21st Century Manufacturing Episode 9 pdf

Output Performs arithmetic operationsSends signals that determine the operation andsequencing of data paths, memory, and use of theinput/output devices 8 Primary main memory; volatile me

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 processorand memory chips) are first assembled onto PCBs (the motherboard and the eight mainmemory boards, assembled vertically on the motherboard) System level packaging on thelower left also shows lhe secondary memory (floppy and hard drive) The schematic showsthe main functional abstractions of the physical devices

Packaged chip

ComponentsLevel 1

Lc\'t'!1

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 operationsSends signals that determine the operation andsequencing of data paths, memory, and use of theinput/output devices

(8) Primary (main) memory; volatile memory of programs

or data being used by the processor(b) Secondary (floppy or hard drive) memory; nonvolatilememory or storage of programs

Includes keyboard, mouse, voice activation, digitalcamera, incoming e-mail, fax, and so forthIncludes screen, printer, outgoing e-mad.fex, andthe like

• Single-sided boards have copper tracks on only one side of an insulating strate

sub-• 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 Adouble-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

6.2.3 B08rd Preparation

The starting board must be prepared for further processing through a variety of

Insulating substrateLands

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 thisfully 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 CNCCNC 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 tional holes provide anchoring locations for heat sinks and connectors.

Addi-These holes, or vias, drilled through any insulative layers are nonconducting.Therefore, conductive pathways must be created between the sides of a double-sidedboard 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 copperions, but without any anode/cathode action Specific details of these reactions aregiven by Nakahara (1996) and Duffek (1996)

6.2.5Circuit Lithography

In this important step, a circuit pattern is transferred to the board's copper 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

sur-foil It is first coated with a polymer resist, sprayed on in liquid form or rolled outover 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 Theexposed resist is then strip-washed away; next, the now-unprotected copper areasare 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-washedaway Thus, at this stage, the board exhibits the exact pattern of the desired tracksplating, 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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(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 areOnce the exposed resist has been removed, that part of the copper is etched away

The final sketch shows the desired layout.

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 individualboards 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 inmultilayerooards.

copper patterns on both sides Precise alignment between each layer is obviouslyessential 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 tions among the inner boards involves a special manufacturing challenge In par-ticular, the creation of buried and blind vias deserves special attention Surfacelaminar circuits (SLCs) are created using intermediate photolithography methods

connec-on the inner boards (Figure 6.5) Inner layer patterns for the ground and powerdistributions 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 vialocations are formed by pbotoexposure, development, and strip-washing Thesevia hole surfaces are coated with copper by either direct metallization or electro-less plating Further inner connections with thicker layers of copper are alsoadded With the constant push for miniaturization, higher speeds, and the use ofsurface-mount components on both outer surfaces of a board, these technologieswill be more in demand

The final phase in the fabrication of a printed circuit board is to test and finishthe 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 electronicand mechanical components attached to it to form a final PCB assembly

6.3 PRINn:D CIRCUIT BOARD ASSEMBLY

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 dering the leads into place on the opposite side of the board

sol-• Surface mount technology (SMT) is now the preference in industry because itallows greater packing densities The SMT method directly solders componentleads to copper lands on the same side of the hoard This approach greatlyreduces the surface areaneeded to fit components (requiring 40% to 80%lessspace than PIH), making it possible to build smaller and higher performancecircuit boards The leads used at the edge of a surface mounted IC typicallyhave 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 byside inside one larger outer package These have the following advantages:closer packing densities; reduced routing needs in the PCB, hence reducing thenumber of layers needed in a multilayer board; reduced power consumption;higher performance dueto tighter noise margins, smaller output drivers, andsmaller 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, wherethe connections are madeunderneath the chip instead of on the perimeter Smallballs of solder make the connections between the chip's underside and the PCB

