Solder-free Connectors Using Buckled Pillars

By Peter Salmon, Salmon Technologies

A particularly versatile form of wafer bumping is stud bumping, using either gold or copper wire. The equipment required is an adaptation of a traditional wire bonder. A ball bond is made at a first contact pad using heat and ultrasonic energy. The wire is extended in a precise direction and a flying lead is created by terminating the wire, for example using electronic flame-off (EFO). This type of stud bump can be used as a buckled pillar connector, enabling electronic assemblies that are put together with screws rather than solder. With appropriate assembly and testing techniques, this can lead to waste-free integrated assemblies; no good parts are thrown away due to compound yield problems, otherwise known as Known good die (KGD) problems.

Manufacturing Sequence for Buckled Pillar Connectors
The animation depicts the manufacturing sequence for a particular case involving a top and a bottom substrate with two layers of ICs in between. The preferred substrates are copper for reasons relating to power dissipation and water channel construction. A copper substrate is shown with interconnection circuits fabricated on top. The interconnection circuits are fabricated on large copper panels using direct laser imaging. The starting point includes a bottom layer of ICs already mounted using buckled pillar connectors and wax, using the method described herein.



figure 1:Manufacturing sequence for buckled pillar connectors.

First, ball bonds, the beginnings of buckled pillars, are placed at 4 locations. Flying leads are created by moving the bonding tool away from the ball bonds, in a direction slightly away from normal, to create a predictable flexure direction for the buckled pillar. Next, electronic flame-off is used to terminate each wire in a spherical ball. An alternative method is to use a spark discharge.

A top-layer IC chip is then flipped and placed on top of the lower IC chip, and similarly constructed wires have are formed. The wire diameters may vary roughly in accordance with their length. The bonding tool is programmed so that all the spherical balls have approximately the same height. Melted wax is applied so as to cover all of the spherical balls, and then hardened. Chemical mechanical planarization (CMP) provides a uniform height for all of the wires; typically the spherical balls are removed in this step. Finally the top substrate is inverted and aligned; contacts are established between the tips of the pillars and corresponding capture points; and the structure is compressed a precise amount using assembly screws (not shown in the figure). The wax is heated to soften it during this assembly process. The buckled pillars so produced are shortened by a few percent in length. They maintain their elastic property and act like springs. The spring action helps to make good contact, and provides some design margin against deviations from co-planarity of the wire tips.

Conclusion
The described method for making buckled pillar connectors draws on mature techniques for making stud bumps, and for planarization using a wax filler. The resulting electronic assembly is solder-free, thus avoiding many of the reliability issues that accompany the use of solder. The technique is versatile and can be adapted to many different substrates and chip stack configurations. As shown in the animation, no through silicon vias (TSVs) are required. By combining these methods with water cooling channels provided between selected ones of the copper substrates, an effective system integration method becomes available, without depending on chip modifications.

Peter C. Salmon, VP can be contacted at Salmon Technologies, LLC, 200 E. Dana St. #8
Mountain View, CA 94041; 650/814-1076; email: [email protected]

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