Categories: Manufacturing

Wire Bonding

TASA ID: 4595

The dominant process for interconnecting semiconductor chips to the outside world is an ultrasonic welding process called wire bonding. More than 90% of the chip interconnections produced annually (more than 15 trillion wires) are produced with this process. Welding is a process where an intermetallic alloy is formed from the materials to be joined. Generally, intermetallic alloys are stronger and also more brittle than their constituents.  Welding is superior to other joining methods, such as soldering which require that a low melting temperature material melt and solidify within the joint. Low melting temperature materials, such as solders have significantly lower strength and are more subject to creep and fatigue failures than intermetallics. There are two major variations of the wire bonding process, ball bonding and wedge bonding. Ball bonding is the larger portion with about 90% of the entire wire bonding market. It is faster, the fastest ball bonders can bond more than 20 wires/second compared to less than 10 wires/sec for wedge bonding. Ball bonding also has more advanced capabilities than wedge bonding. However, ball bonding is limited to wires below approximately 50µm in diameter. All interconnections that require larger diameter wire are produced by wedge bonding Al or Cu, using either round wire or ribbon (a flattened form of round wire).

During the past five years, there has been a major transition in our industry from ball bonding with gold wire to the use of copper, palladium coated copper or silver wire. This year will be the first year where market share for gold wire falls below 50%. Cost, yield and reliability have all played a major part in this transition. In 2009 when gold rose in price above $1000/ Tr. Oz in 2009 and remained there, gold reduction became a mandate in semiconductor packaging. Gold wire represented a large portion of the gold used in semiconductor packaging. Copper had been discussed1 and demonstrated since the early 1980s but had not been widely adopted. Copper was more difficult to bond and had package reliability issues. As these issues (optimum bond pad metallization, encapsulation chemistry for long-term reliability, bonder recipe improvements) were resolved, the transition became a stampede and in five years became a new paradigm. Silver is also less expensive than gold. Silver is used for bonding LED devices because it has better reflectivity properties than either copper or gold. Early problems with silver wire in 850C/85%RH testing were resolved using Ag-Pd alloy wire. Silver market share is now approaching 10%.

Figure 1 is a photo of the bond head with capillary, wire and EFO (Electronic Flame OFF) wand. In ball bonding the tip of a fine diameter metallic wire (protruding from the capillary) is melted by a spark from the EFO. Surface tension in the metallic liquid pulls the liquid into a sphere, the sphere solidifies with more than 80% of the heat transferring back into the wire. This leaves a short region above the ball, called the HAZ (Heat Affected Zone) that has been rapidly heated to just below the melting temperature and then cooled rapidly to near room temperature. The HAZ is the weakest portion of the wire. The bondhead, with capillary and ball dangling below it, descends at a high speed towards the surface (normally the bond pad on a die). At a programmed height above the surface the bond pad velocity transitions to a slower, constant velocity and the bonder begins searching for the surface (surface height can vary due to the many tolerances from material and prior operations). Surface detection can occur by a number of methods including mechanically opening a contact spring as in older machines or high speed sensing of a current rise in a voice coil motor when the coil stalls on contact. After contact detection, the bond head continues downward to apply a programmable force on the ball. Ultrasonic energy from a piezo-electric transducer is added for a programmable time (8-12 mille-seconds is typical for a high-speed ball bonder).  The die and substrate are normally heated to 125-2000C depending on the process and materials. These four factors: ultrasonic energy, bond force, heat and time constitute the principal variables for ultrasonic weld formation.

