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Voids in Electronic Parts Yield to Ultrasound

• Tom Adams Sonoscan Inc. Elk Grove Village, III.

Voids are significant because they can cause electronic part failures. Imaging by acoustic microscope provides a valuable inspection method.

AVoid in an electronic part or component is like an air bubble that appears by various means during raw materials processing that is later cured or hardened. Although it is simplest to think of a void as a trapped air bubble, many voids-depending on the host material and processes-are not formed by entrapment and do not contain air.

Whatever their etiology, voids are significant because they can cause electronic failures . They occur in polymers such as molding compounds and flip chip underfills, in metals such as solder, and in ceramics such as those in ceramic chip capacitors. Often the void that causes an electronic failure is very small and operates as a slow-motion failure mechanism that can lead to unanticipated field failure .

In terms of inspection, voids are very susceptible to imaging by acoustic microscopes because they all contain a solid-to-gas interface. The acoustic properties of any solid-to-gas interface are profoundly different from the properties of solid-to-solid interfaces. A void's

Fig. 1 - Acoustic image of part of a plastic-encapsulated microcircuit. Bright diagonal lines are wires within the encapsulant. White spots near and among the wires are potentially troublesome voids.

Em ADVANCED MATERIALS & PROCESSES· JUNE 20 13

solid-to-gas interface reflects virtually 100% of trate

the ultrasound pulsed into the material ; the men

very high amplitude signal sent back to the inal

acoustic microscope's transducer can become enin

a very bright pixel in the acoustic image. As cont

the transducer scans over the surface of the rosie

part and sends pulses of ultrasound into the void

part, internal voids show up as white (or pseudocolor) features . The acoustic images Fig.

shown in this article were made in Sonoscan's part

various SonoLab offices . may

Voids in encapsulants in 01 To encapsulate a lead frame-mounted inte ','Go-

trates that the voids are close to the wires in all three dimension s. A void position ed between two wires can result in a leakage path for current betwe en the two wires, thr eatening reliability. Over tim e, voids may absorb moisture and contaminants fro m the environment, leading to wire co rrosion and electrical shorting failure modes. Worse yet, if voids impinge on a wire, electr ical failur e is likely.

The mottled pattern of the molding comp o und in Fig. 1 is relat ed to, but does not dir ectl y represent, filler particl es and epoxy. Very tiny featu res that are bright white may well be tiny voids.

Voids can cause failures in plastic -encapsulated devices in othe r ways as well. Examples include connec ting two lead fingers and causing them to short; by appearing in the die attac h beneath the die and blocking heat dissipation; and by lodging on top of th e die at a wire bond (alth ough delaminations are more common here) and cau sing the bond to break.

Var ious types of underfill pro cesses used to protect the solder connections ben eath flip chips (and, mor e recentl y, silicon interposers) are also sub ject to voiding, and resulting failur es. Underf ill voids can block heat d issip ation fro m th e flip chip- because th e so lid- to-gas int er face at the top of a void th at reflects ult rasound so well is also a good reflector of heat. These void s can also create electrica l failures by causing so lder bumps to collapse and break contact.

Figur e 2 shows the aco us tic image of a flip chip section. The ima ge is gated on the solder bumps (small circles) and the underfill surr ounding them . At cente r, the large br ight feature is a void th at makes contac t with at least eight of th e solder bumps. Th e patt ern and color of each of the sm all circles connected to th e void is uniform, and more importan tly is the same as the pattern and color of th ose bumps away from th e void. This means th at no bump has yet become det ached. But th e functioning of th ose bumps partly or wh olly unsupport ed by und er fill is likely to be brief.

The mechanism by which voids destroy solder bumps is shown in the diagram in Fig. 3. Th e void, at right , creates an empty spac e in place of cured und erfill supporting and encasing the bump. With repea ted thermal cycling, the solder begins to move. Pb-sold er is most likely to creep into th e void, while the more rigid Pb-free solder will probably fracture. In either case, the bump may eventually slump to such a deg ree that it will become detached from the bond pad on the flip chip above. Lead-free solde rs tend to deform more slowly than so lde rs containing lead , but both types are susceptible to th is failure mechanism. This type of defect is a key reas on why such large numbers of flip chips and silicon int erposers-the former from both development and production , th e latter chiefly from development-are imaged in Son oscan's laboratories .

Fig. 2 - The large rounded void in this acoustic image of flip chip underfill is in contact with at least eight solder bumps (circular features) and may cause the bumps to lose electrical contact.

Bump

Detachment . ~

(Substrate)

Fig. 3 - Diagram shows solder bump creeping into an adjacent void.

Voids in solder Solder joints connecting the exte rna l leads of a com

ponent to a pr inted circuit board are not imageable by ultrasound because they present no flat sur face; ultrasound merely scatters in many direc tions from their rounded and unpredictable shapes.

