January - February 1999
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Substrate Requirements for Chip-Scale Packaging
By Dr. Rao Mahidhara, Technical Editor
Over the past 30 years, electronic packaging has embraced major changes brought about by changing market forces. First, mainframe computers drove interconnection technology in the 60's and 70's. Then, in the 80's, personal computers again changed interconnect requirements. Today, handheld and portable products lead the way.
The most important factors related to portable consumer products, which bring a new level of complexity, are component density, physical size, weight and performance. This complexity arises, in part, from the growing high-speed performance demands of new CMOS technologies, which exhibit steadily decreasing feature sizes.
The most important factors related to portable consumer products, which bring a new level of complexity, are component density, physical size, weight and performance. This complexity arises, in part, from the growing high-speed performance demands of new CMOS technologies which exhibit steadily decreasing feature sizes.
Board Wiring Density IncreasingAccording to the MCC program, in the year 2000 there will be commodity PWBs with up to 1,153 leads/inch2 connected by 300 inches/inch2 of wiring. Leading-edge products, on the other hand, will boast up to 1,859 leads/inch2 connected by 546 inches/inch2 wiring by 1998. By the year 2000, boards will contain an estimated 17,760 leads/inch2 connected by 2840 inches/inch2 of wiring.1 This wiring density increase represents a 2x hike for 1998 and a 9x increase for advanced boards by the year 2000.
In the next few years, we will see a resurgence of new, space-saving semiconductor packaging technologies in a variety of form factors, including BGAs, CSPs, flip- chips and fine-pitch partsall demanding increased interconnect density.
One of the traditional roles of packaging has been to form the transition from the high-density interconnections at the chip's perimeter to the lower densities available on the board.
Moreover, since lithographic features on PWBs are coarser than those on the ICs, boards typically are multilayer boards (MLBs) and use multiple layers rather than finer features to obtain increased interconnection density.
Multiple Board Layers NeededClearly, as chip sizes decline below 3-mil outer lead pitch (actual or effective), routing interconnections to any chip-scale package will require multiple board layers just to place enough wires under the chip.
Gaining electrical access to the underlying levels of wire requires vertical connectivity, which is provided by the vias. Since via and pad can block routing channels, each year these parts become smaller. The reduction of via and via-pad sizes effectively increases the PWB wireability.
The reduction of vias and via-pads, coupled with the line-width and line- space reduction, gives PWB designers the capability of decreasing size, weight and layer count in new products.
As circuit features shrink, however, commodity PWBs and conventional testing technologies will both be inadequate. All components mounted on a PWB place a wiring demand on the board that must be accommodated by the board's wiring capacity to complete all connections. Greater wiring capacity, however, increases overall cost for the board and, therefore, the package.
Via DensityThe evolution of PWB technologies shows that via density is the major driver. However, time-to-market, improved electrical performance, introduction of new types of area-array packages, increased two-sided SMT densification and RFI/EMI reduction should not be discounted.
Table 1. Design Characteristics by Product Type
The first generation of boards was the single-sided, etched copper board. The next generation was double-sided. Methods employing mechanical drilling (or punching to form vias) and metalizing them through the entire stack advanced board technology into the third generation.
The large size and the low density of these methods lead to a mismatch between horizontal and vertical connectivity that limits routing capabilities. Today's fourth-generation HDI board technology is characterized by blind and buried microvias and sequentially manufactured layers. These layers either stand alone or rest atop conventional third-generation boards.
Table 2. Printed Wired Board Capabilities.
The densification of substrate attachment pads, measured as the number of solder interconnections per square inch, is another indicator of HDI substrates. The Semiconductor Industry Association roadmap suggests that the number of solder interconnections is increasing from more than 200 pads/inch2 in 1995 to over 1,200 pads/inch2 beyond the year 2005.
For example, the Sony Handycam (introduced in October 1996 and currently in high-volume production) exhibits over 500 interconnections/ inch2 and corroborates the SIA roadmap data.
Preferred LaminateBT (bismaleimide triazine), a resin-based laminate, has become the preferred laminate material for many manufacturers, although it is more expensive than the traditional epoxy-based laminates, such as FR-4, because of its high Tg (230ÁC-330ÁC), low dielectric constant (3.8 - 4.3 at 1 MHz) and good insulation characteristics (5 x 1014).
