The latest drivers are conversion cost reduction, NPI time reduction, greater I/O density and environmental compliance.

Board assembly accounts for most of the direct material cost, and is closely associated with component packaging, interconnection, inspection, thermal management, final assembly and environmental technologies. This article highlights a few trends identified by the board assembly chapter of the 2007 iNEMI Roadmap. To recap, the Roadmap uses product emulators to identify technology requirements, primarily from the OEM perspective, for five product sectors: automotive, defense and aerospace, office/large business/communication systems, medical, and portable/consumer. Though diverse in form and function, the product sectors reflect certain common priorities for board assembly.

Reduction in conversion costs. Conversion cost is the cost to take a group of parts and convert them to a functioning electronics assembly. Conversion cost is the price of a completed assembly (including test, material procurement cost, etc.) less material cost. In this way, all cost associated with manufacturing and testing the assembly is considered. All product sectors predict significant decreased conversion costs by 2017. Portable electronics appears to seek the most aggressive reductions, overtaking office systems products after 2011 (Figure 1). Defense products lags behind other sectors because of its focus on reliability over cost. Note that these conversion cost reduction numbers reflect OEMs’ forecasted or expected cost reductions. The industry recognizes the challenges in being able to deliver such cost reductions, given the investment levels required and the increased variable costs accompanying technology transitions, such as the Pb-free conversion.


Lower escape and defect rates. The escape rate metric for board assembly is tied closely to conversion cost. All product sectors project an exponential decrease in DPMO levels. This trend is led by the automotive sector, while significant pressure is on the portable, medical and office equipment industries to improve escape rates. For the defense industry, escape rate reduction is not driven by cost so much as a need for increased quality and reliability. In light of tighter process windows because of the RoHS transition, obtaining such steep escape rate reductions presents yet another challenge. Renewed focus on DfX guidelines and design/manufacturing collaboration is required to reduce escape rates and improve quality levels.

Compounding the issue is the simultaneous need to reduce defect levels while products become smaller. With reduced pitch and increased I/O density trends, board suppliers’ ability to reduce defects will be limited without significant disruptive advancements in interconnect technology. Nanotechnology may provide the advancements needed, enabling (for example) such interconnect concepts as carbon nanotubes or nanowires.

Manufacturing migration. Another significant contributor to conversion cost is the migration of electronics manufacturing to lower-cost geographies. The migration to China appears to be leveling, especially in the high-volume portable/consumer sector. China already produces about half the world’s cellphones and almost two-thirds of the DVD players. EMS companies are now beginning to seek other nearby locations, such as India, both to act as a hedge to China and reach local markets.

NPI time reduction. The new product introduction time is the lapse between a design being released for alpha prototyping and its release to production (assuming prototype parts are available at release). Automotive shows the longest NPI times, followed by communications and defense, with portables, office equipment, and medical (externals only) showing similar behavior (Figure 2). In medical implantables, where regulatory approvals are required, cycle times are much longer.


With projected NPI cycle time reductions of more than 60% by 2017 will come new constraints on manufacturing processes and operations. In general, the projected rapid cycle time reduction will need to be met with business system changes within EMS and OEM organizations. An emerging area that can help with cycle time reduction is the use of DfX by EMS companies. EMS companies are being engaged early in the product development process and are providing upfront input to traditional DfX, such as design for manufacturing, assembly or test, as well as nontraditional DfX, such as design for supply chain, environment and recycling. DfX in the global outsourcing environment requires closer interactions and collaboration across the supply chain, including OEMs, EMS providers and the supply base. Industry standards need to be developed to facilitate and streamline the information flow.

Environmental compliance transition. The most pressing environmental legislation is the European Union’s WEEE and RoHS Directives, which have already taken effect, along with environmental legislation in China and other nations. With defense, automotive and several other sectors forecasting a slower adoption timeline for RoHS compliance and Pb-free board assembly than most (Table 1), component traceability, availability, compatibility, and other issues associated with the coexistence of “dual alloys” (i.e., SnPb and Pb-free materials) have become areas of concern. “Backward compatibility” is one such technical issue under intense study. Over time, Pb-free board assembly adoption is expected to force conversion even in exempt industries, such as aerospace and defense, because of supply base dynamics. This de facto compliance with Pb-free materials for all product sectors may impact assembly reliability and create hidden costs for these industries. Furthermore, with product emulators forecasting increasing design-for-take-back requirements, materials and processes opportunities will result. However, future environmental restrictions may result in even further changes in materials and processes.


