Dec 1, 2015 - Since the introduction of plastic encapsulated microcircuits (PEMs) in the late 1960's, they have ... increased junction temperature generated by applied operating ... Instruments, and National Semiconductor Corporation.
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Plastic-Encapsulated Microcircuits (PEMs): Long Term Dormancy Studies Edward B. Hakim, John Fink , Patrick McCluskey, Michael Pecht Abstract For many years, the concern for the use of plastic encapsulated microcircuits (PEMs) has been their capability to survive in harsh environments over a long term with continuous or intermittent operation. The issues centered about operational life limitation, due to failure mechanisms associated with internal corrosion, wires and wire bonds, and surface effects. It has now been conclusively demonstrated that best commercial practices will ensure that PEMs made using best commercial materials, processes, and quality techniques will permit devices to perform reliably in the most severe environments. Missile systems are low volume production items, which use relatively few microcircuits. They are required to operate for very short times after being unpowered (dormant) for very long times (10 to 20 years) and exposed to humidity, temperature cycle, and mechanical shock. This paper will address reliability concerns and provide data from studies which were performed to determine if PEMs could survive such long term unbiased applications. These studies include analysis of PEMs (some date coded 1968) from inventory or various storage locations and from applications where the electronic modules containing PEMs were stored for 10-12 years in various environments. Regardless of the storage conditions, the significant factor is that these early vintage commercial grade PEMs, without screening or incoming inspection, survived assembly and extended storage. 1. INTRODUCTION Since the introduction of plastic encapsulated microcircuits (PEMs) in the late 1960's, they have become more widely accepted in a growing number of markets. Beginning with applications in the computer industry they have spread to the telecommunications, automotive, and avionics industries so that now plastic encapsulation is used in the production of more than 98% of the microcircuits worldwide. An analysis of the reasons for this increased acceptance and guidelines for the use of PEMs can be found in the book by Pecht, Nguyen, and Hakim [1]. Failure mechanisms that were the major cause of device failure, historically, have been bond pad/interconnect metalization corrosion and die wire bond/wire open circuit or intermittent open circuit. In the early 80's these mechanisms were essentially eliminated for dual-in-line packages (DIPs) by improvement of die surface passivation, lead frame materials and construction, and molding compounds which were low in ionic contaminants (Cl and Na), with higher glass transition temperatures (Tg>150°C), better molding characteristics and coefficient of thermal expansion (CTE) better matched to the silicon die and lead frame. Accelerated laboratory testing has demonstrated reliability which is comparable or superior to the older cavity type DIPs. Field data today reveals that for those major markets, telecommunication, automotive, and avionic, the advantages in size, weight, availability, quality, cost, and reliability justifies the change to this package technology [2]-[4].
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There are applications, unlike the vast majority, that are in equipments which are stored, unpowered, for extended periods of time and then required to operate. Although many argue that dormant storage is a benign condition, others believe that operation is the more benign condition, because the increased junction temperature generated by applied operating voltages reduces the humidity at the chip surface and, therefore, reduces the aluminum bond pad and interconnect corrosion [5]. However, in the non-operating environment, the only electrical potential is that of the device's own built-in field. This is argued to be insufficient to produce a corrosive cell which can cause metalization deterioration which will produce device failure in the useful life of the system. While little information exists on environments consisting purely of dormant storage, many operating environments previously studied, such as automotive or avionics, actually consist of a series of on-off cycles with the majority of the time spent in the OFF state. For example, automotive manufacturers consider approximately 400 hours to be the average ON TIME per year for automotive applications. Since this means that the electronics are powered for less than 5% of the time, it could be argued that this is essentially unpowered storage. Two ongoing studies are investigating the reliability of PEMs stored either as: (1) individual devices, such as from inventory; or (2) assembled on printed circuit boards (PCBs), inserted into a system sent to the field, never used and returned because no longer required. The first program is described in a paper by Anthony Casasnovas and James W. White of The Johns Hopkins University [6]. This was a study of PEMs which were in dormant storage for as many as 28 years. Part