Wafer Level Solder Bumping and Flip Chip Assembly with Solder Balls Down to 30μm

Abstract

The most important technology driver in the electronics industry is miniaturization mainly driven by size reduction on wafer level and cost. One of the interconnection technologies for fine pitch applications with the potential for highest integration and cost savings is Flip Chip technology. The commonly used method of generating fine pitch solder bumps is by electroplating the solder. This process is difficult to control or even impossible if it comes to ternary or quaternary alloys. The work described in this study addresses the limitations of existing bumping technologies by enabling low-cost, fine pitch bumping and the use of a very large variety of solder alloys. This flexibility in the selection of the solder materials and UBM stacks is a large advantage if it is essential to improve temperature cycling resistance, drop test resistance, or to increase electromigration lifetime. The technology allows rapid changeover between different low melting solder alloys. Tighter bump pitches and a better bump quality (no flux entrapment) are achievable than with screen printing of solder paste. Because no solder material is wasted, the material costs for precious metal alloys like Au80Sn20 are much lower than with other bumping processes. Solder bumps with a diameter between to date 30 μm and 500 μm as well as small and large batches can be manufactured with one cost efficient process. To explore this potential, cost-efficient solder bumping and automated assembly technologies for the processing of Flip Chips have been developed and qualified. Flip Chips used in this study are 10 mm by 10 mm in size, have a pitch of 100 μm and a solder ball diameter of 30 μm, 40 μm or 50μm, respectively. Wafer level solder application has been done using wafer level solder sphere transfer process or solder sphere jetting technology, respectively. The latter tool has been used for many years in the wafer level packaging industry for both Flip Chip and chip scale packaging applications. It is commonly known in the industry as a solder ball bumping equipment. For the described work the process was scaled down for processing solder spheres with a diameter of 30 μm what was never done before that way worldwide. The research has shown that the underfill process is one of the most crucial factors when it comes to Flip Chip miniaturization for high reliability applications. Therefore, high performance underfill material was qualified initially [1]. Final long term reliability testing has been done according to MIL-STD883G, method 1010.8, condition B up to thirteen thousand cycles with excellent performance of the highly miniaturized solder joints. SEM/EDX and other analysis techniques will be presented. Additionally, an analysis of the failure mechanism will be given and recommendations for key applications and further miniaturization will be outlined.

Study on electromigration in flip chip lead-free solder connections with 40 μm or 30 μm diameter on thin film ceramic substrates

“…The test samples and their fabrication have been described in detail in [2][3][4]. Specially designed, 10 mm x 10 mm x 0.8 mm large silicon chips were assembled onto thin film ceramic (Al 2 O 3 ) substrates by automatic placement and consecutive reflow soldering.…”

Streamlining Sample Preparation of Semiconductor Materials with a New Site-specific Cleaving Technology

“…One method of inspection is performed by creating a cross section of the solder bumps and inspecting it for voids, cracks, intermetallics, or other abnormalities that could cause the device to be susceptible to failure immediately or after aging [4].…”

Fluxless flip-chip bonding using a lead-free solder bumping technique

“…The cost issues in the photolithographic techniques motivated researchers to investigate alternatives. They studied the new controlled-collapse chip connection process (C4NP) [4], gang-ball placement (GBP) [5], Au-stud [6] and laser-jetting technique [7]. The C4NP and GBP are mass transfer techniques, while Au-stud bumping and laser jetting are mask-less sequential processes placing each bump individually.…”

“…The laser-assisted solder-sphere jetting technique, being mask-less, flux-less and cost effective, is making its way among these technologies. It is a sequential jetting technique having ballplacement rates up to 6-8 balls per second [7]. The solder ball in the capillary is liquified by a laser pulse and jetted to the targeted pad by nitrogen pressure.…”