Apple, Samsung, Phillips, Honeywell, etc. are known to have research and development ..... [8] Wirebonding: Ball bonding vs. Wedge Bonding by Empfasis.
Wire bonding Failure Modes
Digvijay Gusain, M.Sc. Electrical Engineering, Delft University of Technology Introduction The world has grown through leaps and bounds at the turn of twentieth century. Technological innovations have been at the heart of our phenomenal pace of growth. Instruments are continually becoming smaller in size, smarter in function and cheaper in price. This revolution is not only limited to the area of consumer electronics, but also includes many other fields like medical sciences, banking and engineering. The “green chips” are now everywhere one can possible imagine, be it remote controllers, televisions, air conditioners, automobiles, etc. A lot of research goes into making these chips smaller day by day by providing huge financial and technological support infrastructure. Big consumer electronics firms like Apple, Samsung, Phillips, Honeywell, etc. are known to have research and development divisions. These attract a high number of engineers who work hard to make the present technology more efficient, more compact and more useful. A good share of company revenues go to this area. While there is no doubt that there has to be significant investments made into research and development, there is also a need for allocating some of the resources towards another important aspect, namely, FAILURE MODES AND EFFECTS ANALYSIS. Any device is prone to fail at some time once it starts operation and in some cases, even before it starts operation. The
probability of device failures may vary from one to another but there is always a probability that the device may fail at one point or another. As various companies are jostling to get the top spot on consumers’ reputation list, it is imperative that they ensure that the products they sell, work well and without significant problems caused to the consumer within their descripted warranty period and even beyond that. This has to be made possible by ensuring that the production procedure is standardized and performed without any lax. Good quality materials used for production, sophisticated manufacturing techniques are some of the prerequisites for a good quality product. Apart from ensuring good quality raw materials, analysis of various failures is also an important measure to be performed. It is estimated that companies spend around one percent of their revenues on failure mode analysis. That is a significant amount since if a product batch turns out to be faulty and in turn reaches the consumer, it will seriously dent the company’s image in the market and also lead to huge losses in recalling and compensation to the affected consumers. A very good example of this case is Johnson & Johnson’s hip implants. Due to failures in these implants, consumers were reportedly going
for the replacement surgery at a rate which was twice the industry average. The faulty devices cost the firm an estimated $600m that year and had to pay the customers for their operation costs among others. No wonder the reputation of the company also took a hit. This presents an excellent example of how even a small mistake in the manufacturing process or failure of a simple component can cause the companies damages worth in millions of euros and dollars. Wall Street Journal carried this story on their newspaper [1]. This essay is aimed at presenting various failures that may be caused in wire bonding process and it’s effects and prevention. It shall analyze the process with a focus on Copper and Aluminum wire bond process failures. I have used many research papers as reference and also consulted the class lecture notes by A. Bossche and Willem van Driel. Wire Bonding One of the most important processes in electronics manufacturing industry is the wire bonding. It is a very delicate process that requires a huge amount of detail and precision. The typical bond wire size used is 0.025 to 0.05mm. The modern day wire bonding process is capable of bonding chip pads spaced at 0.1mm. Now, since this process is very microscopic in nature, any defect might go unnoticed and may potentially cause a failure of the device. Bonding process used to make use of gold as their primary material since it has desirable properties. These include its ability to not corrode
easily, and being easier to implement in the bonding process. Nowadays, many other materials have been put into use in the industry. The rise of Copper as an alternative to gold wire bonding has been phenomenal. The growth has especially been huge within the last decade. The primary reason for this shift can be attributed to rising gold prices in the international markets. There are generally two types of materials used in the bonding process. One is the gold with 1% copper. Other is the aluminum wire with 1% silicon. Both offer almost similar advantages when it comes to the bonding process.
Schematics of wire-‐bonded device
Gold wire bonding (ball-‐wedge bonding) [6][7] Gold wire bonding, as the name suggests, requires wire made from gold for the purpose of bonding process. The advantages that are offered by gold as a material for wire bonding process are many. These include good resistance to corrosion and high electrical conductivity, softening at very high temperatures and pressures, and resistance to formation of oxides at high temperatures at which bonding process is performed. Despite these advantages offered, there are also some main disadvantages for gold wire bonding. The process generally cannot be done at room temperature. To create a good
connection, a minimum substrate temperature of 120°C to 150°C is required. Gold wire bonding has the same common requirements as aluminum wire bonding that the bond pads cannot have any unevenness or contamination. Sometimes Cu can be used as an alternative since it has superior electrical properties than gold and is considerably less costly.