After completing the ball bond cycle, the bond head rises and a series of very precise coordinated motions occur, forming a loop between the ball bond and the second bond. Loop height and uniformity are very important packaging requirements. The demand for thin packages and stacked die packages that are also, as thin as possible, lead to the development of improved bond head control algorithms and many new loop shape options. Memory devices often have their bond pads located down the center of the die surface rather than around their periphery. This allows better signal and voltage distribution and results in faster devices that command premium values.  The left panel of Figure 2 is a photo of low loop wires for memory. These loops rise to a low height and then travel parallel to the edge of the die where they descend to second bond. Stacked die packages (right panel of figure 2) often employ a hybrid bond called a stand-off- stitch (SOS). In a SOS bond, a ball is formed and bonded with the wire intentionally broken in the HAZ. Another ball is formed and bonded to the substrate side of the package. The stitch (second bond) side of the wire is then bonded on top of the original ball. Because it requires the formation of three bonds rather than two, the SOS bond is approximately 40% slower than a standard bond, but it provides the lowest loop height available. Every smart phone (more than 1 billion annually) has at least one stacked die package. Stacked dies, because each die can contain a separate technology (analog, digital, memory, rf), enable integration of the entire system within the package. Earlier attempts to integrate all of these technologies on the same chip proved costly and decreased reliability. Joining the technologies by stacking them within the package became the dominant method.

Second bond is formed by a different portion of the capillary tip than the ball bond. Figure 3 is an illustration of a capillary tip and the portions of the tip that produce the ball bond, the loop and the second bond. In forming the second bond, the capillary face and outer radius are pressed on the top of a round wire. The combination of ultrasonic energy, bond force, heat and time deform the round wire into the fishtail shape and form the initial intermetallic bond.

The mechanical and other materials properties of the ball and the wire are significantly different. Second bond is more diffusion controlled than the ball bond.

Wire Bond Failure Mechanisms

Semiconductor packages must normally pass a battery of short and long-term reliability testing during package qualification prior to market introduction. Once manufacturing and sales begin, mechanical testing is commonly done on each material lot. Mechanical testing normally consists of both wire bond pull testing and shear testing. Because the weld areas for both the ball bond and the second bond are several times larger in cross section than the wire cross section, the pull test is not capable of testing the strength of either bond (the wire breaks first). However, it is capable of detecting very poor bonds, wire damage, damage to the HAZ or a second bond that has been over-deformed and that has a thin cross section at the heel of the bond. The pull test measurement can be understood from a simple resolution of forces.  However, once a history of data exists and SPC has been established the use of control charts can be a very powerful quality tool. The shear test is capable of measuring ball bond strength and should be a standard test for each lot. Average shear strength of 5.5g/mil2 (85MPa) meets the JESD-22-B116A standard for shear testing required by the automotive industry.

The life, and subsequent failure of gold ball bonds on aluminum bond pads by Kirkendall voiding has been well-documented. At a temperature above 1500C, for some packages this can occur quickly and catastrophically. Bonds literally fall off with almost no stress. New 99.9% gold alloy wires (standard gold bonding wire is 99.99% gold) with additional impurities added to stabilize intermetallic formation can improve reliability.  Gold ball bonds on gold bond pads in high temperature environments do not exhibit the problem.

Analysis of intermetallic coverage and morphology should be a standard part of qualification testing and be repeated periodically through the life of a product. Aluminum bond pads can be easily etched with sodium hydroxide or potassium hydroxide to release the bonded balls. Etching will not remove the intermetallic on the bottom of the balls. The balls can be flipped with a dental pick or the die paddle tie bars can be removed to reveal the bottom side of the balls. Intermetallic coverage should exceed 80% as bonded. Figure 4 demonstrates the evolution of bond coverage as a function of bonding time. After 16ms bond time, the intermetallic coverage is over 80%. To expose the bonds in encapsulated packages, it is often necessary to remove the encapsulating material with hot, fuming nitric acid. This will reveal gold ball bonds but will immediately attack copper bonds. Several techniques, such as laser ablation and very controlled etching in an inert atmosphere have been used for copper ball bonds.

