Issue



Ribbon Wire Bond Testing


11/01/2006







Challenges and Solutions for Process Engineers

By LISA GERBRACHT AND MALCOLM COX, Royce Instruments

emiconductor assembly and packaging process engineers routinely face pressures of evolving technology requirements such as lead-free compliance, product introductions, and customer demands for quality. Since wire-bond quality is a large part of achieving product reliability, process engineers must be highly skilled at setting up and optimizing parameters of wire bonders. The bond tester is a vital tool in bonder optimization.

Historically, the majority of wire bonds in both small-signal and power semiconductors have been circular cross-section gold and aluminum wire. With today’s range of advanced packages, ribbon wire bonding is starting to appear in many applications. In the past, ribbon wire bonding was used mainly in microwave products. But it is becoming more common in power semiconductor devices and high-volume, small-signal products such as wireless device components.

The advantages of using ribbon wire bonding over round wire include lower parasitics at high frequencies, lower current density permitting higher current in power applications, and stable bond loops at very low loop heights. With increasing demands for thinner packages, semiconductor package designers are looking to ribbon bonding to reduce bond loop height and overall package thickness.

Setting Up a Process to Test Ribbon Bonds

The universal bond tester, a workhorse in any wire-bond process monitoring area, is well-suited to test ribbon wire. Additionally, many of the skills used in round-wire testing transfer well to ribbon wire. For example, process engineers will be well-versed in data collection and statistical process control (SPC) methods needed to monitor production properly.


Figure 1. Pull testing thick aluminum ribbon bonds. (Courtesy of Orthodyne Electronics Corp.)
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Deciding which type of test to perform is paramount so as to not compromise product quality. For example, when bonding ribbon that has low material strength relative to the bond strength, process problems with bonds lifting from the bond pad may occur. Pull tests that repeatedly break at ribbon mid-span will not catch these failures and therefore are of no value. In this case, a tweezer or shear test would better expose poor bonds.

Ribbon Pull Test

It often makes sense to use a traditional bond pull test. Analogous to the round-wire pull test, this involves inserting a pull hook underneath the ribbon and pulling upwards until the ribbon breaks (Figure 1). The measured force is recorded for process monitoring purposes.

This test is addressed in the most commonly used standard for bond pull, MIL-STD 883E, method 2023.5. Ribbon test setup is not addressed separately in the standard; however, the process engineer is given a method for calculating the minimum breaking force by using the number from round wire with an equivalent cross-sectional area.

For many applications - especially thin gold ribbon - the bond pull test is adequate. However, ribbon wire introduces a new level of difficulty because the bonds are usually harder to access. For example, microwave applications demand very low loop heights because of impedance requirements. Ribbon loop heights of 50 µm are common. The combination of wide ribbons and low loop heights makes fitting a pull hook underneath the bond tricky.

Hook Tool Selection

Selecting the correct hook tool for the bond loop to be tested is important. Hooks for round-wire testing are usually made of a thin, stiff wire - often pure tungsten because of its high elastic modulus. The end of the wire is bent to form an L-shape. The horizontal part of the hook is called the foot, which is the distance from the inside bend of the hook to the “toe.” For round wire, Mil Spec 883 advises a hook-wire diameter at least twice the bond-wire diameter. Applying this rule to the thickness of ribbon wires can result in a weak, fragile hook which will not generate consistent test results.


Figure 2. Custom pull hook for low-loop gold ribbon wire.
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To determine the right hook tool geometry, three parameters must be considered: loop height, ribbon width, and breaking force. The loop height determines the maximum-diameter hook that can fit underneath the ribbon. Often the underside of the hook can be machined away to create a flat surface. This enables the hook to fit under lower ribbon heights, although at some cost to hook stiffness. The ribbon width determines the minimum foot length of the hook. For example, inserting a hook only part way underneath the ribbon will result in an invalid test. The wider the ribbon, the tougher the challenge, because then force is being applied to the hook at a longer distance from the bend; which means that it may not remain horizontal during the test. If the hook foot flexes by more than a few degrees under the test load, then the distortion of the ribbon will yield unpredictable test results.

