5 years ago

EPP Europe P1.2019

  • Text
  • Qualitaetssicherung
  • Schablonendruck
  • Loetprozess
  • Leiterplattenfertigung
  • Baugruppenfertigung
  • Zukunftstechnologien


PCB + ASSEMBLY ADAS take advantage of the flexible, small form factor approach Solder paste attributes for SiP assembly Due to the rapid development of internet-of-things (IoT) edge devices, the industry has seen a surge in demand for system-in-package (SiP) assemblies, which are capable of increased functionality in a smaller package format 1) . The automotive industry has also started to take advantage of this flexible, small form factor approach in a variety of different system types—especially in advanced automated driver assistance (ADAS) applications. The SiP element places higher requirements on the assembly process, especially solder paste, fluxer chemistry, and reflow. Sze Pei Lim, Kenneth Thum, Dr. Andy C. Mackie, Indium Corporation Fine-feature solder paste printing for passive component sizes, from 01005 (0.4 mm x 0.2 mm) and now down to 008004 (0.25 x 0.125 mm), has become more challenging in SiP assembly. Due to small stencil aperture designs, finer powder size solder pastes, typically type 5 (15 – 25 μm), type 6 (5 – 15 μm), and type 7 (2 – 11 μm), are used for these applications. Since more components are being packed into a SiP, the rheology of the solder paste is an important attribute, as the gap between neighboring pads can be as close as 50 μm. Solder paste with minimal slump behavior will be required to minimize bridging in such designs. Consistency in solder paste printing transfer efficiency, stencil life, good wetting, graping resistance, and minimal voiding are other key attributes of solder paste for SiP assembly as well. Desire for zero failures As the trend toward miniaturization in SiP applications continues, from current 01005 components going down to 008004 (0201 m) or even 0050025 for next generation packages, the printing performance of solder paste becomes critical. The conventional SMT solder paste printing process using type 3 or type 4 powder size has evolved into a more complicated printing process for SiP, using types 5, 6, or even 7 powder size, with much smaller stencil apertures, thinner stencils, and more stringent requirements for allowed paste deposit variability. SiP, and similar forms of heterogeneous integration, are now being adopted in complex automotive systems where space is a premium. For many years there was confusion about the appropriate reliability testing for these packages, but the recent issuing of AEC-Q104 Failure Mechanism Based Stress Test Qualification for Multichip Modules (MCM) in Automotive Applications makes it clear that SiP can be covered within the main AEC-Q100 spec, SiP…. can be qualified within the scope of AEC-Q100 per Section 2.1. The desirability of zero failures for complex systems, ultimately driven by the desire for increased safety, and most especially by Powder size is classified by type according to IPC J-STD-005A. Source: Indium Source: Indium Comparison of the mentioned flux systems. Source: Indium Source: Indium Different solder powder sizes and their stencil requirements. Summary of the paste specimens. 50 EPP EUROPE April 2019

PCB + ASSEMBLY Test vehicle Practical Components board. Source: Indium ISO-26262:2018 (road vehicles: functional safety) has meant a dual approach for automotive system design engineers: design for reliability at the ppb level, and (unfortunately from a cost perspective) build-in redundancy for mission-critical subsystems. Consistent, low-voiding solder joints lead to enhanced reliability. It is the SiP assemblers’ task to create reliable miniature solder joints, even when dealing with fine-pitch solder paste at quantities of tonnes per year. In addition to printing a smaller and thinner solder paste deposit, the gap between neighboring pads is smaller, too. Some customers are already looking at a gap of 50 μm between pads. In order to achieve good and consistent printing performance under such challenging conditions, besides having good printer setup and appropriate stencil technology 2) 3) , the choice of correct powder size, flux system, rheology, and slump behavior of solder pastes is key. Solder powder Powder size is classified by type according to IPC J-STD-005A. The table outlines the different solder powder sizes available in the electronics and semiconductor industries. Although types 3 – 5 are increasingly in use for SMT, some OSATs (outsourced semiconductor assembly and test companies) and others are already using powder size type 6 for SiP with 01005 chips; for next generation packages with 008004 chips, both types 6 and 7 powder sizes are being considered. It is a general industry guideline that in order to achieve consistent solder paste printing performance, it is important to choose the correct powder size so that a minimum of 5 – 6 solder particles (the large particle size of the range) can be maintained across the aperture. According to this rule, there is a recommended minimum stencil aperture for each powder type. Another concern when choosing a smaller powder size is the increase in the powder surface area. As shown in the table, when powder size decreases from type 3 to type 7, the powder surface Source: Indium Different stencil designs were used. Pressure to wipe the stencil surface clean. Source: Indium Source: Indium Both vacuum support and carrier pallet are capable setups. The characteristics of the four tested soldering profiles. Source: Indium EPP EUROPE April 2019 51