Aluminum wire bonding (wedge-‐wedge-‐bonding) [6][8] Aluminum wires are generally too soft to be used for wire bonding process. Therefore, they are sometimes alloyed with small proportions of Si and/or Magnesium. This increases the strength of the wire considerably for industrial use. This wire cannot be used for ball bonding process since it is a heat intensive task that may allow formation of oxides on the surface and hinder the bonding process. Compared with gold wire bonding, the bonding speed with aluminum wire bonding is slower, however, it has a distinct advantage over it that the low-‐cost final surfaces of the bond pads make the final product more affordable. Aluminum wire bonding is pure friction welding. The two pure metals are pressed together using a specific amount of pressure and are friction welded with ultrasonic oscillation that is generated by a transducer. Ball Bonding and Wedge Bonding There are basically two types of bonding processes [2], ball bonding and wedge bonding. Ball bonding requires a combination of heat, pressure and ultrasonic vibrations to form a bond. Hence, this technique is also known as thermosonic or
thermocompressive bonding. Wedge bonding is generally purely ultrasonic since no heat or pressure is required to form the wedge bond. The ultrasonic techniques are highly time consuming and more expensive than the thermosonic techniques. Al bonds use pure ultrasonic bonding, because of the difficulties in forming the ball bonds required for thermosonic techniques due to the presence of coherent oxide, which covers the aluminum. The bonding between the chip and the package may be performed using either a wedge-‐wedge bond or a wedge-‐ball bond (ball-‐snitch bond). Despite the wedge-‐wedge bonds’ distinct advantages over the ball bonding procedure, ball bonds are much more popular than the wedge bonds. This is primarily down to the faster processing speeds, a huge incentive for the chip manufacturers. [8]
Microscopic view of Ball bonds
Microscopic view of Wedge bonds
Failures The growing use of semiconductor technology in critical domains requires us to make this technology as reliable as possible. Computers are perhaps the biggest users of the semiconductors and they find use in various fields. Thus, failures in some of these fields, like, medical analysis systems, banking systems etc. may cause havoc. A failure in personal computer might still be tolerated in some situations. It therefore makes it absolutely necessary to have a detailed understanding of the causes of failures, take preventive actions and ensure that these failures don’t occur again in the future. Failure is a continuing engineering problem. A vast amount of data is present and collected over the years of various failure modes and considerable expertise is available to tackle these failures, but the field isn’t limited to the known data. We continue to witness new failure modes every time. Hence, a more sophisticated and detailed approach is needed to tackle the problem of failures in semiconductor devices as we set ourselves for a more deeply integrated society where computers and other semiconductor devices play a huge and important role.
One aspect of this effort is to know how the failure occurs and the mechanism involved. I shall now concentrate on the wire bonding failures in these devices. Wire bonding is a packaging level job in the semiconductor industry. It is one of the absolute essentials in the packaging of components in a SMT. Wire bonding failures can occur for a variety of reasons – fracture, lift off or shearing. Before we delve deeper into the causes of wire bonding failures, let us take a brief look into a related interconnection failure mode called the BOND PAD CORROSION. Bond Pad Corrosion (BPC) BPC represents one of the significant failure modes in IC packages. In the presence of moisture and halide ions, such as chlorides, the aluminum may corrode which can lead to failure in device. Moisture turns aluminum into aluminum hydroxide, which will react with chloride ions to form aluminum hydroxy chloride. Chlorine ions may also react with aluminum metal in the IC to aluminum tetra chloride ions, which undergo hydrolysis to form aluminum hydroxide. The reaction leading to the process is:
𝐴𝑙(𝑂𝐻)! + 𝐶𝑙 ! → 𝐴𝑙(𝑂𝐻)! 𝐶𝑙 + 𝑂𝐻 !
Hydrolysis of Aluminum chloride is given by:
2𝐴𝑙𝐶𝑙!! + 6𝐻! 𝑂 → 2𝐴𝑙(𝑂𝐻)! + 8𝐶𝑙 ! + 6𝐻 !