evaluation included external visual, radiographic inspection, electrical testing at room temperature and at rated low/high temperatures, weight loss assessment, C-mode scanning acoustic microscopy, and destructive physical analysis (DPA). Ninety-two (92) parts were studied from 12 manufacturers which were 71% analog and 23% digital and mostly 8 and 14 pin dual-in-line packages. The majority of parts ranged from 8 to 22 years old. The environment the parts were stored in was a variable with little detail regarding actual conditions. Results from DPA revealed only two cases of corrosion in PEMs -- both 28 years old. Recent improved technology and processing will minimize corrosion as a failure mode even further [6]. The second program will be described in more detail in the next section. 2. DISCUSSION The systems studied were Navy SONOBUOYS fabricated in the early 1980s. Since that time several million units have been produced. These systems used commercial (0 to 70C) PEMs with no additional testing. Two PCBs per unit contain a total of 10 to 15 DIP PEMs, depending on the unit design. The environment in which the units studied were stored was not possible to determine, however, they could have been in one of several locations. These temperature and humidity extremes range from -40°C to 55°C and 0 to 90% RH. Figure 1 illustrates the typical SONOBUOY construction: (1) is the shipping container; (2) the plastic housing which is shed upon firing from the plane; (3) the metallic container which protects the SONOBUOY on the descent to the ocean surface and houses the parachute at one end and the flotation collar for the transmitter at the other end, and (4) the transmitter, hydrophone assembly and electrical wiring. The two types of transmitter modules studied had 12 PEMs each. The printed circuit assemblies (PCBs) were protected by a plastic housing and the modules sealed with O-rings. Desiccants are not used in SONOBUOY systems. Various module designs are shown in Fig. 2. Details of one module are depicted in Fig. 3. The PCBs were protected by plastic shipping containers,
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with thread end caps. Inside this container was the equipment enclosure which used rubber O-rings as seals. The PCB module also used O-rings to seal the assembly. No conformal coating was used on the PCBs. All PEMs were date coded 1984, 85 and 86. Devices studied were produced by Motorola, Texas Instruments, and National Semiconductor Corporation. Other manufacturers were represented, but these were not investigated because of unavailability of test program sets. Electrical verification testing was an issue. Since these devices, in most cases, were no longer in production, software had to be available in-house, from the supplier, or from a contract test facility. The software had to be in a format compatible with the test hardware. Since the devices were obtained to the supplier's commercial data sheet, the test program was not unique. The electrical performance, to the data sheet, was operational from 0°C to 70°C. The units were purchased as commercial products with no additional tests, such as screening. Failure criteria for this study were to the data sheet minimum/maximum values. Initial test data were not available which did not permit parametric change assessment. Prior to removal of the PEMs for electrical evaluation, the PCBs were studied for solder joint cracking, corrosion, board delamination, and metalization delamination. Two different unit designs, A&B, containing two PCBs each, were evaluated and the results shown in Table 1. A total of 24 units, 12 of each type, were visually examined. After approximately 10 years in this storage environment, very little was observed which could possibly cause circuit failure. Electrical functionality of the units or PCBs was not performed. Removal of PEMs from printed circuit boards was critical. Leads had to be clean of solder, straight, and adaptable to test sockets. These dual-in-line devices had leads which were bent to the back side of the board, prior to soldering. This made device removal very difficult. Successful removal rate of all device types was nearly 100%. To assure failure diagnostics did not induce corrosive effects or remove corrosive products, a dry mechanical package encapsulant removal technique was developed. Figures 4 and 5 illustrate the typical die appearance after molding compound removal. No bond pad or interconnect metalization degradation is evident. No passivation cracking, lifting or separation is obvious. Table 1. Visual examination results* Unit A Unit
Solder Crack
Corrosion
Board Delamination
Metalization Delamination
1
NO
NO
NO
NO
2
NO
NO
NO
NO
3
NO
NO
NO
NO
4
NO
NO
NO
NO
5
NO
NO
NO
NO
6
NO
NO
NO
NO
7
NO
NO
NO
NO
8
NO
POSSIBLE
NO
NO
9
NO
NO
NO
NO
10
NO
POSSIBLE
NO
NO
11
YES
NO
NO
NO
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YES
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NO
NO
NO
Corrosion
Board Delamination
Metalization Delamination
Unit B Unit
Solder Crack
1
NO
NO
NO
NO
2
NO
YES
NO
NO
3
NO
NO
NO
NO
4
NO
NO
NO
NO
5
NO
NO
NO
NO
6
NO
NO
NO
NO
7
NO
NO
NO
NO
8
NO
NO
NO
NO
9
NO
NO
NO
NO
10
NO
NO
NO
NO
11
YES
NO
NO
NO
12
YES
NO
NO
NO
* None of these defects could have caused PCB failures.
