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Wafer Level Packaging

SnAg Composition

Introduction

SnAg Composition

  • Small bumps using Sn rich solder makes the UBM selection more critical
    • Surface tension is increasingly impacted by wettability
  • Reduced solder ball size increases surface to volume ratio
    • Surface tension is increasingly impacted by wettability
    • Past Cu-Cr-based UBM is not compatible
  • Nickel based UBM materials are desired because of the low reaction rate with Sn
    • Many Ni based UBM stacks are being used
  • Electroless acidic hypophosphite baths are used for nickel deposition
    • If the phosphorus content is high (>9.5 atm%), the Ni(P) will be in the amorphous state
    • Suppresses fast grain-boundary diffusion

XRF

XRF Data for UBM and RDL Stacks

  • Typical UBM and RDL layers are multi-layer stacks:
    • Barrier (or Under Metal)/Bulk Metal Layer / Passivation Metal Layer
  • Common UBM Film Stack Variations
    • Al/Ni/Au, Al/Ni/Cu
    • Cu/Ni/Au, Cu/Ni/Pd
    • Ti/Ni/Au, Ti/Cu/Ni/Au, Ti-W/Cu/Cu
    • Cr/Cu/Cu, Cr/Cu/Cu/Ni

Metrology

Meeting Evolving Metrology Needs

WLP represents a rapidly growing market segment due to the need for high-density, high-pin-count, smaller size, stacked and high performance devices. This growth gives rise to new metrology challenges with re-distribution and under bump metallization film stacks. In addition, lead-free bumps also require in-line control of material composition.

Bruker stands uniquely qualified to address these continually evolving metrology needs with industry-leading small spot, high speed X-ray Fluorescence technology.

Micro-XRF

Distribution Analysis with Micro-XRF

As bond pads are getting smaller, the deposited amount of solder and the spatial distribution are an increasingly more complicated to analyze. Micro-XRF allows with spot sizes in the range of a few tens of micrometers to measure solder composition on bond pads down to 30 µm. When there is overlap of lower energetic lines, the determination of of composition can be challenging. But not with micro-XRF because it measures the high energy K-lines when determining the composition. Bruker's unique micro-XRF solutions allows investigating small areas with XTrace detector for SEM, and investigate large areas with M4 TORNADO the benchtop micro-XRF.