Copper-aluminum intermetallic requires both a higher formation temperature and longer time (slower growth rate) than gold aluminum. Therefore, copper ball bonds can be more reliable than gold bonds at high temperature. Encapsulation to protect copper bonds is critical. The presence of Cl- ions is auto catalytic to copper. Chlorine corrodes copper and then is released to continue corrosion. Molding compounds that contain less than 30ppm chlorine and have a controlled pH of 4-6 are now available for copper and are necessary for high reliability products2.

Figure 5 shows SEM photos of two failure modes that can occur as a result of wire bonding. Normally, ultrasonic energy is the most aggressive variable affecting bond pad failures but poor design of the bond pads is also a root cause. Designed in reliability resulting from careful DOEs and the development of internal design guidelines focused on the use of  robust bond pad structures cannot be ignored. Modern bond pads often contain multi-level stacks of metal and dielectric layers. In some cases, low-k dielectrics with poor mechanical stability are required for functionality. Instances of failures in layers below the surface, allowing electro-migration and eventually resulting in interlayer shorts are well-documented. Often these failures can occur while the top metal layers and wire bond are unaffected3. They are difficult to detect and analyze. A team approach, involving FAB, Assembly and Reliability engineers must focus on development of pad structures that not only can achieve electrical design requirements but be robust enough to withstand manufacturing and reliability.


Wire bonding continues to be the lowest cost, highest reliability, most flexible semiconductor interconnection method. It continues to reinvent itself, as new demands are understood machine, wire, tool and end users come together to find solutions that enable successful implementation of the new requirements. Each new generation of devices has required increased capabilities for both manufacturing and metrology. Wire bonding has met these challenges and added the capabilities necessary for its continued growth as the leading semiconductor interconnection method.

[1] M. Sheaffer, L. Levine and B. Schlain, “Optimizing the wire bonding process for copper ball bonding using classic experimental designs,” Proc. IEMT, pp 103-108, Sept. 1986

[2] F. Carson, “Copper wire interconnect has arrived,” Chip Scale Review, Jan/Feb 2011, pg 35-37

[3] S. Hunter, R. Gohnert, R.L. Warnick, A.J. Dutson, “A bond pad’s view of wire bonding,” IMAPS Wirebonding Workshop, San Jose, CA, Jan 2013


This article discusses issues of general interest and does not give any specific legal or business advice pertaining to any specific circumstances.  Before acting upon any of its information, you should obtain appropriate advice from a lawyer or other qualified professional.

This article may not be duplicated, altered, distributed, saved, incorporated into another document or website, or otherwise modified without the permission of TASA.

Previous Carbon Monoxide and Houseboat Deaths
Next At Trial, Don’t Leave Technology Behind: The Trial Consulting Paralegal
Tasa ID4595


Let Us Find Your Expert 

Note: This form is to be completed by legal and insurance professionals ONLY. If you are a party in a case that requires an expert witness, please have your attorney contact TASA at 800-523-2319.


Search Experts

TASA provides a variety of quality, independent experts who meet your case criteria. Search our extensive list of experts now.

Search Experts


  • I think it's always good to have access to experts when [TASA] make[s] the process so easy."

    Scott McIntosh, Lewis McIntosh & Teare, Royersford, PA

  • As a busy practitioner, managing a sizeable caseload, I can use all of the help available to me. If I can outsource a task, particularly one as important as securing a qualified expert, I will jump at the opportunity. I use TASA in nearly every case where I need to find an expert witness, be it an engineer, an architect, an economist, etc. They have thousands of qualified experts to refer in virtually any field. Best of all the process is extremely simple. When I need an expert I simply contact TASA, whose knowledgeable representatives ask you targeted questions about your case, your legal theories, and your goals, in order to find the right expert for your case. I usually receive CVs and calls from the potential expert within hours. If you find the originally selected person is not a good fit – for whatever reason – TASA will work with you to find the right person. I would happily recommend this service to any attorney."

