Design007 Magazine


Issue link:

Contents of this Issue


Page 53 of 119

54 DESIGN007 MAGAZINE I OCTOBER 2019 more uncased die elements (often from mul- tiple sources) within a single package outline, these high-density interposers can significant- ly enhance product performance by enabling much shorter circuit interconnects for critical signal paths. Key issues the designers will need to con- sider when developing the 2.5D interposer is choosing the most suitable base materials for interposer development. The three primary base materials commonly selected for the 2.5D interposer application are epoxy-glass-based organic composites and silicon and glass di- electrics. Designers will also need to learn the basic methodologies and complexities for met- alization and imaging the interconnecting cir- cuit pattern. High-Tg, Low-CTE Organic Base Material The semiconductors function best when they are electrically interconnected to related devices with the shortest path. While many organic dielectric materials have traditionally proved suitable for a broad range of wire-bond package applications, a number of leading sup - pliers have developed a more advanced lam- inate material that closely matches the very low thermal coefficient of expansion (CTE) of the silicon die element through elevated oper- ating temperature as well as meeting the fine- line interconnect challenge for new genera- tions of high-I/O, face-down, mounted semi- conductors. Shinko Electric in Japan, for example, has developed an ultra-low CTE (1.5 ppm/°C) or- ganic substrate material that will provide a sta- ble core layer for buildup, multiple-layer inter- posers (commercial FR-4 laminate has a CTE of ~16 ppm/°C while the silicon-based die el- ement has a CTE of ~3 ppm/°C). Thermal sta- bility of the core material is also a concern. The bismaleimide-triazine (B-T) laminate ma- terial has a rated Tg (glass transition) close to 300°C while the commercial FR-4 Tg is rated at 150–170°C. The B-T laminate material also furnishes a higher elastic modulus at ~40 GPa (megapas- cals) where commercial FR-4 laminate modu- lus is in the range of 10–15 GPa. A real ad- vantage is that organic-based 2.5D interposer fabrication can utilize the existing PCB manu- facturing infrastructure that is already employ- ing direct-imaging laser ablation for via forma- tion, and have the lithographic capability for furnishing fine-line, semi-additive copper cir- cuit processing. Silicon Interposer Base Material A majority of commercial semiconductor manufacturers utilize thin silicon wafers to provide a stable base for integrated circuit pro- cessing. The silicon-based material provides excellent electrical and mechanical properties and is a natural choice for the 2.5D interpos- er because it perfectly matches the CTE of the silicon die element(s) that will be mounted onto its surface. Fabrication of the interposer is commonly performed within the semiconduc- tor foundry environment; however, the pro- cesses for via hole ablation and metalization are very different from the basic semiconduc- tor manufacturing processes (Figure 2). Initially, suppliers utilizing silicon wafers or panels will commonly adopt mass plas- ma ablation technology to first form the via holes and employ a series of copper metaliza- tion processes to provide via filling and enable circuit redistribution layers (RDL) with lines and spaces measured in micrometers (µm). To maximize assembly efficiency, the base mate- rial can be furnished in the traditional 300-mm diameter wafers or in a reconstituted silicon panel format. Current panel variations include 300-mm and 500-mm square panels, but some Key issues the designers will need to consider when developing the 2.5D interposer is choosing the most suitable base materials for interposer development.

Articles in this issue

Archives of this issue

view archives of Design007 Magazine - Design007-Oct2019