Fine-Pitch Direct Die Attach With Reduced Cost & Higher Throughput
- 3 days ago
- 9 min read
Andrew Stemmermann & John Yundt SunRay Scientific Inc.
Wall Township, NJ USA andrew@sunrayscientific.com johny@sunrayscientific.com
SunRay Scientific Inc of Wall Township, NJ, USA continues to develop new and innovative technologies for electronic component assembly with their portfolio of novel ZTACH® ACE anisotropic conductive adhesives, printed conductive inks and dielectric and encapsulation materials.
Introduction
Today’s electronics manufacturers are confronted with a growing need for higher performance, lightweight, cost competitive, printed flexible hybrid electronics. This often requires the bonding of fine pitch components in high density and volumes. Factors for success include the capability of the conductive bonding technology to manage the pitch, bond strength requirements, cost constraints, and manufacturing throughput needs. Most available technologies employed in today’s FHE environment have limitations that can either hamper one or more of these important factors.
Traditional component bonding technologies can struggle with the requirements for printed FHE applications. Solder, while very low cost and highly conductive, has limitations due to the required processing temperatures, brittle properties, and low bond strength. As a result, it will always require secondary encapsulation of the bonded component to ensure structural integrity. Silver-filled conductive adhesives (ECA) require patterning, which drastically limit component size and pitch, are heavily filled with silver powder or flake, which is detrimental to cost, and have very poor structural bond strength. As a result, they will also require a secondary process like underfill and/or edge encapsulant to provide additional mechanical strength and stress reduction. The result is a complex assembly process flow. When moving to high density or high volumes of fine pitch components, which require micro-dot dispensing of the ECA to properly pattern to limit potential shorting, results in very slow processing times for the material to be deposited, which dramatically reduces manufacturing throughput. Finally, Anisotropic Conductive Adhesive (ACA) or Anisotropic Conductive Film (ACF) can manage fine pitch very well, but typically involves the use of thermocompression bonding, an additional process step that could also be damaging to thin silicon chips or printed conductive traces. The equipment to do this can be very costly. The number of thermal compression heads on typical equipment will often limit the number of interconnections that can be bonded to 2-6 components per bond step, which can also drastically limit production throughput.
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ZTACH® ACE, a patented anisotropic conductive adhesive system capable of bonding fine pitch components down to 100 microns at high density, can typically be over 1,000 components per sheet at one time, using a pressure-less, low temperature process which eliminates most of these constraints. Using the ZMAG® Magnetic Pallet to create magnetically aligned columns of unique ferromagnetic and highly conductive particles during the cure process, ZTACH® ACE creates a low contact resistance (.007 - .020 Ωcm), fine-pitch, anisotropic electrical interconnection, anywhere from 5-10x the bond strength of either solder or ECA. An illustration of this novel technology approach is shown in Figure 1 below:
This technology utilizes a platform approach which gives SunRay the ability to utilize the most appropriate adhesive resin system for the requirements of the application. The patented, noble metal-plated, ferromagnetic particles are dispersed into the adhesive resin system; this includes a 2-part catalyzed epoxy for the thermally cured formulation, and a 1-part modified acrylate for the UV cured formula. Once the printed circuits are ready for component placement, either manually, or in an SMT process, ZTACH® ACE is deposited onto the component landing pads using an automated syringe dispensing system or printed with a conventional stainless steel open aperture stencil with no patterning for anode and cathode separation required. Next, as in any conventional SMT process, components are placed on top of ZTACH in a pick and place process and then placed directly onto the ZMAG® Magnetic Pallet where the ferromagnetic particles form z-axis magnetically aligned columns within seconds. The particles are held into fine pitch; tightly formed columns as the resin system is cured and fixed in place during the die-to-substrate cure process without any pressure applied. The formation of the columns during the curing process is illustrated in Figure 2. Because this technology uses such fine, precise particles to create these z-axis columns to form the conductive pathways, the formulations require dramatically less conductive particles (typically between 15-25%) to achieve low contact resistance interconnection. This results in a far higher percentage of adhesive resin, thus drastically higher bond strength than most conventionally used technologies in use today. This technology simplifies the assembly process to a single adhesive application, which provides both electrical interconnection and mechanical reinforcement. No additional underfill material is needed. Fine patterning is not required as the entire area of the component target location is deposited with epoxy. The device alignment process is more forgiving relative to solder ball-to-solder pad alignment. Z-axis columns align after component placement; magnetic pallet placement and cure are achieved. Thermal or UV curing methods complete the component attachment without any thermocompression (cure method is epoxy formulation dependent). Thermal curing occurs within the 80°C to 160°C temperature range.
