Микрочипті қаптамаға қосылу технологиясы

Technique used to connect a microchip to its package

Wire bonding is a method of making interconnections between an integrated circuit (IC) or other semiconductor device and its packaging during semiconductor device fabrication. Wire bonding can also be used to connect an IC to other electronics or to connect from one printed circuit board (PCB) to another, although these are less common. Wire bonding is generally considered the most cost effective and flexible interconnect technology and is used to assemble the vast majority of semiconductor packages. Wire bonding can be used at frequencies above 100 GHz.

There are multiple challenges when it comes to wire bond manufacturing and reliability. These challenges tend of be a function of several parameters such as the material systems, bonding parameters, and use environment. Different wire bond bond pad metal systems such as Aluminium Aluminium (Al Al), Gold Aluminium (Au Al), and Copper Aluminium (Cu Al) require different manufacturing parameters and behave differently under the same use environments.

Much work has been done to characterize various metal systems, review critical manufacturing parameters, and identify typical reliability issues that occur in wire bonding. When it comes to material selection, the application and use environment will dictate the metal system. Often the electrical properties, mechanical properties, and cost are taken into account when making a decision. For example, a high current device for a space application might require a large diameter aluminium wire bond in a hermetically sealed ceramic package. If cost is a large constraint, then avoiding gold wire bonds may be a necessity. Some recent work has been done to look at copper wire bonds in automotive applications. This is only a small sampling, as there is a vast body of work reviewing and testing what material systems work best in different applications. From a manufacturing perspective, the bonding parameters play a critical role in bond formation and bond quality. Parameters such bond force, ultrasonic energy, temperature, and loop geometry, to name a few, can have a significant effect on bond quality. There are various wire bonding techniques (thermosonic bonding, ultrasonic bonding, thermocompression bonding) and types of wire bonds (ball bonding, wedge bonding) that affect susceptibility to manufacturing defects and reliability issues. Certain materials and wire diameters are more practical for fine pitch or complex layouts. The bond pad also plays an important role as the metallization and barrier layer(s) stackup will impact the bond formation. Typical failure modes that result from poor bond quality and manufacturing defects include: fracture at the ball bond neck, heel cracking (wedge bonds), pad liftoff, pad peel, overcompression, and improper intermetallic formation. A combination of wire bond pull/shear testing, nondestructive testing, and destructive physical analysis (DPA) can be used to screen manufacturing and quality issues.

While wirebond manufacturing tends to focus on bond quality, it often does not account for wearout mechanisms related to wire bond reliability. In this case, an understanding of the application and use environment can help prevent reliability issues. Common examples of environments that lead to wire bond failures include elevated temperature, humidity, and temperature cycling. Under elevated temperatures, excessive intermetallics (IMC) growth can create brittle points of fracture. Much work that has been done to characterize the intermetallic formation and aging for various metal systems. This not a problem in metal systems where the wire bond and bond pad are the same material such as Al Al. This does become a concern in dissimilar metal systems. One of the most well known examples is the brittle intermetallics formed in gold aluminium IMCs such as purple plague. Additionally, diffusion related issues, such as Kirkendall voiding and Horsting voiding, can also lead to wire bond failures. Under elevated temperature and humidity environments, corrosion can be a concern. This is most common in Au Al metal systems and is driven by galvanic corrosion. The presence of halides such as chlorine can accelerate this behavior. This Au Al corrosion is often characterized with Peck's law for temperature and humidity. This is not as common in other metal systems. Under temperature cycling, thermomechanical stress is generated in the wire bond as a result of coefficient of thermal expansion (CTE) mismatch between the epoxy molding compound (EMC), the leadframe, the die, the die adhesive, and the wire bond. This leads to low cycle fatigue due to shear or tensile stresses in the wire bond. Various fatigue models have been used to predict the fatigue life of wire bonds under such conditions. Proper understanding of the use environment and metal systems are often the most important factors for increasing wire bond reliability.

While there are some wire bond pull and shear testing techniques such as MIL STD 883, ASTM F459 13, and JESD22 B116, these tend to be applicable for manufacturing quality rather than reliability. They are often monotonic overstress techniques, where peak force and fracture location are the critical outputs. In this case the damage is plasticity dominated, and does not reflect some wearout mechanisms that might be seen under environmental conditions. Wire pull testing applies an upward force under the wire, effectively pulling it away from the substrate or die. The purpose of the test is as MIL STD 883 2011.9 describes it: "To measure bond strengths, evaluate bond strength distributions, or determine compliance with specified bond strength requirements". A wire can be pulled to destruction, but there are also non destructive variants whereby one tests whether the wire can withstand a certain force. Non destructive test methods are typically used for 100% testing of safety critical, high quality and high cost products, avoiding damage to the acceptable wired bonds tested. The term wire pull usually refers to the act of pulling a wire with a hook mounted on a pull sensor on a bond tester. However, to promote certain failure modes, wires can be cut and then pulled by tweezers, also mounted on a pull sensor on a bond tester. Usually wires up to 75 μm diameter (3 mil) are classified as thin wire. Beyond that size, we speak about thick wire testing.