21st Century Manufacturing Episode 2 Part 3 ppsx

The interconnections are provided by coppertracksthat are applied to the circuit board in aseries of additive or subtractive processing steps similar to those used nections to individual

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

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Output Interface' Compiler

TABLE 6.1 Key Functional Abstractions of a Computer System

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

6.2.1Introduction

Printed circuit boards (PCBs) provide the foundation for the interconnections among subcomponents The interconnections are provided by coppertracksthat are applied to the circuit board in aseries of additive or subtractive processing steps similar to those used 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 mouse The earliest circuit-laying methods used screen printing techniques, from which the termprinted circuit board orprinted 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 sides of 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 is a flat laminated "sandwich." A thin substrate (0.25 to 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

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'lame6.1 Three types of printed circuitboardstructures:(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 platesor 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

Insulating substrate Lands

Via hole Insertion hole

Plated through-hole Buried via hole 7 -Partially buried via hole

Copper

size for the final computerfelectronic equipment Second, tooling or alignment holes, 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 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

8.2.4Hole Drilling, Punehing, and Plating

Additional holes are then created in the board Automatic hole punchers or CNC CNC drills can also drill a stack of several panels, thereby increasing productivity

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)

6.2.5Circuit 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,

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

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 plating, the board is shielded under the "hills" of remaining photoresist Meanwhile, the copper is added-that is-c-electroplated, into the exposed "valleys," creating the

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(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

6.2.6Muttllayer 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-Butlerooat

Starting board

F1pre6.5 Surfacelaminar circuitscreated for blind and buried via boles in 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

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

Probimer<!lS2SmaU Probimer®52 Fine Plated Cu 2 0 = Surface thin photo via curtain-coated pitch conductor 5() 6()0 shielding buildup (127"un) thin dielectric (l60).tm) I ,

on any given hoard The list thatfollows 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 holesdrilled 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 areaneeded 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 dueto 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 madeunderneath the chip instead of on the perimeter Small balls of solder make the connections between the chip's underside and the PCB

• Flip chiptechnology (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 ringcreate 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 butcan 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 Allthese 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 alsoshare 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,i1illhard to differentiate where the Iepackage ends and where the PCB begins, and so some further

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"pmprocess This involves inserting the leads of each component into the holes that have been predrilled in the board during 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 insertedpmcomponents 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

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

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

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 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 for one to three days sometimes at high temperatures

Rework is the final step that will commonly be seen in any factory tour of a

sub-contract board assembly operation Because of the high value of electronic compo-feasible to repair defects than to discard the entire board Rework is always a skilled 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-ning through the board There are two primary methods shown in Figures 6.9 and 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!

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