    Patrick K. Gibson, Gibson & Perkins PC, Media, PA

  • Ms. Darlie I. McDonald RN was awesome on the witness stand, and we prevailed in our case to the tune of  [a] (highly unusual [amount] for a medical malpractice [case] in our area).  I'd highly recommend her."

    Shane Reed, Shane A. Reed Law Office, Jacksonville, OR

  • I appreciate your inquiries and offers of assistance as well as the consistently high-quality, well-organized, and erudite TASA webinars, which invariably have excellent presenters."

    Maurice S. Kane, Cummings McClorey Davis Acho and Associates PC, Riverside, CA

  • Steven Kursh was an outstanding technical expert on our ecommerce IP lawsuit. He completed a massive amount of work on extremely complicated material, in a very short period of time. His work product was first rate and I think he would have done a terrific job if the case proceeded to trial. He is very articulate and helped us. I only wish we had gotten him involved sooner in the litigation."

    Daniel J. Brown, Reiss Sheppe LLP, New York, NY

  • I thank you all for the response to my request for an expert witness...Both Mr. Scott and Mr. Bianchi appear to be well-qualified for this case, but we have hired another expert. As always, I was impressed by TASA's ability to produce exceptionally well-qualified candidates with great speed."

    John Thomas Dzialo, The Law Offices of John Thomas Dzialo, Santa Ana, CA

  • Thank you for your quick response and the names of the two proposed experts. The situation that gave rise to our search for these experts has resolved and we will not need to retain them. However, we will continue to keep TASA in mind as these needs arise from time to time as your breadth of coverage for experts of all types is unparalleled, in my experience."

    Bart W. Brizzee, County of San Bernadino, San Bernadino, CA

  • I have used TASA for the last five years for locating an expert for many personal injury cases. On each and every occasion, TASA was able to find me more than one qualified expert. With such a variety of experts, I was able to select one who met my client's needs in prosecuting these claims. I found the experts TASA referred not only qualified, but available on a moment's notice. Your fees are reasonable and fair, and I will continue to use TASA for the remainder of my career."

    Robert Oushalem, Esq., The Law Office of Robert Oushalem, San Jose, CA

  • I recently used TASA for the first time to locate an expert to testify in a case requiring rather unusual expertise and where there were no applicable regulations or standards for guidance. TASA referred an expert in California who was everything a lawyer looks for in a forensic expert. He was promptly available for consultation, efficiently prepared for deposition and trial and very persuasive and credible with the jury. TASA's administrative services and assistance in locating this expert were excellent, and we would certainly use both the expert and TASA in the future."

    Theodore Phillips, Miller Hauser Law Group, LLP, Placerville, CA

  • TASA has always given me first-class service, but in a recent matter, TASA found the 'needle-in-the-haystack' expert witness we feared didn't exist. We needed an expert for a very narrow and limited issue in a very narrow and limited industry. Because TASA has an extensive expert witness database, it was able to give us a referral almost on the spot. It's why I always turn to TASA first."

    Kathleen A. Herdell, Law Offices of Kathleen A. Herdell, St. Helena, CA

  • There are numerous companies that provide litigation experts. However, I always choose the TASA Group because of their quick response in finding a qualified expert for my particular case. I have extreme confidence in the TASA Group and will continue to use their services in the future."

    Katie A. Killion, Esq., Chiurazzi & Mengine, LLC, Pittsburgh, PA

  • I spent hours trying to locate an expert in a very technical case involving a defect in a medical device. I could have saved a lot of time by calling TASA first. Within hours, I was supplied with the name of an engineer who had more than 30 years of job training, education and expertise in the precise area involving the device. Bravo TASA!"

    Timothy W. Peach, Partner, Peach & Weathers, San Bernardino, CA

  • We were involved in a case pending for more than five years with seven parties from three states. Three mediations failed before we looked to TASA for an expert. TASA referred an expert who clearly understood the complexity of the project and could effectively support his opinion. If it weren't for his expert advice and deposition testimony, the case would not have settled. Interestingly, the case settled within 90 days from the date this expert began."

