However, the key difference is that the conductive metal of a via-through-pad TSV penetrates through the thickness of the bond pad to form a circumferential edge contact. This is structurally identical to the well-established wafer-level edge contact technology and has the same inherent reliability. Because the act of penetrating the bond pad exposes fresh metal at the circumference, this solution obviates the difficulty of making ohmic contact between the conductive metal of the via and the bond pad, when the bond pad is the closed end of a high aspect ratio pipe. The end result is higher process yield and product reliability.
ACCELERATED RELIABILITY TESTING
Evaluating the reliability of semiconductor packages is a time-consuming process. Many companies use quick screening tests, which typically give a good indication of the likely outcome. Solid-state imagers and MEMS packages that have a sealed cavity can use the following test:
Autoclave: 120°C; 100% RH for 168 hrs; followed immediately by lead-free solder reflow: step heat/cool 25°C to 280°C to 25°C (30 sec dwell) x 20 cycles
Wafer-level cavity packages with via-through-pad TSVs will pass this test.
PACKAGE-LEVEL RELIABILITY
Components intended for integration into products must be applicable for that purpose. Meeting this criterion is usually demonstrated by subjecting batches of component parts to various environmental regimes, which must be survived. For conventional semiconductor parts, these tests are defined by industry standards, one of the most arduous of which applies to automotive applications. (Table 1) summarizes the principal package-level tests and accompanying environmental parameters.
A typical test sample comprises a minimum of three lots of 77 parts taken from production batches manufactured on different days, by different shifts, on the full range of equipment available. As can be seen from (Table 2), imager die protected by wafer-level packages using via-through-pad interconnects exhibit a margin of safety of more than two and often substantially more over the required reliability standards.
BOARD-LEVEL RELIABILITY
The test vehicle selected for this application measured 6 x 7 mm. It was packaged at the wafer-level with via-through-pad TSVs and equipped with a µBGA interface of 200-µm-diameter solder balls consisting of SAC305 composition. The chip-scale packages (CSPs) were attached to eight-layer, 1-mm-thick FR4 board having 350-µm-diameter lands for each solder ball. SAC305 solder paste was used for package attach.
The wafer-level packages measured 600 µm in thickness. Prior to attachment to the PCB, the packages were pre-conditioned to MSL1. That is, a bake in air at 125°C for 24 hours, followed by a bake in a damp environment at 85°C with 85% relative humidity for 168 hours, and finally exposure to three consecutive thermal excursions, each representative of a lead-free solder reflow cycle.
After attachment to the PCB, the µBGA interface collapsed slightly, giving a package-to-board spacing of approximately 150 µm. Underfill material (Namic SUF 1570) is introduced into this gap, which was then cured in accordance with the manufacturers recommendations. A total of 50 parts were manufactured for this trial.
The components were subjected to automotive board-level environmental tests (Table 3), which included the most aggressive of tests for electronic assemblies, namely cyclic thermal shock. This comprised a dual chamber system, with the cold chamber set at -40°C and the hot chamber at +85°C. The dwell time in each chamber was 5 minutes and the transfer time less than seven seconds.
The results, which are also incorporated in Table 3, show that no failures were recorded at all. In the case of thermal shock, no failures occurred even after testing was extended to 1250 cycles.
CONCLUSION
TSVs can be used to provide circuit-to-circuit and die-to-package interconnects. While many variations of TSVs have been developed over the years, the applications remain limited because of high cost and/or poor reliability.
Via-through-pad TSVs have been engineered for the specific application of wafer-level packages for solid-state imagers. Focusing on one product type has made it possible to engineer a TSV solution that works for a specific application, is very low cost, and extremely reliable. Indeed, the package using via-through-pad TSVs will comfortably pass automotive component- and board-level reliability tests.