Table 3. Typical Design Rules for Printed Circuits
Mitsubishi Gas and Chemical owns the lion's share of the world BT market, supplying about 100,000 m2/month.2 Meanwhile, General Electric, Hitachi Chemical and Matsushita have been promoting their alternate materials, which are, respectively, GETEK, MegTran and MCL-679. These alternates are epoxy-based, high-temperature materials which Hitachi also calls FR-4.5. Currently, BT substrates for BGAs and CSPs (pitches 0.5 mm to 2.54 mm) are fabricated by conventional PWB technologies involving hole formation by conventional drilling.
A higher drilling capacity will be a requirement for future advanced substrates for BGAs and CSPs. A panel made of BT resin is hard and is not conductive to drilling small holes. Considerable time will be required to drill substrates made from BT resin to meet the hole-density needs (500,000 small holes/m2) of future substrates for new BGAs and CSPs.
Design RulesFour categories are used by the PWB industry: conventional, advanced, leading-edge and state-of-the-art.4 The design rules for these categories are shown in Table 3. (See Table 2 for the capabilities of conventional PWBs and future requirements for 1999 and beyond.) Conventional boards cannot handle grid arrays with high I/O or pitches below 0.5 mm.
Newer substrate technologies are based on build-up multilayer boards with microvia holes (MVHs) featuring an aspect ratio of about 1:1. The last two categories in Table 3 are specific to HDI substrates and are the basis of MVH technologies.
When board thickness does not decrease in proportion to the hole diameter, higher-aspect-ratio holes result, which can lead to reduced reliability. Again, the solution is to employ blind vias in the context of HDI printed circuits. (All the various MVH technologies provide approximately the same design rules.)
Obviously, the use of PWBs with design rules appropriate to advanced packages is critical. For example, lower I/O (<400), larger pitch (1.27 mm), peripheral-row array (up to four) BGA packages can employ less elaborate design rules and four-layer MLBs.)
If more or finer-pitch I/Os are required, the complexity of the PWBs escalates and may require microvia-based, HDI boards. The use of CSPs or direct-attach flip-chips usually requires HDI printed circuits.
MVH may take the form of SBVH (surface-blind via holes) and IVH (inner via holes). In some cases, SBVH may reach the third or fourth layer from the top. Build-up technologies are always accompanied by microvia-hole technologies. Thus, they are really synonymous with MVH methods.
Microvia technologies may be grouped into three fabrication methods:
The cost of drilling holes less than 0.2 mm and down to 0.15 mm (also called MVHs), is prohibitively higher with mechanical drilling and can add to the overall PWB cost.5
Among the newer drilling techniques, laser (CO2, UV/Nd:YAG and excimer) and photochemical techniques are being actively pursued by several companies, with the prime objective to reduce the overall substrate processing cost.
Drilling speed using the CO2 laser is about 230 - 250 holes per second. In addition, new T-CO2 lasers can produce more than 30,000 SBVHs per minute, unlike the UV/Nd:YAG laser.
Table 4. Key High-Density Substrate Technologies
An elegant example of MVGP is IBM's Surface Laminar Circuit or SLC technology. SLC was developed and is being manufactured as a typical sequential build-up board with a liquid photo-imageable dielectric resin in layers.
When the number of holes required is very high, photovia processes, such as SLC, are more economical than laser drilling, because many vias can be produced in a batch photolithographic step. A number of board makers are now mass-producing MLBs with MVHs by a photovia process (summarized in Table 4).
In a market dominated by rigid substrates for BGA and CSP applications, a new breed of substrates, which employs polyimide film as the circuitizing material (also called flexible tape), is rapidly expanding into the substrate market for assembly of fine-pitch packages.
Lightweight, thin, reliable 3-D interconnection and a host of other positive attributes have been used to describe the enabling quality of flexible circuits.
Among flexible circuits' attributes is that the flex-tape manufacturing is quite similar to that of the rigid substrate fabrication, except that flex-tape is handled roll-to-roll. Hole formation is by punching or laser ablation.
ConclusionThe complex job of selecting design rules and structures for PWBs is being driven by newer, high-density components. In addition, materials technology is at the heart of "build-up" technology and will enable the the next generation of PWBs with higher density, suitable to chip-scale packages.
The materials used in almost all these new PWB technologies share the same characteristics of non-glass reinforcement. This makes via generation by plasma etching, photolithography or laser ablation.
Next-generation build-up technologies will provide many challenges for board fabricators and their suppliers. Design tools and tooling techniques will have to be modifiedwhich again warrants additional capital investment.
And new processes will have to be evaluated and installed. No matter what the change is, competitive costs will be the deciding factor. However, it will take several years before any single substrate technology emerges as the undisputed leader.
AcknowledgmentThe author thanks Happy Holden of TechLead Corp., Boulder, Colo., for helpful advice during the preparation of this manuscript.
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