But solder is often used to attach a heat sink to a single component such as a flip chip, or to an assembly such as a pr inted circuit board or an isolated -gate bipolar transistor (IGBT). In these applicatio ns, the int egrity of th e solder is important, primarily to ensure that voids do not prevent heat dissip at ion and thereby cause chips to overh eat.

Figur e 4 shows the aco ustic image of a metal heat sink portion th at was sold ered to one side of a printed circuit board. Th e weave of th e • oboard and a few other fea Q • tures can be d iscerned in the background. The red- yellow features ar e ultrasonically highly reflective voids in the solder. Th ese may simply be pockets of air tr apped when the solder was applied; one of

Fig. 4 - Pseudocolor acoustic image shows numerous voids

(red, yellow) in the solder bonding a metal heat sink to a - o

printed circuit board.

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ADVANCED MATERIALS & PROCESSES· JUNE 20 13 m

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Fig. 5 - Acoustic image of a multilayer ceramic chip capaci tor shows many tiny voids (bright spo ts) and a single relatively huge void that once held an airborne fiber that was incinerated during firing.

th e large voids see ms to have taken on th e shape of a printed circ uit board depression th at matches a component on th e other side of the board. Collecti vely the large and sma ll vo ids cover a large eno ugh percentage of the heat sink to make overheating a concern . To some degree, they also wea ken th e attachment of th e heat sink to the board .

More cr itical voids occur in the solder th at bonds heat sinks to IGBT modules. These power devices typically generate high heat levels. and do not tolerate high levels of voids in the heat sink solder. Acoustic imaging throu gh the heat sink not only finds voids, but also can map so lder th ickness at the same time. Irregular thickness ofte n means th at cera mic plates und er th e solde r are tilted .

Voids in ceramics into Many of th e void s imaged by acoustic microscope s the (

reside in multilayer ce ra mic chip capacit or s. A void in a isher cerami c ca paci to r is confi ne d to a single layer of dielec muc tric materi al. Such voids are formed during fabricat ion. are ti Cracks and delam inat ion s are probably both more fre elect qu ent cau ses of elect ro nic failure in ceramic capacito rs than are void s, but void s could serve as crack in it iators. pear After failure occ urs , th e void site that caused th e failure elect may, either optica lly or acoust ically, res emble a crac k. layer Voids can also occ ur in electro de layers, where th ey pose appli little danger to re liab ility. large

Militar y inspec tio n standards define any cera mic chip weal capacitor in which there is a void whose dia meter is more tion than half the th ickness of the dielectric as a rejec t. A void with a diameter grea ter than this may be susce pti ble to a mak leakage cur rent. This curre nt may be small initially, bu t it Sonc may eventually degrade the dielectric until th ere is a leak lows age path betw een the two adjacent electrodes. In a low matt power enviro nment, a void of this typ e may sho w no and symptoms for a relatively long time; in a high-power envi is nc ronment . failur e is likely to occur more quickly. faces

Figure 5 shows the acoustic image of a multilayer ceramic indk chip capacitor containing an unusual type of void. The long, ages straight white feature is a void created not by air, moistur e, or just volatiles. but by a single fiber dust particle that found its way othe

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disk and th e complex anomalies within the disk can simplify th e pro cess of learning how voids form ed.

Voids occur in many other electronic parts. On e example is dir ect bonded wafers, where a void between th e two wafers can cause silicon to fly loose as one wafer is thinned. Many failur es are not so immediate, and a sma ll molding co mpo und void that is not near another surface may pose no risk at all. It is the potential for voids to gener ate unan ticipated field failures that makes them such frequ ent subject s for aco ustic micro imaging. a

For more information: Tom Adams is a consultant for Sonoscan Inc., 2149 E. Pratt Blvd., Elk Grove Village. IL 60007, 847/437-6400. info@sonoscan.com, www.sonoscan.com.

Fig. 6 - 3-D acoust ic image of a ceramic disk shows a complex void (or group of voids) at right and a few scattered voids. The disk's top surface was retained for orientation; other surfaces were subtracted from the image.

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into the layers of the capacitor. W hen the capacitor was fired, the fiber vanished, leaving the void. The dozens of much smaller bright pinpoint featur es are tiny voids in the various layers of dielectric in the capacitor.

Much larger voids some times appear in monolith ic ceramic parts in electronic assemblies, where there is no layered structure. Dep end ing on the applicatio n in which the part is used, large voids may result in structura l weakness or an imbalance caused by the irregular distribution of weight in the part.

A flat, perforated ceramic disk is sho wn in Fig. 6. To make thi s ima ge, a 3-D acoustic method developed by Sonosca n for its C-SAM systems was used. Th e method allows the op erator to remove echoes pertaining to specifi c material int erfaces from th e acoustic image. Th e side walls and bottom surface were removed . The bulk of the ceramic is not ima ged because, being mon olith ic, it had no interfaces to reflect ultrasound. Th e top surface was ret ain ed to indicate the sha pe of the sample and the much brighter images of internal voids. Th e bottom surface of the disk was just below the large void gro up; not e a few sma ller voids in ot her location s. Being able to see the 3-D structure of the