Consider these eight issues where the packaging team should be closely involved with the circuit design team.
BY JOHN T. MACKAY, Semi-Pac, Inc., Sunnyvale, CA
Today’s integrated circuit designs are driven by size, performance, cost, reliability, and time- to-market. In order to optimize these design drivers, the requirements of the entire system should be considered at the beginning of the design cycle—from the end system product down to the chips and their packages. Failure to include packaging in this holistic view can result in missing market windows or getting to market with a product that is more costly and problematic to build than an optimized product.
As a starting consideration, chip packaging strategies should be developed prior to chip design completion. System timing budgets, power management, and thermal behavior can be defined at the beginning of the design cycle, eliminating the sometimes impossible constraints that are given to the package engineering team at the end of the design. In many instances chip designs end up being unnecessarily difficult to manufacture, have higher than necessary assembly costs and have reduced manufacturing yields because the chip design team used minimum design rules when looser rules could have been used.
Examples of these are using minimum pad-to-pad spacing when the pads could have been spread out or using unnecessary minimum metal to pad clearance (FIGURE 1). These hard taught lessons are well understood by the large chip manufacturers, yet often resurface with newer companies and design teams that have not experienced these lessons. Using design rule minimums puts unnecessary pressure on the manufacturing process resulting in lower overall manufacturing yields.
Semiconductor packaging has often been seen as a necessary evil, with most chip designers relying on existing packages rather than package customization for optimal performance. Wafer level and chipscale packaging methods have further perpetuated the belief that the package is less important and can be eliminated, saving cost and improving performance. The real fact is that the semiconductor package provides six essential functions: power in, heat out, signal I/O, environmental protection, fan-out/compatibility to surface mounting (SMD), and managing reliability. These functions do not disappear with the implementation of chipscale packaging, they only transfer over to the printed circuit board (PCB) designer. Passing the buck does not solve the problem since the PCB designers and their tools are not usually expected to provide optimal consideration to the essential semiconductor die requirements.
Packaging technology has considerably evolved over the past 40 years. The evolution has kept pace with Moore’s Law increasing density while at the same time reducing cost and size. Hermetic pin grid arrays (PGAs) and side-brazed packages have mostly been replaced by the lead-frame-based plastic quad flat packs (QFP). Following those developments, laminate based ball grid arrays (BGA), quad flat pack no leads (QFN), chip scale and flip-chip direct attach became the dominate choice for packages.
The next generation of packages will employ through-silicon vias to allow 3D packaging with chip-on-chip or chip-on-interposer stacking. Such approaches promise to solve many of the packaging problems and usher in a new era. The reality is that each package type has its benefits and drawbacks and no package type ever seems to be completely extinct. The designer needs to have an in-depth understand of all of the packaging options to determine how each die design might benefit or suffer drawbacks from the use of any particular package type. If the designer does not have this expertise, it is wise to call in a packaging team that possesses this expertise.
The push to put more and more electronics into a smaller space can inadvertently lead to unnec- essary packaging complications. The ever increasing push to produce thinner packages is a compromise against reliability and manufacturability. Putting unpackaged die on the board definitely saves space and can produce thinner assemblies such as smart card applications. This chip-on-board (COB) approach often has problems since the die are difficult to bond because of their tight proximity to other components or have unnecessarily long bond wires or wires at acute angles that can cause shorts as PCB designers attempt to accommodate both board manufacturing line and space realities with wire bond requirements.
Additionally, the use of minimum PCB design rules can complicate the assembly process since the PCB etch-process variations must be accommodated. Picking the right PCB manufacturer is important too as laminate substrate manufacturers and standard PCB shops are most often seen as equals by many users. Often, designers will use material selections and metal systems that were designed for surface mounting but turn out to be difficult to wire bond. Picking a supplier that makes the right metallization tradeoffs and process disciplines is important in order to maximize manufacturing yields
Power distribution, including decoupling capaci- tance and copper ground and power planes have been mostly a job for the PCB designer. This is a wonder to most users as to why decoupling is rarely embedded into the package as a complete unit. Cost or package size limitations are typically the reasons cited as to why this isn’t done. The reality is that semiconductor component suppliers usually don’t know the system requirements, power fluctuation tolerance and switching noise mitigation in any particular installation. Therefore power management is left to the system designer at the board level.
Miniaturization results in less volume and heat spreading to dissipate heat. Often, there is no room or project funds available for heat sinks. Managing junction temperature has always been the job of the packaging engineer who must balance operating and ambient temperatures and packaging heat flow.
Once again, it is important to develop a thermal strategy early in the design cycle that includes die specifics, die attachment material specification, heat spreading die attachment pad, thermal balls on BGA and direct thermal pad attachment during surface mount.
Managing signal integrity has always been the primary concern of the packaging engineer. Minimizing parasitics, crosstalk, impedance mismatch, transmission line effects and signal atten- uation are all challenges that must be addressed. The package must handle the input/output signal requirements at the desired operating frequencies without a significant decrease in signal integrity. All packages have signal characteristics specific to the materials and package designs.
There are a number of factors that impact perfor- mance including: on-chip drivers, impedance matching, crosstalk, power supply shielding, noise and PCB materials to name a few. The performance goals must be defined at the beginning of the design cycle and tradeoffs made throughout the design process.
The designer must also be aware that packaging choices have an impact on protecting the die from environmental contamination and/or damage. Next- generation chip-scale packaging (CSP) and flip chip technologies can expose the die to contami- nation. While the fab, packaging and manufacturing engineers are responsible for coming up with solutions that protect the die, the design engineer needs to understand the impact that these packaging technologies have on manufacturing yields and long-term reliability.
Involve your packaging team
Hopefully, these points have provided some insights on how packaging impacts many aspects of design and should not be relegated to just picking the right package at the end of the chip design. It is important that your packaging team be involved in the design process from initial specification through the final design review.
In today’s fast moving markets, market windows are shrinking so time to market is often the important differentiator between success and failure. Not involving your packaging team early in the design cycle can result in costly rework cycles at the end of the project, having manufacturing issues that delay the product introduction or, even worse, having impossible problems to solve that could have been eliminated had packaging been considered at the beginning of the design cycle.
System design incorporates many different design disciplines. Most designers are proficient in their domain specialty and not all domains. An important byproduct of these cross-functional teams is the spreading of design knowledge throughout the teams, resulting in more robust and cost effective designs.