Ultra low loop wire bonding of 20 μm palladium coated copper wire for very thin packages

摘要

In 1998, Carsem introduced the MLP (Micro Leadless Package). Today, the MLP has become the semiconductor industry's package of choice for many devices with low-to-medium I/O count. Coincidentally, the MLP came out at the same time that smartphones were just starting to take off. Following the tremendous increase in market demand for smartphones, tablets and other handheld devices, the MLP soon outpaced the growth (in billions of units shipped) of every other leadframe based package in the industry. This phenomenal growth is greatly attributed to the MLP's low cost, small size, and excellent thermal & electrical performance characteristics. Consumer handheld devices are constantly getting thinner, smaller and cheaper. In recent years, there has been a significant trend towards thinner and thinner packages. Nowadays, the MLP package is available in different package thicknesses ranging from 0.9 mm to as thin as 0.3 mm. The trend for thinner MLP presents several challenges in the choice of packaging materials, most especially in the choice of interconnect wire material. Gold wire has been widely used by the industry to interconnect the silicon chip to the metal leadframe. Thin MLPs will require wire bonds with lower loop profiles. Numerous innovations have been implemented in the gold wire bonding process. For instance, forward bond loops with 50 μm loop height are already in mass production. Other innovations such as die to die bonding, cascade bonding, and bonding overhang die are also being implemented. But as the price of gold continues to sky rocket, copper wire will gain widespread use, especially for price-sensitive consumer electronic devices. Therefore, it is only inevitable that the technological developments made on low loop wire bonding of gold wires will have to be extended to copper wires as well. This paper presents the development work done on Ultra Low Loop wire bonding using 20 μm (0.8 mil) palladium coated copper wire. Actual - esults will be shared to demonstrate capability to achieve a maximum loop height of 63.5 μm (2.5 mils). This paper will also discuss details on the loop type selection, the appropriate test vehicles used, and the relevant output responses after wirebond. And lastly, results from the reliability stress tests will also be discussed.