• Flip chiptechnology (Fer) extends SMTIBGA for even greater packing sity In this case, the IC is turned over and placed face down on the board Asearlier, solder balls and a perimeter solder ringcreate the circuit connections

den-to the board Additional mechanical bonding with epoxy is required.Just as SMT has gradually replaced PIH for many applications due to theincreased 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 coursehigher 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 iscleaned, tested, and if necessary, reworked The key differences lie in the method forplacing and soldering components on the board; there are also some differences inthe subsequent testing and reworking steps Most SMT components alsoshare thc

"real estate" on a multilayer board with PIH components This complicates theassembly sequence, but the basic processing steps do not change

IBack_end packaging was already introduced in Chapter 5 However, with continuing tion,i1illhard to differentiate where theIepackage ends and where the PCB begins, and so some further

miruaturiza-I1pre 6Ji Ie packaging famiIiClI and trends (from PrintN CircuiJs Handbook byayde 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 theleads of each component into the holes that have been predrilled in the board duringaxial 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 rightangles to be inserted in the board Preforming is thus required so that componentleads, which are straight, are bent into a U shape (Figure 6.7) Light-emitting diodesand fuse holders are common radial lead components with parallel leads radiatingfrom 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 suchthat 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 veyor After preheating, the board and the projecting leads of the components meetthe agitation wave that "wets" and cleans the surfaces The final laminar wave creates

con-by forcing the liquid solder to flow into the clearances between the leads and holes Figure 6.8c shows that there are design rules (layout rules) fur this process thatmust be followed to ensure correct flow and filling and to avoid "shadowing."

through-Cleaningand testingfollow the wave soldering The PCBs are degreased to remuve

contaminants such as flux, oil, and dirt that might chemically degrade the assembly orinterfere 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.Testvidual components, subcircuits, and the entire circuit.The assembly may also be pluggedinto 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 theassemblies 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 feasible to repair defects than to discard the entire board Rework is always a skilledmissing components, or repair of the copper substrate

compo-6.3.3 Fabricating with Surface Mount Technology ISMT)

As mentioned, surface mount technology uses an assembly method in which ning through the board There are two primary methods shown in Figures 6.9 and 6.10:

compo-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 automaticallyplaced 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, andJorinfrared radiation to bond the components to the PCB surface The board is thenwave soldered as described in the PIH method The difference is that in SMTassembly, the components are first shielded before passing them through the moltensolder 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, tocreate strong adhesion, is to heat the solder paste in a solder reflow oven Boardsmove on conveyors through heated chambers under controlled conditions This stepremelts the solder sufficiently to form a high-quality mechanical and electrical jointbetween the component leads and the board's circuit lands Finally, whicheverattachment process is used, the board is put through the standard test/inspection!

AgitatingLaminarwave wave

Fipre6.10 Solderpaste and reflew method: (1) solderpaste applied to land

areas, (2) components placed on board, (3) paste baked, and (4) solder reflew (courtesy of Groover, 1996)

6.3.4 Ball Grid Arrays (BGA)

Faster, more versatile computing with miniaturized devices drives the demand formore input/output (110)counts between the functional blocks of the circuit Thismeans there is a greater demand for a product in which the leads have less spacingbetween them This may be referred to as fine pitch technology (FPT), which allows,say, the standard size quad flat packIewith many "gull wing" or "J-leads" to be sur-face mounted onto the PCB Unfortunately, there is a natural limit to how close these

Board surface

"O"Od~ ~

Solder BT epoxy ThermaV Solder bump

Fipre 6.11 The ball grid array (from Prill~ CircuitsHtmdbook by Clyde F.