The remainder of this article will outline recent project-based findings for high density LED and related component placement for backlighting on flexible films.
Case Study 1
Printed Electronics contract manufacturers want to produce more complex, Flexible Hybrid Electronics devices, which need to meet both high-volume production speeds, while achieving strict mechanical and electrical requirements. The initial capabilities of ZTACH® ACE to successfully run product in a high-speed contract manufacturing SMT process for a wide range of printed FHE applications were initially validated on the production lines at a major global printed electronics contract manufacturer. Taking an existing product being run with a low-temperature solder process to bond 8 x 0402 LEDs onto a multi-layer silver ink printed flex circuit, the SunRay High-Throughput thermally cured formulation of ZTACH® ACE was put into the validation process. The incumbent low-temperature solder process was run at production speeds equating to 60 sheets per hour, 4 parts up per sheet, for a total of 32 LEDs per sheet. This required a reflow oven profile of 160°C, which equated to a tunnel transition time of 7 minutes. Once bonded, the parts would then be encapsulated with a standard UV cured glob-top encapsulant and put through a series of electrical and mechanical testing. Once the placement, stencil, and reflow processes were optimized for ZTACH® ACE, the final reflow profile was determined to be 135°C, for the same dwell time and production rate of 60 sheets per hour: a significant reduction in processing temperature required.
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Testing included component voltage sweep to confirm electrical interconnections are meeting specifications: 100% of the LED’s lit with =/< 12% in voltage shift. An initial mechanical test was then conducted which included rolling a completed part under power, over a ¼ inch steel mandrel with a 1 kg weight attached at one end for 10 cycles. The specification was 100% of all LEDs remained illuminated throughout and after mandrel testing, with zero interruption to the fully illuminated state of all LEDs.
Next, a comprehensive set of environmental and mechanical testing was conducted by a third-party testing lab on all parts built. These tests included salt fog (MIL STD-202F, 101D), heat and humidity (MIL-STD-202F, 103B), Thermal shock (MIL-STD-202F, 107G, and thermal aging (MIL-STD-202F, 108A), Mechanical vibration (MIL-STD-202F, 201A) and mechanical shock (MIL STD-202F, 107G). For all tests, the specification was zero failures of the bonded LEDs on each part.
ZTACH® ACE passed all testing criteria and was qualified for manufacturing production at scale. But what was most compelling about these results where what ZTACH® ACE showed it could accomplish outside the specification. The existing requirements for this CM’s current production process called for all components to be encapsulated, because they reported unencapsulated parts built with their current process would typically FAIL nearly all the above testing anywhere from 50% to 100%. The decision was made to produce half the total parts built with ZTACH® ACE without the required encapsulation. Of those parts, 94% passed ¼ inch mandrel bend testing, 100% passed 85/85 testing, 100% passed mechanical shock, and 80% passed mechanical vibration, thermal aging, and salt-fog testing. This would be virtually impossible with unencapsulated solder or ECA as a bonding method.
Case Study 2
As the automobile industry increasingly shifts to hybrid and fully electrified vehicles, the need to conserve space and weight becomes more critical for manufacturers and drivers. According to a recent article in Semiconductor Engineering titled “Shedding Pounds in Automotive Electronics”,the traditional wire harnessing takes up a lot of space and weight even in compact cars. “The wiring harness is one of three heaviest subsystems in many vehicles—as much as 150 lbs. in highly contented vehicles—and it’s very typical for the average vehicle to have 100–120 lbs. of wire harness in the vehicle. These vehicles weigh on average around 3,500 lbs,” said Mentor’s Burcicki. “Today’s luxury cars contain some 1,500–2000 copper wires—totaling over 1 mile in length. To put that into perspective, in 1948, the average family car contained only about 55 wires, amounting to total length of 150 feet.” [1]
With the growth in e-vehicle adoption and the weight those battery systems can reach, the need to trim weight to maintain or increase vehicle efficiency becomes even more focused. Like the traditional commercial aerospace concern…every ounce and inch cost money. The more automotive designers and manufacturers can leverage lighter weight, smaller, and lower current draw printed electronics into their vehicles, the more cost efficient they can be without sacrificing performance and design. Recent demand for printed flexible hybrid electronics in areas like Automotive interiors for Human Machine Interface (HMI) like capacitive touch controls, flexible, light weight heating, or flexible, light weight ambient lighting, is creating new and exciting opportunities for the printed electronics industry. Notable advances have been made with printed and in-mold electronics to be adapted into the vehicle compartment for these applications. However, the ability to make high reliability component attachments in a manner that can meet manufacturing throughput requirements remains an obstacle.
