 6.5). Inner layer patterns for the ground and power distributions are first created on the inner boards, and then the board is oxidized. Next, an insulating photosensitive resin is coated over the panel. The desired via locations are formed by pbotoexposure, development, and strip-washing. These via hole surfaces are coated with copper by either direct metallization or electro- less plating. Further inner connections with thicker layers of copper are also added. With the constant push for miniaturization, higher speeds, and the use of surface-mount components on both outer surfaces of a board, these technologies will be more in demand. The final phase in the fabrication of a printed circuit board is to test and finish the circuitry. Both visual and electronic test methods are used to check the function- ality of the copper wiring. Detailed information on testing is given by Andrade (1996).The last step isto screen print a legend guiding the placement of components on the board, as well as a bar code.The finished board is now ready to have electronic and mechanical components attached to it to form a final PCB assembly. 6.3 PRINn:D CIRCUIT BOARD ASSEMBLY 6.3.1 Overview A multilayer PCB will probably contain hundreds of individual components ranging from sophisticated ICs to rather ordinary heat sinks and perimeter connec- tions. As a result several, or perhaps all, of the following assembly styles will be used Probimer<!lS2SmaU Probimer®52 Fine PlatedCu 2 0 = Surface thin photo via curtain-coated pitch conductor 5() 6()0 shielding buildup (127"un) thin dielectric (l60).tm) I , layers I (4C!IJ.ITI) I 240 Computer Manufacturing Chap. 6 on any given hoard. The list that follows gives a summary, and the details are shown in Figures 6.6 to 6.13. • Pin-in-hole (PIH) is the older classical method. It involves inserting the leads of standard components into holes drilled in the board, then clipping and sol- dering the leads into place on the opposite side of the board. • Surface mount technology (SMT) is now the preference in industry because it allows greater packing densities. The SMT method directly solders component leads to copper lands on the same side of the hoard. This approach greatly reduces the surface area needed to fit components (requiring 40% to 80% less space than PIH), making it possible to build smaller and higher performance circuit boards. The leads used at the edge of a surface mounted IC typically have the "gull wing" or a "J-lead" shape shown in the diagrams toward the end of Chapter 5. 1 • Multichip modules (MCM) consist of several SMT chips all mounted side by side inside one larger outer package. These have the following advantages: closer packing densities; reduced routing needs in the PCB, hence reducing the number of layers needed in a multilayer board; reduced power consumption; higher performance due to tighter noise margins, smaller output drivers, and smaller die sizes; and lower overall packaging costs. An excellent review may be found in Green (1996). • Ball grid array (BGA) is a development of individual SMT components, where the connections are made underneath the chip instead of on the perimeter. Small balls of solder make the connections between the chip's underside and the PCB. • Flip chip technology (Fer) extends SMTIBGA for even greater packing den- sity In this case, the IC is turned over and placed face down on the board. As earlier, solder balls and a perimeter solder ring create the circuit connections to the board. Additional mechanical bonding with epoxy is required. Just as SMT has gradually replaced PIH for many applications due to the increased packing densities it offers, BGA and FfC have been growing in popularity in comparison to standard SMT. The costs of these newer methods are of course higher but can be justified in certain devices such as cellular phones where minia- turization is key to market leadership. Figure 6.6 shows many of these trends. All these assembly methods involve similar basic processing steps. Compo- nents are first soldered into place on the board, and then the whole assembly is cleaned, tested, and. if necessary, reworked. The key differences lie in the method for placing and soldering components on the board; there are also some differences in the subsequent testing and reworking steps. Most SMT components also share thc "real estate" on a multilayer board with PIH components. This complicates the assembly sequence, but the basic processing steps do not change. IBack_end packaging was already introduced in Chapter 5.However, with continuing miruaturiza- tion,i1 ill hard to differentiate where the Ie package ends and where the PCB begins, and so some further discussion is warranted from a PCB perspective. 6.3 Printed Circuit Board Assembly 241 I1pre 6Ji Ie packaging famiIiClI and trends (from PrintN CircuiJs Handbook by ayde F. Coombs, C 1996.Reprinted by pennission oftbe McGraw-Hill Companies). 