Acoustic Methods Cut Supply Chain Risks
A smoothly running supply chain is not easy to achieve. Although much of supply chain management consists of sourcing and receiving parts and services in a timely fashion and at an acceptable cost, there are often other factors that complicate management. Outwardly similar manufacturers may wind up evolving very different supply chain management styles.
A smoothly running supply chain is not easy to achieve. Although much of supply chain management consists of sourcing and receiving parts and services in a timely fashion and at an acceptable cost, there are often other factors that complicate management. Outwardly similar manufacturers may wind up evolving very different supply chain management styles.
One factor that can greatly impact the supply chain is the presence of unseen defects in plastic ICs and related parts. Such defects can greatly affect product reliability, but significant numbers of such defects occur only intermittently, which results in differing methods for handling the problem. Manufacturers of high-end products that can be heavily impacted by defects resulting from relatively minor process drift are likely to use a method known as acoustic micro imaging as part of process control or as part of final inspection to remove bad parts before getting shipped. Some fabless manufacturers try to keep costs very low by using acoustic inspection only as a failure analysis tool for field returns. Between these two extremes there are many other solutions to the problem of poor reliability caused by hidden internal packaging defects.
The Hidden World Of Packaging Defects
The growing use of acoustic inspection of plastic-encapsulated ICs and other parts is based on the need to avoid sudden and perhaps widespread product failures. The facts are these: a certain small percentage of parts contain hidden flaws such as delaminations, cracks and voids. These flaws may seem harmless since they may not immediately disturb the electrical functioning of the part.
Some of these flaws do grow over time in service – a disturbing concept because the flawed parts cannot initially be distinguished electrically from flawless parts. The problem is that small mechanical packaging anomalies may turn into larger anomalies that in turn cause electrical failures. One common scenario: a delamination between the die face and the molding compound, initially harmless, gradually expands through normal thermal cycling in service until a tiny wire pulls off the die or breaks due to the relative motion between the die and the plastic encapsulant during thermal expansion. Another common mechanism: delamination along a lead finger enables ionic contaminants to reach the bond pads and cause them to corrode. There are many other possible mechanisms, but if a lot or a series of lots contains anomalous parts, it requires only bad luck [Figure 1] for undetected anomalies to turn into a large-scale product recall, and to damage the manufacturer’s reputation.
Without the use of acoustic microscopes, these anomalies are essentially invisible. A few can be discovered, at some effort, by x-ray systems if there is a large distortion of the molding compound and wide open air gap associated with the delamination. But these anomalies are most compatible with acoustic microscopes for the basic reason that ultrasound propagates easily through production materials and is reflected very strongly from very thin (below 0.1 micron) internal gaps – meaning delaminations, cracks and voids.
There are probably several reasons for the recent dramatic increase in acoustic inspection. One reason is simply a wider - in fact, global - understanding of the many intricacies of supply chain management including the need to remotely control quality and reliability at a subcontractor. A second is the blunt reality of greater competition and thinner profit margins: recovery from a product failure or recall is more difficult than it once was, and is especially harmful to smaller fabless companies trying to establish themselves. To some extent, implementation has been helped by the greater throughput speed of acoustic inspection that has recently been made possible by Sonoscan’s proprietary UPH-Turbo™ technology. Higher throughput brings down the unit cost of inspection or screening and makes flying blind a less attractive option.
Implementing Acoustic Inspection
Exactly how acoustic inspection is used to maintain the integrity of a supply chain varies from one manufacturer to another. The most intense users of the technology are semiconductor packaging foundries who produce high-end plastic encapsulated integrated circuits or flip chip parts for critical-use applications. Many such assemblers use automated acoustic systems to inspect 100% of parts. The dimensions of the defects they need to eliminate from production can be less than a thousandth of an inch. These intense users are paid back by achieving extremely high product reliability, by gaining customer satisfaction, and - on the unusual occasions when a defect is found - by feedback that can be used to make their processes even tighter. 
In less rigorous environments it is more feasible to inspect only a percentage of parts from each lot, and to increase that percentage when a defect is found. Depending on volume, this inspection may be automated, semi-automated or manual. Automated inspection means that if the parts can be loaded into JEDEC-style trays, they are conveyed, unstacked, inspected, separated by quality and restacked by automated handlers integrated into the acoustic inspection stations [Figure 2]. Purely manual inspection means that a human operator handles the parts and inspects them individually. Semi-automated inspection can take various forms, but generally these parts are in trays, the trays are handled manually, and the imaging process is programmed. At the end of any level of inspection, parts deemed defective are removed either automatically or manually [Figure 3].
