Comparison of Drilling Rates and Tolerances of Laser-Drilled holes in Silicon Nitride and Polyimide Vertical Probe Cards

Silicon Nitride has been a popular choice of material for vertical probe cards because of its excellent thermal, mechanical and electrical properties. In recent years, polyimide has become available in thick wafers (up to 0.125”) such as Cirlex®. This material also has good properties for use in Vertical Probe cards. However, previously the only drilling method was mechanical drilling for which drill wear and positioning reduce production rates and yields. We report on a laser drilling method that produces high quality holes at high production rates and compare this with laser drilling of Silicon Nitride. In both materials, high quality holes with diameters as small as 1.2 mil (30 microns) have been produced

Polyimide is an easier material to drill with mechanical methods than silicon nitride. For laser drilling it shows a lower ablation threshold than silicon nitride. This facilitates drilling of deeper holes than is normal in silicon nitride but also presents certain challenges in terms of increased taper and damage to the material. We have developed a new laser drilling process using the standard ProbeDrill355 system which overcomes these challenges. The figure shows a 2 mil (50 micron) diameter hole drilled through 30 mil (762 micron) thick Cirlex® with an aspect ratio of 15:1. It can be seen that the hole is very parallel with no significant taper and free of damage. Typical drill times for these holes are in the range 1 – 3 seconds depending on the specific geometrical requirements and tolerances. Probe card plates using Silicon Nitride are typically 10 mil (250 micron) thick and in this case typical drill times are in the range of 2.5 – 6 seconds again depending on the specific geometrical requirements and tolerances. Thus polyimide offers the possibility of a X2 increase in production throughput. This can be very significant in 300mm wafer probing where pin count can reach 25,000. Drilling of polyimide requires a very different set of laser tool parameters to silicon nitride. However, these parameters are stored on the tool and enable easy switch-over between the two materials. The quality of the holes in terms of roundness and diameter repeatability is broadly similar and detailed differences will be discussed.