6.3.2 Fabricating with Pin~ln·Hol. Technology IPIH) Insertion is the first step in the "old classic" pm process. This involves inserting the leads of each component into the holes that have been predrilled in the board during fabrication. The insertion method depends on the type of component. For example, axial components-oommonly including resistors, capacitors, and diodes-are cylin- drical in shape, and their leads project from each end; the leads must be bent at right angles to be inserted in the board. Preforming is thus required so that component leads, which are straight, are bent into a U shape (Figure 6.7). Light-emitting diodes and fuse holders are common radial lead components. with parallel leads radiating from the component body, and require a different type of work head and preforming. Wave soldering is the next major step in manufacturing. For example, a PCB with inserted pm components is passed over a standing wave of molten solder such that the solder just touches the bent leads on the underside of the board. Figure 6.8a «~~ ~ W 00 W W I1pre (,.7 Affixing a component to a PCB with the "old clasaic" PIH method: (1) an uial c:omponent is first 1Dsertcd;(2) bendina and Cl'<lppinain-ro. (a) mel outward (b) (cnurtesyof Groover. 1996). - ,~ Computer Manufacturing Chap. a. shows that flux is applied to the underside of the board at the beginning of the con- veyor. After preheating, the board and the projecting leads of the components meet the agitation wave that "wets" and cleans the surfaces.The final laminar wave creates the joints at the temperatures shown in Figure 6.8b.This process forms solder joints by forcing the liquid solder to flow into the clearances between the leads and through- holes. Figure 6.8c shows that there are design rules (layout rules) fur this process that must be followed to ensure correct flow and filling and to avoid "shadowing." Cleaningand testingfollow the wave soldering.The PCBs are degreased to remuve contaminants such as flux, oil, and dirt that might chemically degrade the assembly or interfere with the electronic functions of the circuitry. Boards are visually inspected (hwnan and computer vision systems are used) for a variety of potential quality defects, including substrate damage, missing or damaged components, and soldering faults.Test points are also designed into each circuit from the CAD phase. Contact probes test indi- vidual components, subcircuits,and the entire circuit.The assembly may also be plugged into a working system and powered up to test its functionality. Most PCBs are also sub- jected to burn-in tests that force early failure of weak assemblies;this test operates the assemblies forone to three days. sometimes at high temperatures. Rework is the final step that willcommonly be seen in any factory tour of asub- contract board assembly operation. Because of the high value of electronic compo- nents, as well as the cost of board fabrication and assembly, it is economically more feasible to repair defects than to discard the entire board. Rework is always a skilled manual operation involving manual solder touch-up, replacement of defective or missing components, or repair of the copper substrate. 6.3.3 Fabricating with Surface Mount Technology ISMT) As mentioned, surface mount technology uses an assembly method in which compo- nent leads are soldered to lands on the surface of the board rather than into holes run- ning through the board. There are two primary methods shown in Figures 6.9and 6.10: For adhesive bonding and wave soldering, epoxy or acrylic is first dispensed through a stencil onto the desired locations on the board. Components are then automatically placed on the board surface by a computer-controlled "onsertion'' machine at a rate of up to several components a second. The adhesive is cured with heat, UV, andJor infrared radiation to bond the components to the PCB surface. The board is then wave soldered as described in the PIH method. The difference is that in SMT assembly, the components are first shielded before passing them through the molten solder wave. The ref/ow method is a more common method that first stencils down the solder paste and a flux binder on the lands of the PCB. Next, the components are "onserted.' The flux binder is then baked at low temperatures. The final step, to create strong adhesion, is to heat the solder paste in a solder reflow oven. Boards move on conveyors through heated chambers under controlled conditions. This step remelts the solder sufficiently to form a high-quality mechanical and electrical joint between the component leads and the board's circuit lands. Finally, whichever attachment process is used, the board is put through the standard test/inspection! rework operations described earlier. Agitating Laminar wave wave ('J 24O"e il i 140°C 8 ~ ~~\\ Fluxing Preheat Agitating Laminar wave wave Time (b) ,c_ Shadowing bd placement not ' Fl recommended [[]]~OrientatiOWlnot t::::I~ I recommended bd W brjbd [] bdbdbd~ Recommended orientations (0) 111-6.1 (a> WavellOklerios equipment, (b) temperature profile, and (c) design rules. IWave contact line Preheaters "Fluxer Conveyor Entrance [...]