Technology Trends

PWBs. Some of the key PWB technology trends affecting board assembly include:

• Higher use of flexible (especially for portables) and low-loss materials (especially for communications and medical products).
• Availability of a low-cost board technology to handle very fine pitch, high I/O devices.
• Smaller pad diameters impacting second-level assembly reliability.
• Transition to embedded passives (in portables).

Flexible substrate materials adoption is driven by electronics penetrating every aspect of our lives. Portables forecast increased use of flexible substrate materials. Flexible materials’ higher application rate offers unique challenges when coupled with high-I/O density devices and Pb-free applications. For board assembly, process and handling technique development to support these materials will be paramount to successful technology introduction.

One of the greatest challenges PWB suppliers face is low-cost board technology that enables routing out of high pin count devices with tight pitches. As this challenge is met, second-level assembly will see a rapid decrease in component pitch and an associated increase in package I/O count.

The third major implication that board technology changes are having on second-level assembly is the impact on second-level reliability. With continued reduction in pitch, and likewise standoff, interactions between the PWB and the substrate will challenge the solder interconnect. Again, this trend, coupled with higher reflow temperatures and (potentially) increased voiding of Pb-free solder joints, will offer unique challenges for assemblers.

Components. Component technology advances continue to provide challenges to board assembly. All the product emulators show significant maximum I/O density increases over time. Portables show the steepest increase, whereas I/O density plateaus in office equipment and defense sectors after 2009 (likely driven by the high cost of fine-line PWBs and die size flattening). The automotive and communication equipment sectors show consistent increases in maximum I/O density. Such increases will demand a further reduction in device pitch. For example, the portables sector predicts the maximum I/O package on the board will use a 0.4 mm pitch until 2009 and a 0.3 mm pitch by 2011.

Over time, more functionality will be integrated through system-on-chip (SoC) and system-in-package (SiP) solutions. The level at which functionality is integrated (i.e., silicon, package or board) will depend on the product and market maturity, design envelope, manufacturing flexibility and manufacturability, unit volume, performance and overall cost.

Assembly materials. Increased component complexity will be a driver for advances in new materials. Increased package density will challenge cleaning requirements and rework, as smaller components with lower standoffs are more difficult to clean. Low component standoff height will also challenge underfill chemistries in meeting fill time and voiding requirements. Lower joint standoff’s negative impacts on solder joint reliability may create opportunities for new interconnect technologies and materials. This reliability challenge is made more complicated by automotive and other product sectors’ stringent reliability requirements.

SMT process. Board assembly conversion cost targets drive the need for greater SMT process efficiency. Lean manufacturing methodologies adoption in high-mix, low-volume manufacturing is providing an impetus to reduce changeover/setup times and process cycle times, aided by flexible tooling solutions, optimized production equipment sets, and optimized production line configurations. Other key drivers include reducing the total cost of ownership (including energy cost, nitrogen cost, and maintenance cost), improving flux management systems and improving traceability.

The transition to mixed technology and Pb-free production requires tightly controlled production floor processes to handle added supply chain complexity. Higher component moisture sensitivity levels as a result of reflow temperatures of 260oC may negatively affect SMT cycle times (due to longer bake-out schedules) and possibly yields, product reliability, and board conversion costs (resulting from moisture damage to production parts). Pb-free assembly requires tighter reflow process control to minimize temperature deltas across assemblies and ensure maximum assembly temperatures are not exceeded. Mixed assembly (Pb-free components in a SnPb assembly) reliability is a subject of substantial study.