Another problem arises if the ribbon is very thin (on the order of 12 µm). If the hook is too small, it may actually cut through the thin ribbon. In that case only the strength of the material itself is being measured, which is seldom an indication of a good bond between the ribbon and the bond pad. If the L-shaped round-wire hook cannot be used, special pull hooks may be designed that solve these problems (Figure 2).

This style of hook’s material is usually stiffer than standard pull-hook wire, and the part that contacts the underside of the wire is flatter. The hook shape is such that the line of force is vertical between the ribbon center and the hook’s center of rotation. The hook is extremely thin for insertion under very low ribbon loops. This style of hook may be difficult to fabricate, depending on the dimensions of the ribbon and the loop height.

In some applications, accessibility of the bonds is difficult due to deep packages or modules. A longer hook may be used to reach the ribbon bond.

Non-destructive Testing

Many high-reliability applications require non-destructive bond pull testing on each bond in the module. Process engineers may be asked to set up a program that involves both destructive (on a sample basis) and non-destructive testing on every ribbon bond. The parameters of the non-destructive test must be optimized to meet both industry standards and customer expectations.


Figure 3. Shear testing thick aluminum ribbon bonds. (Courtesy of Orthodyne Electronics Corp.)
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A gold ribbon wire that is 1 × 5 mils may have a breaking force of approximately 20 g. Non-destructive testing of this ribbon wire is generally done at around 10% of the breaking force; or 2 g. If there is a defective connection, it can be caught and re-worked before shipping. This is much more common in high-reliability industries, such as military, space, and medical products. Generally, hook geometry selection considerations for non-destructive testing are the same as for destructive testing, although less demanding since test loads are usually 1/10 as large.

Shear Tests

Shear testing is an excellent method for evaluating the bond strength of ribbon wires. A shear test is done with a chisel-shaped tool which applies force to the bond at a specific height in a direction perpendicular to the ribbon. It is possible to shear parallel to the ribbon; although extra care must be taken with tool selection.1 Shear testing offers a good opportunity for inspecting the test site afterwards, and measuring the bonded area. In some applications, however, data can be misleading. If the heel area tends to be over-bonded, the shear forces may be high when the bond quality is actually low. This would be a source of localized high current density which could compromise device reliability. A combination of shear and pull tests would overcome this.

Shear testing is especially useful if thick aluminum ribbon is used, as often seen in power applications. This type of ribbon bond can be pulled with a hook and also sheared at the wedge bond.

Ribbon bond shear test forces are usually much higher than with heavy round-wire bonding because the bond area is many times larger. It is important that the shear height be highly repeatable across the bond site. Modern universal bond testers have excellent shear height repeatability.

Single-ended Gold Ribbon Testing

One common practice is to bond a thin gold ribbon at one end and complete the second bond later in the assembly process. To properly monitor the quality of that first bond, tests are needed. A single-ended ribbon cannot be pull tested with a hook. The two remaining choices are to shear the bond or pull the ribbon with a set of tweezers. A tweezer pull test on this type of sample may yield a number of different failure modes. One common result is that the ribbon peels from the bond pad, in some cases completely separating. This indicates a poor bond. If the bond is good, the ribbon will break just above the wedge bond.


Figure 4. Single-ended tweezer pull test of gold ribbon.
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The most complete approach is to perform both types of tests. Shear tests can be used to optimize the bond parameters at the start of process development. The tested area is then inspected to ensure that a maximum bond area is being achieved. Once this is dialed in, a tweezer pull test can be used to verify that the heel of the wedge bond meets specifications.

Conclusions

Testing ribbon wires is relatively straightforward, but may require different test procedures to monitor bond quality than conventional round bond wires. Much of the knowledge-base from round-wire testing transfers well to ribbon wire, such as evaluating trends in the data and monitoring the overall process; however, there are so many new applications that the trickiest part is deciding which test to use. Ribbon bonding covers a wide range of geometries. Test tools can be specified to meet the requirements of demanding applications.

References and Acknowldgements

Contact the authors for a complete list of references and acknowledgements.


LISA GERBRACHT, head of applications engineering; and MALCOLM COX, founder and CTO, may be contacted at Royce Instruments Inc., 500 Gateway Drive, Napa, CA 94558; 707/ 255-9078; E-mail: lgerbracht@royceinstruments.com, mcox@royceinstruments.com.