High-density Package Design
MEETING THE DEMAND
BY JAVED SANDHU AND ED VARGA
The world’s smallest book measures 0.9 × 0.9 mm and contains 30 pages, according to the Guinness Book of World Records. The type is so small that it cannot be read by the naked eye. Although impractical and merely a curiosity, it is analogous to the real issues placed before today’s package designers with the growing demand for high-density packages.
Driven by the shrinking real estate in end-user applications, package designers are asked to push the envelope in high-density designs, particularly the very-fine-pitch BGA (ball pitch of 0.5 mm). Although the handheld electronics industry continues to demand more functions in increasingly smaller spaces, cost continues to be the operating variable in developing solutions to high-density packages. Customers will search out semiconductor subcontractors that are set up to handle the resulting issues associated with high density, including the areas of substrate technology, assembly and manufacturing, electrical performance, design cycle times, and ultimately the experience and cross-discipline function of designers.
Higher density demands specific substrate technologies, which in turn, pushes the substrate industry to explore new solutions. To fill today’s demands, substrate vendors must have these capabilities, so subcontractors are re-evaluating their relationships with vendors to connect with those who have the experience and technology.
Figure 1. Example of high-density finger pitch, trace pitch, and via hole spacing.
Finer line spaces (less than 80-µm trace pitch), smaller vias, and thinner copper require a different substrate manufacturing process, a “semi-additive,” as opposed to a “subtractive” process (Figure 1). Substrate vendors also must use a thinner substrate material (core) to maintain an optimized drill aspect ratio. This reduces the overall substrate thickness, which conceivably adds handling issues for the subcontractor.
Assembly & Manufacturing
A good designer will design for manufacturability, meaning that the designer knows what the assembly process can handle in terms of wire bonding, over-mold, solder ball attachment, etc. In the case of high density, wire bonding will be extremely tight. Thinner wires are required (thinner than 1.0 mil; between 0.7 and 0.8 mil), which is driven by the bond finger pitch on the substrate and not the die pad pitch on the IC. Both the designer and the assembly engineers must consider the issue of wire sweep when using tighter, thinner wires. The over-molding process and material used must decrease wire sweep issues. Therefore, materials with low viscosity must be used.
Solder ball attachment becomes another issue for consideration. As the number of balls increase in a single package, the number of balls on one strip (for example, saw singulated fpBGAs) increases exponentially. It is possible that there will not be enough vacuum to pick up the required number of balls for placement. The designer must be able to calculate the ball count on one strip to know exactly how many units to put it. Other possible solutions include doing it in steps, or purchasing new equipment that can handle increased solder ball attachment. Again, the creative and problem-solving abilities of the designer are crucial to maximize the process without adding cost-prohibitive measures.
Electrical considerations are crucial to the reliability of the package. As previously mentioned, higher density results in thinner wires, closer together. In the case of high-density packages, the cross-section area of the conductors becomes smaller as well, and this could also add significantly to the overall inductance of the signal. Designers must be aware of these electrical impacts during the design process.
Figure 2. Design optimization of fpBGA using electrical modeling during design process.
The designers can help reduce parasitics by reducing the bond wire lengths, and on the substrate by reducing trace lengths on critical signals. It is critical that the IC designers give electrical requirements at the start of the process so the design can be optimized from the beginning. Also critical, is the presence of electrical modeling during the design process. Figure 2 shows an example of how electrical modeling was able to assist in design optimization to meet electrical requirements.
Design Cycle Time
With the increased need for greater performance in smaller spaces, comes the increasing time it takes for designers to tackle such complex designs. Other factors play into the cycle time, including the increase in communication between the designer, customer, vendors, and assembly. A collaborative environment is crucial to these complex designs, which provides a net benefit. A possible downside, however, is the time needed for such communication. For customers, developing a partnership with a subcontractor with an experienced and locally based design team is critical.
Experience & Function of Today’s Designers
Concurrent design interaction is the key to success in the world of customer-driven package development. The designer becomes the “thru-way” that the various contact points are connected. As the world of packages shrink, so too does the world of the designer. Designers will have to become more deeply involved in every aspect of the process, from customer contact to final assembly.
Figure 3. Designers can be the ‘hub’ in the process.
The location of design centers is a vital consideration. Subcontractors will have design centers near the customer base, as well as near the assembly process. The obvious benefits to local interaction between customers and designers are added to by the portability of today’s designers. With advancements in design software and high-speed hardware, working through design issues at a customers’ site is now common.
Figure 4. Example of high-density design.
Today’s designers must also understand the manufacturing process, what works and what doesn’t. Competent knowledge of substrate materials and technology is not an option. Often, the designer is that first warning between the customer and assembly, thus saving time and money to all involved.
Experienced, well-rounded designers are in demand today. In the era of outsourcing, design is still a vital part of an in-house process, using highly qualified individuals is key.
We have discussed the many challenges that higher performance in smaller packages pose for design and assembly. These challenges push the industry into discovering new technologies and processes, and also require the experience and capabilities of subcontractors. Offering state-of-the-art manufacturing to meet the challenges of high-density assembly is key to the ever-present concern of cost management. Last, but not least, using a design team and offering software support, as well as the ability to incorporate electronic simulation and provide tight match pair impedance, is pivotal.
JAVED SANDHU, senior manager of Design Services, and ED VARGA, communications coordinator, may be contacted at ASAT Inc., 6701 Koll Center Parkway, Suite 200, Pleasanton, CA 94566; (925) 398-0400; e-mail: firstname.lastname@example.org, email@example.com.