Increasing component complexity (with projected component pin counts above 8000 and component pitches below 300 µm at the second-level interconnect and below 70 µm at the SIP interconnect) and larger-sized assemblies (with thicker board structures) are challenging printing, reflow and dispensing first-pass yields. Advanced PWB materials (such as flexible circuits and low-loss materials) use may present unique handling and reflow challenges. Embedded passives adoption will require new line configurations and operating methodologies for board assembly operations.

Wave and selective soldering. PWBs are being designed with PTH and surface mount devices. In general, PTH is used less than 5% in PWB designs for commercial products, and 10 to 15% in aerospace products. PTH use is expected to fall slightly (to less than 5%) as OEMs migrate to contact interconnect devices (such as press-fit) for cost savings. Wave and selective soldering processes will continue to be used for soldering PTH components. Currently, conventional wave soldering is the dominant process (about 80%), and it is expected that the use of selective soldering will slightly increase, to about 30% by 2011. As PTH technology typically requires manual labor for assembly, these products will continue to migrate to countries with abundant labor markets.

Two alloy systems are considered leading candidates for Pb-free wave and selective soldering: SnAgCu and SnCu (and their variants). The adoption of these Pb-free solder alloys (and associated flux systems) requires reassessment and re-optimization of wave and selective soldering process steps, as well as equipment upgrades. Water-soluble (WS) and no-clean (NC) Pb-free compatible flux development is a key focus of materials suppliers. Achieving acceptable hole-fill for thick boards, while minimizing copper dissolution, poses a challenge for Pb-free wave and selective soldering processes.

Rework and repair. As components become smaller, they will be physically impossible to rework by hand soldering. The trend toward tighter component pitches is requiring increased placement accuracy for rework. With the projected reductions in the land pattern diameter comes unique challenges in rework, from site dressing to defect-free localized reflow. High component pin counts and larger component body sizes will challenge current rework placement and reflow techniques, and impact yields. Higher Pb-free process temperatures narrow the rework process window, particularly for larger products such as networking products.

Summary

The 2007 iNEMI Roadmap highlights four main board assembly development drivers: conversion cost reduction, NPI time reduction, increased component I/O density, and environmental and regulatory compliance. The industry faces several key business issues, such as the supply chain’s ability to support Pb-containing and Pb-free BoMs, and the need for government, academia and industry consortia to adopt and develop emerging technologies (such as nanotechnology) into the board assembly process. Another area of importance is DfX in the global outsourcing environment, requiring closer interaction and collaboration across the supply chain and industry standards to facilitate and streamline the information flow. One of the profound business environment impacts is the higher level of service demands or opportunities placed on EMS providers. Today’s EMS companies are expanding offerings to include services in a wider range of a product’s lifecycle.

For the supply chain, several critical technical areas are identified for future development, including:

• PWB/substrate to provide a low-cost, fine-line technology for higher density.
• Flexible/low-loss substrates material handling equipment.
• Availability of 01005s with required values.
• Solder materials to replace (expensive) Ag-containing alloys for certain cost-sensitive applications, coupled with the need for low-temperature attachment requirements for new polymer-based products.
• Pb-free solder alloys to overcome several critical concerns (such as copper dissolution during wave/selective soldering, reliability under mechanical shock, etc.).

For the board assembly process, a number of key technology gaps have been identified:

• Nontraditional technologies for solder paste deposition to meet, with consistency, the widening range of required paste volumes deposited on mixed technology assemblies.
• Creative engineered solutions to support cost-reduction targets with the Pb-free transition.
• Inspection/test technologies to keep up with increasing board density and component complexity.
• Innovation in every step of the board assembly process for 3-D assembly.

Acknowledgments
The Board Assembly chapter is the result of work done over eight months by a team of 85 contributors from 45 companies, located in several continents representing different segments of the supply chain. Their hard work, dedication and contributions are gratefully acknowledged.

Dr. Dongkai Shangguan is vice president, assembly technology & platform realization at Flextronics (flextronics.com), and chair of the 2007 iNEMI Roadmap Board Assembly chapter; dongkai.shangguan@flextronics.com. For information about ordering the iNEMI Roadmap, visit inemi.org/cms/roadmapping/roadmaporder.html.

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