Results of device electrical evaluation on 193 PEMs are shown in Table 2. Although these PEMs were purchased to a 0°C to 70°C specification, the testing was performed 0 to 70C and either 40°C to 85°C or 55°C to 125°C. In all cases the devices passed 0°C to 70°C. Only one unit would not function at the +85°C limit. It is believed that this device did not degrade with storage, but would not have passed this test if performed initially. Table 2. Test results after storage Device Type Supplier Quantity TL071CP
T.I.
70
TL082CP
T.I.
2
TL062CP
T.I.
6
SN74LS92N Motorola
8
SN74HC74N Motorola
3
SN74LS00N Motorola
3
SN74LS393N Motorola
3
MC145155
Motorola
20
MC145156
Motorola
39
MC120156P Motorola
25
MC14093U
Motorola
3
MC14066
Motorola
6
MN74C906N National
5
One anomaly was found of the many devices destructively analyzed. Figure 6 is a device functionally acceptable, but with a residue on the bond pad and adjacent passivation.
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Other passivation sites had this same appearance. SEM X-ray analysis of these areas detected large amounts of sodium, chloride, potassium, and oxygen. It is now believed that these sites are the result of spittle generated at wafer fabrication. Although this device was fabricated in 1985, no adverse effects are apparent due to the spittle. After a plasma etch and a DI water rinse, there is no evidence of the spittle on the aluminum bond pad metal or passivation (Fig. 7). This effect is wafer fabrication related and is not related to the encapsulation process or package. 3. CONCLUSIONS PEMs and PCBs stored in unbiased dormant applications for at least 10 years have shown no device or circuit card failures. The storage locations for the SONOBUOYS investigated are throughout the world and are typical of SONOBUOY environments. These studies are in agreement with the finds at the Johns Hopkins University [6] on unused stored PEMs. The Panama Canal Zone study conducted between 1970 and 1980 on PEMs and plastic encapsulated transistors was the incentive to insert these high volume, low cost, size, and weight components into operational systems [7]. In that study data indicated that identical device types which passed up to 10 years of biased testing also survived unbiased extended testing. Device types that failed extended bias testing also failed unbiased testing. It is proposed that an input to the determination of the "goodness" of a device for long term unbiased applications should be the capability of a part to pass Highly Accelerated Stress Tests (HAST) or Temperature Humidity Bias (THB) stressing. These tests in conjunction with other tests used to assess possible failure mechanisms should be used to accept devices for missile applications. ACKNOWLEDGMENTS The following individuals have provided outstanding support to this program: K. Cuneo, Army Research Laboratory, Fort Monmouth, NJ, M. Cooper, Litton, Inc., Woodland Hills, CA, Sun Man Tam, Texas Instruments, Lewisville, TX, and George Wolfe, NSWC, Crane, IN. REFERENCES [1] M.G. Pecht, L.T. Nguyen, and E.B. Hakim, "Plastic encapsulated microelectronics - materials, processes, quality, reliability, and applications," John Wiley & Sons, Inc., New York, 1995 [2] L.W. Condra, G.A. Kromholtz, M.G. Pecht, and E.B. Hakim, "Using plastic encapsulated microcircuits in high reliability applications," Proceedings Annual Reliability and Maintainability Symposium, pp 481-488, 1994 [3] M.G. Pecht, R. Agarwal, and D. Quearry, "Plastic packaged microcircuits: quality, reliability and issues," IEEE Transactions on Reliability, Vol 42, No. 4, pp 513-517, Dec 1993 [4] I. Weil, M. Pecht, and E. Hakim, "Reliability evaluation of plastic encapsulated parts," IEEE Transactions on Reliability, Vol 42, No. 4, pp 536-540, Dec 1993 [5] J. Pecht and M. Pecht, "Long term non-operating reliability of electronic products," CRC Press, 1995 [6] A. Casasnovas and J.W. White, "The Navy F/A-18 program and plastic encapsulated microcircuits: long term dormant storage study," Proceedings 1996 Advanced Electronics Acquisition, Qualification and Reliability Workshop, Aug 1996
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[7] E.B. Hakim, "US Army Panama field test of plastic encapsulated devices," Microelectronic Reliability, Vol 17, pp 387-392, 1978 Copyright © 1997 by CALCE and the University of Maryland, All Rights Reserved
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