Fabless companies probably face more dangers than other manufacturers simply because their dependence on the integrity of parts and services is typically spread over a widely distributed network. Fabless companies may also assess risks differently. For some fabless companies, it makes economic sense to keep costs down by bypassing acoustic inspection of parts, which would have to be done by a supplier, who would raise prices. To compensate, the fabless company may perform its own acoustic inspection of components that have failed functional test, or it may hire a third-party lab to perform the inspection. This approach keeps costs down, and helps to some extent to keep bad product out of the hands of end users, but can become expensive if the number of failed parts is very large.
Other fabless companies use acoustic inspection as a tool to monitor their suppliers. One example: a fabless company had placed a qualifying trial order with a remote supplier for 5,000 of a fairly common type of IC. This order was understood by both parties to be the precursor to a much larger order. At the receiving end, acoustic screening showed that about 2% of the IC packaged received had internal delaminations. Some of the delaminations were unquestionably harmless, but production could not ramp up with the knowledge that any percentage of these packages was defective. After fairly brief negotiation, the supplier used the acoustic data to find and eliminate the cause of the delaminations (aerosol contamination), the hundred or so bad components were removed from the lot of 5,000, and production moved ahead. No further evidence occurred in the production lot, though inspection continued as a monitor, and the data obtained was sent back to the packaging house.
Figure 4 attempts to put this complexity into a graphic form. Marked on this chart are the sites at which acoustic inspection may be beneficial. It also suggests the advantages that can be gained from shared acoustic data - for example, if the contract assembler sends acoustic data to the packaging house to enable the packaging house to modify its processes 
Benefits Of Early Detection
Across the industry, the revelation that a given lot contains bad parts may come about in a variety of ways. Companies that have their own acoustic systems are likely to make this discovery earlier and to quantify its impact more easily. Some defect trends are identified only by electrical testing, by physical sectioning of components, or by outright product failure.
The impact on production varies from plant to plant. An assembler who practices “just-in-time” inventory control may find that the only available inventory of a given component contains defects. If the discovery is made as the parts are delivered, the damage will be minimized, and will largely involve screening out the bad parts and finding good replacements for them.
An assembler who maintains a larger inventory and who does not screen incoming parts may wind up in the uncomfortable position of having a sizeable inventory of parts that cannot be used. In situations like this, urgent questions tend to be asked. Is there an alternate supplier of the component? If there is an alternate supplier, can he actually supply an identical component? Or does he make a very similar component that may have a different landing pattern?
The acoustic discovery of flawed parts has a way of revealing problems not merely of processing technology but also of management. One assembler discovered a failure on a high percentage of assemblies via functional board test. The failures were all in a single IC and were a surprising discovery, because the failed IC had been coming from the same supplier for years with no significant problems. Acoustic imaging of both mounted and unmounted components eventually led to questions for the supplier of the IC: had there been, for instance, any recent changes in process parameters, or equipment, or materials? After a bit of delay, it was revealed that the supplier had recently outsourced the construction of that component to a third party.
A key challenge when a significant number of defective components are discovered is to prevent production from grinding to a halt. Often it is possible to acoustically screen the parts on hand in sufficient quantities to maintain at least partial production while a more thorough solution is found. Large lots of questionable parts give assemblers real headaches - only 5% of a lot may be bad, but which 5% is it? - and create the need for rapid mass screening that is frequently carried out at the various Sonoscan laboratories, where screenings of lots of 100,000 or more components are not unusual.
The overall purpose of acoustic screening at various appropriate points in the supply chain is to avoid problems that will have a large downside - or, since such problems cannot be realistically eliminated in their entirety, to minimize the damage they do to the supply chain as a whole and a manufacturer’s reputation and profitability.
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Text 1697 words. 10.25.05
Figure captions for Acoustic View Cuts Supply Chain Risks:
Figure 1 Red-yellow areas at left and right center are delaminations of the molding compound from the tie bar in this PQFP. These delaminations reach, or nearly reach, the exterior of the package. At the same time the molding compound is delaminated (yellow at center of image) from the die paddle. The red-yellow tie bar delaminations appear to end near the die paddle (they become black) – but this is just where the tie bar plunges vertically downward, and this vertical portion may be delaminated as well. Diagram at bottom shows cross section at green arrows.
Figure 2 A JEDEC-style tray of TQFPs, imaged acoustically. Part of a larger lot where the percentage of parts having internal packaging defects threatened product reliability.
Figure 3 After mass acoustic screening, a technician compares the tray to its acoustic image to identify and remove defective ICs.
Figure 4 Flow of materials, parts and data in the supply chain, including fabless companies. Locations where acoustic systems can add stability are marked in green.
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Contact information:
Sonoscan, Inc.
2149 E. Pratt Blvd.
Elk Grove Village IL USA 60007
Phone: 847 437-6400
Fax: 847 437-1550
E-mail: info@sonoscan.com
Website: www.sonoscan.com
