Despite considerable skepticism when 0201 devices were first introduced to high density board assembly a few years ago, they are now common. This time, it is the turn of even smaller 01005 components. In number, by far most components are chip resistors and capacitors, and so equipment manufacturers can use these 01005 types to significantly increase board densities once more.
At just 0.4 mm long by 0.2 mm wide, 01005 chip components place new demands on assembly and interconnection processes. Work is needed on the stencil printing process, the placement process itself, and soldering conditions since the components cannot be used in mass production until defect per million levels are close to single digit figures. Assembléon has performed exhaustive tests to find whether 01005 components are ready yet. The conclusion is that – as long as some basic rules are followed – they are.
INCREASING COMPONENT DENSITY
The dominant passive sizes from the 1990s have changed. The 1206 (3.2 x 1.6 mm) and 0805 (2.0 x 1.25 mm) chips no longer make up the majority. These formats have generally migrated to 0603 (1.6 x 0.8 mm), 0402 (1.0 x 0.5 mm) and 0201 (0.6 x 0.3 mm) respectively. We have therefore seen a considerable reduction in occupied board area.
It is portable equipment that has really driven component density, now already at over 50 components/cm2 and predicted to rise to near 80 over the next 10 years. 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 now, they are expected to claim an increasing market share, with the ITRS roadmap predicting that 01005 will remain the minimum size until at least 2012.
Interspacings also determine board density
The minimum interspacings of 0402 components have to be at least 150µm, so even these relatively small components are limited to around 100 components/cm2. Smaller 0201 types allow tighter 100µm interspacings to triple the density to 300/cm2. And, with 01005 components on 50µm interspacings, the maximum density à could double again to 600/cm2. Such densities are needed for integrating passive chip components into semiconductor “modules” or SiPs. These semiconductor modules can be found mainly in handheld applications (mobile phone, MP3 players etc.) and offer even more functions at reduced dimensions. Component placement at small interspacing, however, has a much more critical process window.
Solder stencils determined by 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 will 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” reflow process to reduce flux evaporation, while large components need a “slow” reflow process since more time is needed for heating up to reflow temperature.
All this suggests that 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” PCBs, with the smallest 01005 components migrating to modules. This is a good trend for manufacturers since, rather than upgrading the whole line, the ultra-high precision assembly is needed only for the modules.
Migrating from microchips to modules
The SiP approach brings equipment manufacturers several benefits. It starts by keeping signal lines short and local for best frequency response. That makes for more flexibility in mobile phone (for example) design, by increasi ng 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 off-the-shelf items. 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, it is expected that In 3 to 5 years this ratio will shift to 40/60.
PLACING 01005 COMPONENTS
The 01005 components must be placed on SiPs reliably, placement after placement. This also makes phenomenal demands on the Pick & Place machines, with accuracy and reliability of 01005 component placements being perhaps the two ultimate tests for a machine.
In full production runs, Assembléon’s A-Series of Pick & Place machines have single digit dpm (defects per million) figures at an accuracy of 40µm – currently the industry benchmark. Getting figures this low means performing the same action time after time with minimum possible variation, so the placement process itself must be ultra repeatable. Rather than placing components sequentially using large overhead gantry robots, as is the more usual Pick & Place operation, the A-Series has multiple modules placing components in parallel.
Parallel placement with multiple heads improves process control
Parallel placement means much smaller accelerations and decelerations. It gives the individual xy robots much more time to settle, and also to check the placement process. During the pick cycle, the A-Series 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.
Placement force is also much more accurate with a controlled Pick & Place operation. Connectors and larger components can need up to 40N for reliable placement, while small components generally need down to 1.5N (an extra risk with ultra-small components below 0603 is component cracking because of excessive impact force). So placement forces need to be precise, and adjustable.
Testing 01005 placement reliability
Assembléon has performed several large-scale tests of 01005 placements to decide on the manufacturability of the components. Perhaps the most important questions for manufacturers are which solder footprint to use, and the minimum layout interspacings for reliable placements. These were tested with a DEK Infinity stencil printer, an Assembléon AX-301 Pick & Place machine, and a Vitronics Soltec Model 7038 solder reflow oven.
Four stencil sizes from smallest A to largest D (Figure 1) were tested. Interspacings ranged from an ultra conservative 200µm right down to 30µm. We now recommend Aperture A at interspacings of 30, 40, 50 and 60µm, and Aperture B at interspacings of 70, 80, 90 and 100µm (Figure 2).
Slightly larger components like 0201 types show considerable resilience in full production, largely because of their tendency to self-align when placed on the solder footprint. Our tests do suggest 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 it could be better to align the components with the solder paste, as a small solder paste offset could result in the component not touching the paste, and hence a defect.
We found that, when placing tiny components, and when there is a risk of a large paste offset (larger than 50µm), solder paste fiducials are advisable to prevent “tombstoning” (components where one end lifts off). When the paste offset is small (50µm or less), either fiducial type will work.
And the final major production stage is reflow soldering. Here, we found process errors to be either solder paste printing related or interspacing related. Solder paste related errors divide into tombstoning and opens. Interspacing related errors divide into contacts and shorts.
01005 proves to be ready for production at 60µm interspacing
As a whole, we found solder paste printing to be perhaps the most critical stage, with thin vulnerable stencils needing frequent cleaning to prevent the stencil aperture blocking. If paste offsets are too large, 01005 components cannot self-align which makes production vulnerable to tombstoning. If these factors are taken into account, we have found placement of 01005 size components to already be a robust process. For larger interspacings (90, 100 and 200µm) we recommend an 80µm stencil thickness, while for 60, 70 and 80µm we recommend a stencil thickness of 50µm – which will allow 0402, 0201 and 01005 components to be combined. With a placement accuracy of within 50µm at 3-sigma, our tests showed the minimum interspacing for 01005 components to be 60µm. The figures show that the soldering processes gave virtually 100% good joints for both stencil sizes A and B. This translates into a placement quality of far below 20 defects per million. Assembléon’s A Series actually has a 40µm accuracy at 3-sigma (even 20µm for the AX-201), which improves placement quality even further – to nearly 10 defects per million, which easily classes as “production ready”.