Solder is cost effective and very conductive. It’s heavy, not environmentally friendly, requires higher temps to process which limits substrate options, and lacks structural integrity. Traditional silver filled conductive epoxies (ECAs) require precise patterning that cannot support placement of smaller fine-pitch components, or requires very precise, micro-dot dispensing that requires exceptionally long dispensing times, and the cure times can be too long for high throughput mfg.
Additionally, due to the high percentage of conductive particles needed in ECAs, they also tend to be very low bond strength materials which require secondary encapsulation processes to ensure the structural integrity of bond. There is a cost issue with ECAs. The high loading percentage of silver used in most ECAs can lead to costs that exceed $2 or $3 per gram.
This was prior to silver going from a 10-year average of ~$25 per ounce, to its current price of $67 per ounce, and the record high of $118 hit earlier in January of this year. The need for a more cost stable, lighter weight, higher bond strength adhesive system that can easily manage finer pitch, smaller components and be processed at the high-volume throughput rates is required by the automotive world.
ZTACH® ACE, a pressure-less, anisotropic conductive adhesive system, can solve each of these issues. With a bond strength ranging from 5-10x that of solder or ECA, ZTACH® ACE is an exceptionally reliable electrical interconnect solution. It does not require patterning for small components with fine pitch, so standard screen depositing with a stainless-steel stencil makes very high-density, high-volume deposition in a matter of seconds a reality. SunRay has repeatedly demonstrated that this technology can handle over 1000 component placements per sheet in a typically configured production SMT process. The high throughput formulation can manage production rates of 60 sheets per hour at low process parameters of 135°C for 7 minutes. The conductive particle make up of ZTACH is in the range of 25-35% and of that, only 5-7% is silver, the cost of the material is not dependent on the fluctuation of silver cost. The density is so low that even when underfilling the entire component, the weight of ZTACH is dramatically less. This case study conducted by SunRay compared a typical ECA used for bonding a total of 100 Automotive LEDs (8 electrical pads each) - 100 × 0603 capacitors (2 terminations each) - Total interconnect locations: ~1,000 electrical joints onto a silver ink printed circuit part with a configuration of 400 mm x 500 mm for flexible ambient LED lighting. This resulted in considerable materials cost savings over traditional ECA materials, with lower contact resistance, greater throughput, and higher component bond integrity as illustrated in Table 2.
Conclusions
The ability to deposit a novel anisotropic conductive adhesive via high-speed stencil printing for the bonding of fine pitch components in high density for increasingly complex FHE applications manufactured at high throughput is more feasible than ever before. ZTACH® ACE helps to enable the easier, low-cost adoption of this approach for the next generation of printed electronics. SunRay Scientific can provide technical and application support for new projects or applications with their experienced Engineering Team and full-scale SMT line capabilities. SunRay can also sell ZTACH® ACE Developer Kits that put this technology and the in-person training to deploy into your product development in your facility. These are being utilized now, enabling their customers to design and validate the processes and design rules allowing them to integrate ZTACH® ACE into their products for improvements in:
Finer pitch interconnections that enable more functionality in smaller space
Higher throughput for complex assemblies
Higher density component placement
Increased environmental and mechanical durability of electrical interconnections
Ability to make complex bare die attach a reality for printed FHE
New technology integration into your existing portfolio of capabilities
[1] Semiconductor Engineering, March 12, 2019, Shedding Pounds in Automotive Electronics – SE Staff
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