Other halogen species inside the IC may arise in form of bromides, which are found in fire retarding agents in plastics as Bromine. At high temperatures, it can undergo decomposition reactions to give rise
to haloalkanes and hydrogen halides that are corrosive in nature. [3] As already discussed, wire bonds are formed with either Aluminum or Gold wires on aluminum or gold plated pads. Solid state inter diffusion of elements is generally considered as the source of the problem. The interface of the bond pad consists of Al2Au and Al. At high temperatures, Al diffuses into AuAl2 creating voids. These voids are called Kirkendahl voids. As diffusion continues, these voids continuously keep forming and coalesce giving rise to an open connection. It should be noted that presence of intermetallics does not imply a failure, but it is a good measure to know that a problem is impending. They point towards an imminent failure of not dealt with properly. Ball bond fractures are also known to result from either thermal shear or tensile stresses or from encapsulation flow induced stresses during molding operation. This is known as wire wash or wire sweep. Wedge bond fails have been known to occur as a result of using improper process parameters. Excessive pressure at the wedge can squeeze enough material at the heel of the bond resulting in a failure. [2] Having discussed the various causes briefly, we now discuss these in further detail individually [3]. There are 6 main causes of bonding failures in devices: 1) Formation of intermetallics. As discussed earlier, formation of intermetallics due to gold aluminum inter-‐ diffusion at the bond wires to
bond pad interface occurs due to dissimilarity of the metals involved. Au! Al formation is necessary to ensure a good contact with the bond pad. However, if the alloying is not controlled, the result is a purple plague. Purple plague is a particular aluminum gold alloy. The presence of this alloy indicates the presence of other aluminum gold alloys that are generally not easily visible, such as white plague, and it is the presence of all these different kind of alloys that ultimately cause the wire bond failure. Apart from these alloy formations, creation of voids at the base of bond wire is also observed sometimes due to different interdiffusion rates of gold and aluminum during intermetallic formations. The migration of these voids, as has already been mentioned is termed as Kirkendahl voiding. These are the primary source of bond lifts that undermine the quality of the bonds. 2) Whisker Growth or dendrite growth, as it is called, occurs at the bond pad in order to equalize the compressive stress. Dendrite growth has also been reported due to contamination-‐aided migrations. 3) Bonding pressure affects bond integrity, with low bonding pressure giving rise to low fracture strengths in the neck and the heel, while a high bonding pressure can cause failures such as bond looping, whisker growth and
wire sweeping. These failures are often seen after a temperature cycling test. 4) Bond looping and lagging. In the bonding process, a loop is formed between a semiconductor and the lead frame. If this loop lags too much then the device is susceptible to failures as a result of short circuits between adjacent bond wires. On the other hand, if the loop is too tight, the tension created at the heel and neck of the bond and in the wire itself leads to fracturing and slippage of the bond metal. Military specifications are generally considered a standard for failure specifications. The military standards define a maximum length of 2.5mm for the wire to prevent lagging. Automated wire bonders are capable of making good bonds with lengths up to 5mm. 5) Bond Integrity, which loosely defines the goodness of bonding wire, is a very important reliability concern. Metal migration along the bond, which is aided by moisture, is one problem, which has been very commonly observed. Contaminations such as carbon inclusions, and the presence of flame retardants such as bromine, will also reduce the bond integrity by expediting voiding, ball lifting, metal thinning and breakage, as has been briefly described above. Many heel failures have been reported at locations where cracks
have been initiated during bonding and exacerbated by current cycling. Improvement can be made to bond integrity by better surface cleaning using plasma methods. 6) Wire sweep occurs during molding process for plastic encapsulation. The bond wires are forced against each other and can cause short circuits. Wire sweep can be limited by controlling the length of the wire. Apart from these, the bonding process is prone to other failures due to errors in manufacturing process. Misplaced bonds or incorrect bonds cause numerous failures. Excessive bonding pressure may also result in cracking of the passivation at bond pad edge or may even lead to cracking of the substrate. Partial detachment of silicon from the lead carrier has also been reported. The most common failure mode observed is that of open circuits due to bond lifting. Others, like, formation of intermetallics can lead to high resistances in the bond wire connection, wire sweeping due to excessive lagging can result in short circuits while whisker growths, also lead to short circuit failures. Thinning of Al wires can result in localized heating in regions of high resistance along the wire. The thinning of wire is due to oxidation of the wire, which reduces the effective cross section of the wire. The high current densities lead to electrical overstress straining the thermal limits eventually leading to open circuits. Of bonding failures, it has been observed that while heel of the bond is expected to give way
first, breaks in the bond itself due to tensile stress are very common. Thinning of wire, especially Aluminum wires, can lead to localized heating in the regions of high resistance along the wire. The high current densities lead to an electrical overstress type of thermal melting and eventual open circuits. Bonding failures are typically of wear out nature and found in that region in the reliability curve. Those that account for infant mortality should be detected first by tests before they reach the customers. Poor quality bonds that escape detection are either intermittent failures or inadequately stressed during screening. Preventing wire-‐bond failures [3] Several tests are made on the bonds before the device actually reaches the consumer. These involve various stress tests. Screening for defective bonds is performed by means of a bond pull or wire shear test. In the first test, the wire is pulled until the wire breaks, while shear test involves pushing against the ball bond until it shears from the pad. The number of grams needed to shear or pull the bond away from the pad is measured. The force varies for different quality metals/bonds. Gold bonds of about 50𝜇m have a typical breaking strength of 200-‐300g. the breaking strength is a function of bonding process and the wire diameter. The test is usually performed after a burn in at 250 ℃ for 48 hours as described in MIL-‐ STD 883 method 2023*. It has been reported that, by using bond-‐ shearing tests during assembly, bonding parameters can be
controlled to ensure good wire bond quality. Additionally, thermal shocks, which are exposure to alternate extremes in temperature ranging from -‐55 ℃ to 125 ℃ , mechanical shock centrifuge, and excess vibration, are all recommended as screens for weaker bonds. The use of bond metals other than gold, like aluminum, copper, palladium coated copper, have been considered with the intention of eliminating purple plague and Kirkendahl voiding. Each of these metals has it’s own pros and cons. These are listed below: Pros Cons Pure Al is very • Low processing Al soft and hence temperature (at needs to be ambient alloyed. temperature) • Higher pull test strength
• Eliminates purple
Gold
plague seen in gold wire bonding.