... so-called nanosliders-c-are extremely Disc-drive capacity, gigabytes 10 Log scale 14/10.8 inch 35i~/ 0.1 ~ %:::orCmVS) om 198 6 8788 89 F\gure 6.15 90 91 92 93 94 95 96 Disc drive capacity over the last decade Figure6.!6 megabyte 12 1 0.5 ~~ ~~ ~~ ~ ~ ~~ ~~ ~ Year 97 Average price per Computer Manufacturing 250 Chap 6 it is performed by human operators viewing the components through stereomicroscopes The assembly... Pennumverocessor 1E+6 'DEC1264 ~ PPC620 PPC604 ~ vrPC601 Spare MIPS4400 Transistors per chip DEC21064 80286 1E+5 8086 lE+4 lE+3 197 0 197 5 198 0 198 5 199 0 Yo ar 199 5 2000 2005 Flpre 6.21 The transistor count in the central processing unit (CPU) from 197 0 and projected to 2005 Computer Manufacturing 258 Chap 6 6.5.2 The Present From a management of technology point of view, the computer industry is about coping-or... CD-ROM, TV, telephone, workstation, and wireless communications technology 6.5.4.4 The Integrated Man-Machine Age (2002 to 2020 and Beyond) For 199 9, PCs-17% The Economist ( 199 9) states that U.S consumers mure than in 199 8 raisiug household penetration purchased 16 .9 million to 52% However, the same and other observers indicate a possible reduction over the next few years due to several factors: (a) overcapacity;... only the beginning of a new age of man-machine devices Wearable computers are already established devices in advanced applications Weiss ( 199 9) provides a popular review Akella and associates ( 199 2), Smailagic and Siewiorek ( 199 3), and Finger and colleagues ( 199 6) provide more scientific details Extrapolating from these existing prototypes, how might the following list of technical developments influence... upgrades-many users have "powerful enough" machines; and (c) the rise of PDAs, smart cellular phones, and networked computers (see Red Herring, 199 8) Obviously the PC "ruled" in the desktop age ( 198 1- 199 1) and was the key workhorse or platform for the World Wide Web age ( 199 2-2001) However, in the new age of man-machine devices, distinctions and interfaces between human beings and their communication devices... most significant breakthrough in computing did not occur until the development of the microprocessor Throughout the 196 0s, Computer Manufacturing 256 TABLE 6.2 Some of the Milestones Decade Prototype 194 " ENlAC EDVAC EDSAC Transistor 195 0s UNIVAC1 1_ 650 and 701 IC f360Series !'n" 19' " 199 0 -2000 PDPS CDC 6600 4004 Apple II Internet IBM PC in the Development Chap 6 of the Electronic Computer Comments... chronological "eras" that summarize the rise of the computer from the early mechanical computers, to the vacuum tube era, to the Ie, and to the microprocessor (e.g., see Stem, 198 0; Bell, 198 4; Patterson and Hennessy, 199 6a; Economist, 199 6) Partially based on these other writings, the present text hypothesizes that the history and anticipated future of commercial computers may be divided into four distinct... automated bonding, and flip chip techndlogy (from Primed Circum Handbook by Clyde F Coombs, © 199 6 Reprinted by pennission of the McGraw-Hill Companies), 248 Computer Manufacturing (c) Chap 6 (d) F1gIft 6.14 Folding configurations for flexible circuits (adapted from literature of ShcldahI.Inc.,l 996 ) 6.4 HARD DRIVE 8.4.1 MANUFACTURING Introduction At the beginning of Chapter 5 the Ie, especially the central... development to design, manufacturing, and marketing The recent history and some of the present situations in the computer industry illustrate how difficult this can be, even for some top performers • IBM and DEC suffered during the early 199 08 because they were large conservative organizations, overcommitted to a mainframe philosophy • Apple lost market share throughout the 199 0s Analysts and economic... 194 7.Usually, the commercial development phase is 5 to 10 years behind the scientific discovery phase and prototype use by the academic community This is certainly true of the World Wide Web (see BernersLee, 198 9) Actually, this particular gap is 25 years if today's "dot-com-fever" is measured from the beginning of the DARPAnet and its use in the academic community 6.5.4.1 The Iron Age ( 195 3 to 198 0) . gigabytes 10 Log scale 14/10.8inch 35i~/ ~ %:::orCmVS) 0.1 198 6 8788 89 90 91 92 93 94 95 96 97 om 12 Fgure 6.15 Disc drive capacity over the last decade. Figure6.!6 Average price per megabyte. 1 0.5 ~~~~~~~~~~~~ ~ Year 250 Computer Manufacturing. Leicht, 199 5) see them becoming the major attach- ment method in these next few years. FCT is the next generation after BGA and indeed is a fast-growing category. However, Messner ( 199 6) observes. any anode/cathode action. Specific details of these reactions are given by Nakahara ( 199 6) and Duffek ( 199 6). 6.2.5 Circuit Lithography In this important step, a circuit pattern is transferred