Originally Published MEM Fall 2008

MINIATURIZATION

High-density manufacture is essential to keep ahead of an increasingly competitive market. Product size is often a key sales driver and, while greatly influenced by processors and large complex components, packing densities are just as strongly determined by the size and interspacing of passive components. This is especially true for handheld and portable medical electronics. Besides the space savings that the size and interspacing of these passive components bring on the main board, passive devices are increasingly being concentrated into system-in-package (SiP) modules.

Despite considerable skepticism when 0201 devices were first introduced to high-density board assembly a few years ago, they are now commonly used in medical electronics. Now, even-smaller 01005 components are available. Most components are chip resistors and capacitors, and, therefore, medical equipment manufacturers can use these 01005 devices to significantly increase board densities.

At just 0.4 mm long by 0.2 mm wide, 01005 chip components place new demands on assembly and interconnection processes. However, work is needed on the stencil printing process, the placement process itself, and soldering conditions. The components cannot be used in mass production until defect-per-million (dpm) levels are close to single-digit figures. This article discusses exhaustive tests that have been performed to determine whether 01005 components are ready for integration into devices. The conclusion is that as long as some basic rules are followed, they are ready.

Increasing Component Density

The dominant passive component sizes have changed since the 1990s. The 1206 (3.2 × 1.6 mm) and 0805 (2.0 × 1.25 mm) chips no longer make up the majority. These formats have generally migrated to 0603 (1.6 × 0.8 mm), 0402 (1.0 × 0.5 mm), and 0201 (0.6 × 0.3 mm). This change has led to a considerable reduction in occupied board area.

Portable equipment has driven component density, which is now already at more than 50 components/cm² and predicted to rise to nearly 80 components/cm² over the next 10 years.¹ In response, the 0603 and 0402 types are now themselves migrating to 0201 and 01005 types in high-density applications. Although these two packages are only beginning to emerge, they are expected to claim an increasing market share, with the International Technology Roadmap for Semiconductors (ITRS) predicting that 01005 types will remain the minimum size until at least 2012.

Interspacings and Board Density

The minimum interspacings of 0402 components are 150 µm, so even these relatively small components are limited to around 100 components per cm². Smaller 0201 types allow tighter interspacings at 100 µm to triple the density to 300 per cm². And, with 01005 components on 50-µm interspacings, the maximum density could double again to 600 per cm².

Such densities are needed for integrating passive chip components into semiconductor or SiP modules. The semiconductor modules can be found mainly in handheld applications and offer even more functions at reduced dimensions for devices such as hearing aids, implantables, and endoscopy cameras. Component placement at such small interspacing, however, has a much more critical process window.

Solder Stencils and Component Size

The first of the interspacing-related issues is the solder stencil. Large components need thick stencils to deliver enough solder to make the chip attach properly. Small components need thin stencils. Unfortunately, these do not carry enough solder paste to form a good joint for large components.

The solder process itself is critical, too. Small components need a fast (2 per second) reflow process to reduce flux evaporation, while large components need a slow (1 per second) reflow process since more time is needed for heating up to reflow temperature.

In short, large and small components need to be treated differently. We are indeed seeing a trend to have the larger components (down to 0402 or 0201) on main printed circuit boards (PCBs), with the smallest 01005 components migrating to modules. This is a positive trend for manufacturers because, rather than upgrading a whole line, ultra-high-precision assembly is needed only for the modules.

Migrating from Microchips to Modules

The SiP approach brings equipment manufacturers several benefits. For starters, it keeps signal lines short and local for best frequency response. That makes for more design flexibility, by increasing diversity from a modular architecture. High-frequency signals can be kept within the modules (where track lengths are short), and less RF-specific design competence is needed for the main board.

Such RF modules are increasingly becoming available as off-the-shelf products. It is estimated that around 70% of ICs are now placed on the main board, with 30% in modules. As more and more functions become standard in medical electronics, it is expected that in 3–5 years, this ratio will shift to 40:60.

Placing 01005 Components

The 01005 components must be placed on SiPs reliably and consistently. This also makes phenomenal demands on pick-and-place machines, with accuracy and reliability of 01005 component placements being perhaps the two ultimate tests for a machine.

In full production runs, parallel placement pick-and-place machines have single-digit dpm figures at an accuracy of 40 µm, currently the industry benchmark. Getting figures this low means performing the same action time after time with minimal variation, so the placement process itself must be ultrarepeatable. Rather than placing components sequentially using large overhead gantry robots, as do the more common pick-and-place operations, these new machines have multiple modules placing components in parallel.