of the silicon IC and the board

The ball grid array (BOA) and flip chip technology both overcome these culties (Figures 6.11 and 6.12) Here, the leads that would otherwise be sticking outfrom the side of the quad flat pack (QFP) are replaced by an array of solder balls onthe underside surface These are originally made by silk screening, and then thesolder is reOowed to create the balls Leicht (1995) predicts that soon BGA ICs willlargely replace QFP ICs with side leads The trade literature indicates that somemanufacturers are presenting their standard quotations only in the BOA format.Mitt and associates (1995) describe the future need for micro-BOAs with anexample of a BGA that has a 125 micron pitch and 576 locations for pins in a packagethat is only 21 x 21 millimeters in size Evidently, a number of rival techniques willcontinue to emerge to address this issue

diffi-8.3.5 Direct Chip Attachment Methods InclUding Flip Chip

Technology IFCTI

It is understandable that products like today's cellular phones, PDAs, and thin-formatlaptops relentlessly push for small chips and smaller, more efficient PCBs The res nit

one step further The following three methods are extensively used for mounting abare chip directly onto a PCB:

• Direct wire bondingattaches the chip to the landson the board The wirebonding requires more assembly costs, but the reliability is high once the man-ufacturing processes are established

• Tape automated bonding (TAB) also attaches the chip directly to the lands on theboard The chips are firstheld by a metal lead frame that is printed onto apolyamide film that looks like a roll of photographic film with sprockets along itstin "bumps" on the IIOs of theIe(center Figure 6.13f).In later assembly,this pre-fabricated tape indexes passed the desired locations, and the outer leads of thepads on the interconnects of the PCB The thermode is shown in FIgure 6.13f

• Flip chip technology (FCT) represents today's culmination of methods that

package ICs and provide connections to the PCB Pads on the top of the chippad array downward onto the corresponding pads on the PCB The literatureshows that IBM tried these methods in the 1960s (see Gilleo et al., 1996),together with a few peripheral contacts between the chip and the board Aschematic of theFCTprocess is shown in FIgure 6.12 It is emphasized that theepoxy underfill in the fourth sketch is used not just to fix the chip on moredistribution layer, the difference in coefficient of thermal expansion betweensilicon chip and copper coated PCB can cause damage to the individual con-sumption and clock speed may also improve with the flip chip style because thebetter quality than the aluminum interconnects on a chip

6.3.6Summary

Figure 6.13 summarizes the major designs The trend to SMTusing BGA is well lished, and some observers (see Leicht, 1995) see them becoming the major attach-ment method in these next few years FCT is the next generation after BGA andindeed is a fast-growing category However, Messner (1996) observes that they stillrepresent only a small percentage of alliCs used and, for economic reasons, are usedonly in situations in which a large110count or small pitches are needed Messner pos-

estab-tulates that around the year 2000, the ICs with over 100 lIDs (the type where FCT

cernes into play) amount to only 10% of the total worldwideIeconsumption Ofcourse as FCT matures, a range of techniques is being investigated to make themeasier to use and more cost effective For example, new attachment methods willincrease reliability and circuit efficiency These include deposited metal, mechanicalattachment, and conductive anisotropic adhesive (see Palmer et at, 1997)

A final note on flexible circuits-made from roUed copper, polyamide, or ester film-is added for completeness Figure 6.14 illustrates four of the typical

i """, _ ","", "'"" _Substrate

Die attachment using wider bumps

(from Primed Circum Handbook by Clyde F Coombs, © 1996 Reprinted by

Wedge bond-on arcabout first bond -

(c) (d)F1gIft 6.14 Folding configurationsfor flexible circuits (adapted from literature ofShcldahI.Inc.,l996).

6.4 HARD DRIVE MANUFACTURING

it needs to be kept in a nonvolatile format

Since 1965,magnetic discs (or disks) with surfaces that can be magneticallyrecorded over (just like audiotapes) have been used for permanent memory In addi-ally called heads) into their hard disc drives for the recording heads These extremelysmall electromagnetic coils art: one of the standard technologies that can be used towrite data to, and read data from, the hard disc on a computer They are the criticallink.between a computer's virtual desktop and the files stored more permanently onthe hard disc As a user opens, closes, saves, and transfers files, the gentle whirringsounds that can be beard come from these heads and discs doing their job.The markets for such miniaturized heads and discs are driven by the associatedmarkets for small, high-capacity storage devices for personal computers (PCS),