    Renee Colbert, Esq., Corporate Counsel, W.G. Tomko, Inc., Finleyville, PA

  • Using TASA to find experts for defending our client in a negligent homicide case ended up being one of the most important decisions we made in trial preparation. The experts they suggested were exactly what we needed for the case. I truly did not expect to find experts that would be such a perfect fit for the nature of case. TASA provided us with highly qualified experts in somewhat narrow fields of expertise. A large percentage of our victory is due to the experts recommended by TASA."

    Marta Farmer, Esq., Carl S. White Law Office, Haver, MT

  • I have used TASA's services since the 1980's and have never been disappointed. TASA is indispensable for locating that hard-to-find expert. TASA representatives have always been courteous and pleasant, with the attitude that they cannot do enough to help. I expect to continue using TASA throughout my career."

    Brad W. Greenberg, Esq., Smyth Law Offices, P.C., Brockton, MA

  • I needed to retain a multitude of scientists from a variety of disciplines for a complex litigation. Initially, I went through a series of interviews with an extremely knowledgeable and professional team of TASA advisors. They were able to find highly qualified experts in the specific fields, all of whom turned out to be superior in qualification and area of expertise to my adversary’s experts. I am a TASA believer!"

    Nooshin Namazi, Partner, Nicoletti Hornig & Sweeney, New York, NY

  • TASA always comes through in the difficult IP cases. Their representatives work with you to refine the search criteria and quickly send you a list of very qualified experts."

    Timothy L. Boller, Principal, Seed Intellectual Property Law Group, PLLC, Seattle, WA

  • Special thanks to our TASA referral advisor for her quick response to our initial request—we were extremely happy with how fast TASA was able to assist us! Your group does excellent work, and it is always my first stop when looking for an expert."

    Susanne K. Sullivan, Senior Attorney, Southwest Airlines Corporation, Dallas, TX

  • When we needed an expert in a patent infringement lawsuit, we turned to TASA. We were looking for a witness qualified in two unrelated technical areas, and TASA worked with us to identify and refine our requirements. TASA performed well, promptly providing us with several excellent candidates to consider, one of whom we retained."

    Joseph T. Miotke, Partner, IP Practice Group, Michael Best & Friedrich LLP, Milwaukee, WI

  • Our team had a very positive experience with TASA. The Expert was professional, efficient, and certainly an expert in his field. His work and testimony contributed to a winning decision for our client! We will recommend the Expert and TASA whenever appropriate."

    Stephanie Sprague, Esq., CT

  • (The Expert) WAS A PERFECT FIT for my case: qualified, competent, easy to work with, attentive to detail, knowledgeable, smart, communicative, enthusiastic, resourceful—have I left anything out? I highly recommend TASA and would be happy to share my experience with anyone else. Thank you!"

    Michael Porrazzo, Esq., The Porrazzo Law Firm, Mission Viejo, CA

  • The expert was very thorough. TASA was quick to respond with an answer to my request. I have used TASA in the past under various other law firms and have been pleased. TASA continues to live up to expectations and then some."

    Anne Desormier-Cartwright, Esq., Jupiter, FL

  • Your organization found us an appropriate expert witness in less than one day. This was excellent service. The expert you found was excellent and a pleasure to work with."

    William A. Ehrlich, Esq., Allentown, PA

  • (The Expert)…accomplished exactly what we wanted. TASA was very prompt and efficient in locating him. All fees were reasonable."

    J. Michael Lehman, Esq., Bruce, Bruce, & Lehman, Wichita, KS

  • We needed an Internet expert right away to meet a deadline. One phone call to TASA, and in less than a day, TASA called back with a list of 8-10 experts who were exactly what I needed. The TASA expert I chose knew the business and mechanics of the Internet so well—he was a PhD and professor who had written a book on the subject—that he put the fear of truth in the defendant that caused him to settle. When I get the kind of service that I did from TASA, I stick with it and use it again and again."

