Tutorial #101
George Riley
February 2010

Buckled pillars are an interesting development for solder-free connections in a module, using a modified stud bump construction. The resulting assembly does not require underfill, and may be reworked at any stage. Inability to rework by removing a failed device has generally limited the number of devices used in a module, since failure of one device means discarding all. Buckled pillars would remove that limit.

Construction

Figure 1 is a conceptual drawing of a multi-chip assembly, showing how a variety of devices might be assembled. Pillars are created on each device by an extension of the well-established stud bump process. Stud bumping uses a modified thermosonic wire ball bonder to place a “bump” on a pad. In stud bumping, the wire is broken or sheared immediately above the bump.

Figure 1. Multi-chip Buckled Pillar Assembly

For buckled pillar creation, instead of shearing the wire at the bump, it is extended upwards as a flying lead at a small, controlled angle from the vertical. The wire is terminated at the desired length by electronic flame-off (EFO), forming a new ball. The bonder is programmed so that the balls of all the chips connecting to the same substrate surface are approximately coplanar.

When all flying leads of the chips connecting to one substrate are completed, the multi-chip assembly is filled with wax up to a level covering all of the balls. Lapping or chemical-mechanical polishing (CMP) removes the excess wax and the balls, planarizing the wire tips of all the chips at a common level for connection to the substrate. As shown in figure 1, different wire diameters may be required for different pillar lengths.

The substrate conducting metal layer is patterned with receiving cups matching the pillars. Assembly is completed by aligning pillars and cups, then tightening the assembly screws to slightly compress the pillars. The wax is softened by preheating heating during this assembly step. Compression is limited by fixed spacers to stay within the elastic range of the pillars, so that they will act as springs. The compression also compensates for any non-coplanarity of the pillar tips.

As shown in figure 2, the assembly is adaptable to a variety of external connections. Shown here are BGA, buckled pillar to PC board, and buckled pillar to flexible circuit connections.

Figure 2.   Examples of external connections.

Benefits

In addition to reworkability, potential buckled pillar benefits include:

• Eliminating solder avoids the added cost of solder, the limitations of lead-free solders, and reliability concerns such as solder voids and solder cracks.
• Eliminating underfill allows easy removal of components if rework is required, and avoids the time and cost of underfill dispensing and curing.
• Providing an alternative to expensive TSVs allows standard die without redesign or exotic processes.
• Allowing easy channels for water cooling, enabling thermal control of high power devices.
• Replacing costly and complex assembly equipment with simple, proven wire bonders.
• Increasing versatility by interfacing with a wide variety of devices, substrates, chip stacks, and external connections.

Conclusion

Buckled pillar connectors draws on mature techniques for forming and planarizing flying leads. The resulting solder-free and underfill-free electronic assembly avoids many of the reliability issues that accompany the use of solder, and allows easy rework. The technique is adaptable to many different substrates and chip stack configurations, without expensive TSVs. With buckled pillars, effective system integration does not require chip modifications.

FOR MORE INFORMATION

Contact Peter C. Salmon
Salmon Technologies, LLC
Phone (650) 814-1076       Email:peter@salmontech.com