-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueIPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
The Cost of Rework
In this issue, we investigate rework's current state of the art. What are the root causes and how are they resolved? What is the financial impact of rework, and is it possible to eliminate it entirely without sacrificing your yields?
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Non-Contact Streaming Technology Enhances Dispensing
December 31, 1969 |Estimated reading time: 6 minutes
The dispensing industry within electronics manufacturing represents a diverse marketplace; different materials can be applied in different ways. One high-growth area in this market is underfill - driven by explosive demand for handheld devices.
BY Hugh Read, Speedline Technologies
The handheld-device segment comprises popular consumer goods, such as cell phones, MP3 players, GPS navigators, PDAs, portable games, and ultra-mobile PCs. One non-contact dispense technology, streaming, recently has been introduced to address the incumbent needs associated with underfill. While streaming has attributes similar to other dispensing technologies, it offers some advantages, particularly with underfill. This technique has been proven under extensive testing and stands to be a factor in improving throughput and yield in underfill applications.
Consumer demand and component miniaturization continue to shape this emerging segment; the technology and functionality of individual handheld devices are converging into a single device: the smartphone. Underfill is an important step in manufacturing handheld products, so the industry welcomes improved speed and accuracy in this process.
Design Drivers
Prior to development, interviews were conducted with current and potential users of dispense systems to determine their needs. Speed enhancement, while maintaining high yields, was the most common response. This is highlighted by the 2007 iNEMI Technology Roadmap (Table 1). The underfill section of this report was compiled with feedback from both PCB and capital equipment manufacturers. Four key metrics relating to underfill dispensing are identified, as are current and future production capabilities from an equipment perspective. A shortfall in 2007 is already evident from a throughput standpoint, and that likely will increase. Equipment manufacturers must focus platform and dispense technology development on this requirement, while simultaneously improving capability in control of volume, keep-out zones, and process management.
Streaming Pump Technology
Streaming is accomplished using a patent-pending pump technology. What differentiates streaming from other non-contact technologies is that striking a piston to a seat or nozzle is eliminated. This increases longevity of the related parts, reducing wear and resulting in quieter operation. Carbide and sapphire materials are used in the pump design. Underfill can have a filler powder blended into the material; commonly this is silica, but other materials such as alumina, carbon, or aluminum nitride also are used. The primary function of the filler is to reduce the coefficient of thermal expansion (CTE) mismatch between substrate and device, and reduce stress on solder joints. As the filler is abrasive, it can cause most metal contact surfaces to wear. Carbide, however, resists abrasion from silica, so it is used for the piston and barrel assembly. The piston’s action expels the material through a nozzle, so this must also be wear-resistant. The nozzle orifice may be one of several small diameters (typically 200 μm), and would be cost-prohibitive if made of carbide. Instead, an industrial sapphire orifice is bonded and roll-swaged into a steel holder (Figure 1). Although sapphire can be expensive, nozzles are comparably priced to dispense needles.
Figure 1. An industrial sapphire orifice is bonded and roll-swaged into a steel holder.
In operation, each pump cycle displaces a column of material rather than a single sphere. The unit cycles at a frequency that creates a “stream,” which can deliver increased flow rates up to 70 μl/sec. A linear motor/encoder assembly drives the carbide-displacement piston, controlling its upstroke to regulate the charge level. The piston is then accelerated to a high speed on the down stroke, its bottom position governed by an external collar. Combining advantages associated with both positive displacement and jetting technologies allows streaming to deliver precise volumes from a preferred distance above the substrate.
Pump unit design enables easy removal and cleaning of the fluidic module that contains all wetted parts. Only o-rings require periodic replacement, so downtime and associated costs are minimal.
Results
There are two major dispense methods in the market - needle- and non-contact-based technologies. The latter has been documented to offer throughput advantages over needle-based processes. This advantage is most apparent in applications involving dispensing materials into tight gaps, or those with restrictions on the wet-out area. When addressed with needle-based dispensing equipment, a small I/D (<0.010") is mandated, which will restrict flow and, consequently, could decrease units per hour (UPH) throughput.
In some cases, dispense rates of application vary by the speed of the capillary action after the material has been applied. For example, the material or process may have slow capillary action due to the underfill viscosity and/or a small stand-off gap underneath the component. Applying the underfill too quickly may cause material to build up and wet to the top or side, contaminating adjacent components. In this case, the pump - needle- or non-contact-based - would be tuned to accommodate the capillary action.
For many underfill applications, however, non-contact technology does not necessarily dispense material at higher flow rates than a needle. It is the process steps in and around the dispense step to which we attribute generally increased productivity. In these applications, throughput improvements come from two areas:
- Z-travel of the dispense head is virtually eliminated as non-contact dispensing expels the material from a nozzle 2-3 mm above the substrate. Needles, on the other hand, must be approximately 0.25 mm above the surface to ensure wetting. Z-motion is required to move down to the dispense position, and then move upward after dispensing to “snap off” the material from the tip. The needle also is raised to physically clear components when moving to the next location.
- Height sensing is used to reference the distance at the dispense area from substrate to dispense nozzle/needle. This measurement is taken either by mechanical probe or laser. With needle-based technology, height above the substrate is a critical parameter in maintaining a reliable process. Typically, height sensing is performed once per component (flip chip, chip-scale package [CSP], or BGA), or at least once per circuit. Dispense height, however, is less critical for non-contact processes, because the technology is more forgiving, requiring only a single height measurement per panel.
To better understand and quantify UPH improvements, application trials were run using non-contact and needle-based methods. The intent was to highlight the difference between technologies when operating at identical flow rates. Needle-based trials were conducted using a positive displacement, multi-piston pump, and non-contact tests used the streaming pump. In both cases, the pumps were installed on a commercially available, standard dispensing system.
A time comparison study was run on three different applications: a cell phone, a high-end MP3 device, and a GPS navigator. Both pumps were set to run with identical 45-μl/sec. material flow rates. With the needle-based trials, all programs used the same lift height between dispense commands, thus accurately identifying the differences in Z operation and height sensing. A typical cell phone application was emulated using underfill dispensing through a hole in the RF shield. The MP3 device dispense pattern used a combination of single lines, “L” passes, and dots around components. A combination of through-shield and single-line dispenses was used on the GPS navigator. For all three devices, each component-dispense operation was done in a single pass (Table 2).
ConclusionResults from the trials show that each product assembly process benefits from the use of streaming. The more dispense positions for an application, the greater the difference between the two technologies. Additional improvements are realized as less height sensing is required. On average, streaming showed a 25% time reduction over the needle-based method relative to the Z-height and sensing operations. This figure can be considered fairly representative for these types of applications, and should correspond to savings achievable in typical production environments. Speed advantages, in addition to those outlined, may also be obtained. For example, needle dispensing may require the use of parameters proven unnecessary with the streaming approach. “On-delay” and “dwell” both pause the needle at either the beginning or end of a dispense line or dot, adding to the cycle time.
The demand for higher-throughput dispense solutions is ever-increasing. Continued focus and development in underfill applications is fundamental to keep pace with the explosive growth of handheld consumer devices.
Acknowledgements
iNEMI 2007 Technology Roadmap
Hugh Read, product manager, dispensers, Speedline Technologies, may be contacted at (508) 541-4836, HRead@speedlinetech.com