Parallel Placement with Multiple Heads

Parallel placement means much smaller accelerations and decelerations. It gives the individual x-y robots much more time to settle and to check the placement process. During the pick cycle, a parallel placement device checks the pick height, checks that a component is present, and corrects for any misalignments in component position. During the place cycle, it checks that the component is still there, checks alignment, detects any on-edge components, places it with correct force, and inspects the placed component. For parallel placement, typical acceleration is 10–15 m/sec; for sequential placement, typical acceleration is 30–40 m/sec.

Placement force is also much more accurate with a controlled pick-and-place operation. Connectors and large components (25 × 25 mm) can need up to 40 N for reliable placement, while small components (1 × 1 mm) generally need down to 1.5 N. (An extra risk with ultrasmall components below 0603 is component cracking because of excessive impact force.) Therefore, placement forces need to be precise and adjustable.

(click to enlarge) Figure 1. Tests examined four stencil sizes from the smallest (A) to the largest (D) at interspacings down to 30μm.

Testing 01005 Placement Reliability

Several large-scale tests of 01005 placements have been performed to determine whether these components are suitable for integration into devices.

Perhaps the most important determinations for manufacturers are the appropriate solder footprint and the minimum layout interspacings for reliable placement. These options were tested with a DEK Infinity stencil printer, an Assembléon AX-301 pick-and-place machine, and a Vitronics Soltec Model 7038 solder reflow oven.

Four stencil sizes ranging from A to D (smallest to largest) were tested (see Figure 1). Interspacings ranged from an ultraconservative 200 µm down to 30 µm. We now recommend Aperture A at interspacings of 30, 40, 50, and 60 µm, and Aperture B at interspacings of 70, 80, 90, and 100 µm (see Figure 2).

(click to enlarge) Figure 2. As a result of the tests, a minimum interspacing of 60 μm is recommended.

Slightly larger components such as 0201 types show considerable resilience in full production, largely because of their tendency to self-align when placed on the solder footprint. These tests indicated that they are better at self-aligning than 01005 components.

Choosing between SMD and SP Fiducials

When placing components on solder-mask-defined pads, solder-mask-defined fiducials normally ensure that components are properly aligned with pads. However, for very small components (0201 or 01005 sizes), it could be better to align the components with the solder paste, because a small solder-paste offset could prevent the component from touching the paste, which could result in a defect.

This study found that when placing these tiny components, and when there is a risk of a large paste offset (more than 50 µm), solder-paste fiducials are advisable to prevent tombstoning, a condition in which one end of a component lifts off of the board. When the paste offset is small (50 µm or less), either fiducial type will work.

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The final major production stage is reflow soldering. Here, the study found process errors to be either solder-paste-printing related or interspacing related. Solder-paste-related errors are split into tombstoning and opens (open circuits). Interspacing-related errors are divided into contacts (possible contact problems—either short or no contact—between the component and the board) and short circuits.

The 01005: Ready for Production

Overall, the study found solder-paste printing to be perhaps the most critical stage, with thin, vulnerable stencils needing frequent cleaning to prevent the stencil aperture from being blocked. If paste offsets are too large (>50 µm), 01005 components cannot self-align, which makes production vulnerable to tombstoning.

Taking these factors into account, the study found that placement of 01005-size components is already a robust process. For larger interspacings (90, 100, and 200 µm), we recommend an 80-µm stencil thickness, whereas for 60, 70, and 80 µm, we recommend a stencil thickness of 50 µm, which will allow 0402, 0201, and 01005 components to be combined.

Conclusion

Exhaustive tests were performed to determine whether 01005 components are ready for integration into devices. With a placement accuracy of within 50 µm at 3-sigma, these tests showed the minimum interspacing for 01005 components to be 60 µm. The soldering processes gave virtually 100% good joints for both stencil sizes A and B. These results translate into a placement quality of far below 20 defects per million. Parallel placement pick-and-place machines have a 40-µm accuracy at 3-sigma, which improves placement quality down to nearly 10 defects per million, which easily classifies as production ready. Medical electronics manufacturers can use these tiny 01005 components to significantly increase board densities.

Reference

1. iNEMI Roadmap (Herndon, VA: International Electronics Manufacturing Initiative, 2007); available from Internet: www.nemi.org/cms/roadmapping/roadmaporder.html.

Sjef van Gastel is manager of advanced development for Assembléon Netherlands B.V. (Veldhoven, The Netherlands).