• Good conductivity
• Good strength
• Gold wire bonded to Al pads can give rise to a few microns thick gold aluminide layer within few hours of heating.
• Excess
Copper
layers lead to Kirkendahl voids. • Can easily • Better form oxides, conductivity so storage • Cheaper and shelf life • Does not form are a aluminide at problem. higher • Has to be temperatures. coated with Palladium, which turns out to be two to three times more expensive than gold wires. [9]
Another option is to use reaction inhibitors. These include use of “nitrogen blanket” during the bonding process in case of Copper wires. [8] MIL-‐STD 883 [5] MIL-‐STD 883 refers to the Military Standard that establishes uniform methods, controls and procedures for testing microelectronic systems suitable for use within military and aerospace electronic systems. These involve basic stress tests to determine the resistance of systems to various harsh effects faced by military and aerospace systems. These are subdivided into • Environmental tests, methods (1001-‐1034) that include tests like immersion (1002), insulation resistance (1003), Thermal characteristics (1012.1) etc. • Mechanical tests, methods 2001-‐2036, which includes vibrational fatigue (2005.2), resistance to solvents (2015.11), ultrasonic inspection of die attach (2030) etc. • Electrical tests, methods 3001-‐3024, which include, power supply current (3005.1), terminal capacitance (3012.1) etc. • Electrical tests (linear) methods 4001-‐4007 • Test procedures (5001-‐ 5013) Conclusion Wire bonding failure forms one of the most common failures in the device. Although ignored in many low cost cheap devices sold in the market, big companies have to make
sure that their devices do not fail for such trivial failures. We discussed in this essay, the types of wire bonds that are used in the industry, the metals utilized for making bonds, their properties and their advantages for using them over others. We discussed the Kirkendahl voiding and the purple plague. Further, various causes of the wire bonding failures were discussed in detail. A table highlighting the pros and cons of various metals used primarily for wire bonding process was made. Next, the effects of these failures were highlighted and then the prevention of these wire bonds was discussed. A reference was made to MIL-‐STD 883 which was discussed in a little bit further detail at last since the military standard 883 forms a global standard for reliability tests for microelectronic devices. References [1] Wall Street Journal reference: http://www.wsj.com/articles/SB10 0014240527487039597045754534 92107751662 [2]Failure Mechanisms in Semiconductor Devices by E. Ajith Amerasekera and Farid N. Najm. Wiley Publications. ISBN 0 471 95482 9. [3]Failure Modes and Mechanisms in Electronic Packages by Puligandla Vishwanadham and Pratap Singh. Chapman and Hall. ISBN 0 412 10591 8 90000 [4] Images 1 and 2 taken from the following URLs: http://www.atotech.com/uploads/p ics/el_sf_ebafa9_01.jpg and http://www.ami.ac.uk/courses/topi cs/0268_wb/images/m0pfg5cfe.gif ; image 3 taken from the book in [3].
[5] Wikipedia article on MIL-‐STD 883. [6] Information about gold wire bonds and aluminum wire bonds taken from http://www.we-‐ online.com/web/en/leiterplatten/pr odukte_/bonden/verfahren/Verfahr en.php [7]http://www.azom.com/article.as px?ArticleID=4999 [8] Wirebonding: Ball bonding vs Wedge Bonding by Empfasis. http://www.empf.org/empfasis/ma y05/bond505.htm [9]Palladium surface finishes for copper wire bonding (Part I: The selection of surface finishes) by Ozkok, M.; Atotech Deutschland GmbH, Berlin, Germany ; Kao, B. ; Clauberg, H. Publisher: IEEE, E-‐ISBN: 978-‐1-‐4577-‐1388-‐0