    Philip Green, Attorney at Law, Green and Green, San Rafael and San Francisco, CA

  • Excellent—in a word. I just do not have the time to hunt for experts. (The Expert) was fantastic. Thank you for providing such a quality service."

    Francesca Carinci, Esq., Steubenville, OH

  • TASA stands for Tops At Serving Attorneys. It’s always rewarding working with TASA."

    Marshall A. Bernstein, Esq., Philadelphia, PA.

  • That was, however, one of the best and most interesting webinars I've seen in the last few years.  Thank you for hosting it and introducing me to such a knowledgeable and caring person." - Referencing the Medicolegal Consequences of Post-Traumatic Stress Disorder in Civilian and Military Populations webinar. 

    Lori Bauer Apodaca, The Law Office of Lori Bauer Apodaca, Los Lunas, NM 

  • I needed a dental malpractice expert to assist me in a complex negligence claim. The very able staff at TASA had no difficulty identifying a knowledgeable professional who rendered a reasonable opinion in support of the case, which aided our client in receiving a fair amount of compensation. I am grateful to TASA for its invaluable assistance!"

    John Hermina, Hermina Law Group, New York, Pennsylvania, Maryland & Washington, DC

  • For many years I have relied upon TASAmed to provide excellent medical malpractice experts. As a sole practitioner, I find it reassuring to know that a seasoned expert is just a call away. Usually, TASAmed has found just the right expert in a day or two. The support and guidance I receive from TASAmed is a vital part of my law practice, and I have come to expect both great service and high rewards from my TASA cases."

    Thomas J. Massey, The Thomas J. Massey Law Firm, Fallbrook, CA

  • The caliber of physicians that TASAmed has referred to us is superb. Prior referral groups used the same experts over and over again. With TASAmed I have access to experts all over the United States. I’m not limited to the same experts. The TASAmed staff is easy to work with and very professional, with an established track record. When I call for a medical expert, I’m called back the same day, and I often have an idea of what expert will be contacted before my first call is completed."

    Kari Alexander, Certified Legal Nurse Consultant, Texas

  • We hadn’t been able to find the medical expert we needed, and, frankly, I didn’t think we’d find one in that field. TASAmed was able to find us an expert with the exact expertise and medical experience we needed. Your referral advisor was very helpful and found our expert in one day."

    Kurt Osenbaugh, Partner, Alston & Bird, Los Angeles, CA

  • TASAmed’s service was prompt and efficient in connecting us with the right person. The expert was so cooperative and helpful. With how challenging it is to find a narrow area of medical expertise, it’s extra helpful to have your TASAmed pool to plug into instantaneously."

    Greg Roosevelt, Esq., Law Office of Greg Roosevelt, Edwardsville, IL

  • TASAmed has connected me to credible experts in four medical cases just this year. TASAmed and the referred experts respond quickly, the fees are reasonable, and the referrals are well tuned to the fields I request. Since the experts are already associated with TASAmed, they are comfortable having substantial conversations about the case, both before and after record review."

    Martin A. Cannon, Esq., Cannon Law Offices, Crescent, IA

  • I have used TASAmed a number of times and have always been happy with your give-and-take timeliness. Once I requested a medical expert in a particular field, but, after speaking with your referral advisor, we concluded that an expert in another field would be more effective. That same day, I spoke to two experts the advisor gave me, and I retained one."

    Mark A. Lope, Esq., Lope and Honlihan, Butler, PA

  • Very close to the time of trial, the TASAmed advisor quickly referred me to several experienced ER trauma physicians to review medical records and prepare me for cross-examination. After selecting my expert, I over-nighted records for review, and the doctor found valuable information for my client's defense. Thank you, TASAmed, for this timely, specific, valuable referral."

    Charles Morgan, Esq., Law Office of Charles L. Morgan, Jr., Charlotte, NC