Despite the current economic downturn, the relentless push for performance improvement continues. While shrinking line sizes are improving performance at the die level, improvements must also occur at the package level, so that overall device performance will not be impeded. The substrate is at the heart of high-performance packaging. Ceramic substrates have been popular for years, because they offer the capability of achieving fine pitches, and allowing for high I/O density. Cracking Not an Issue In addition, ceramic's CTE is similar to that of the die, so cracking the die during thermal cycling is not an issue. The fact that ceramic does not have any outgassing issues makes this material ideal for hermetically sealed applications. However, the ceramic CTE does not match the PWB's, which potentially may cause problems between the package and the PWB if the package is not socketed. Ceramic substrates are also expensive, and can be in short supply. The development of organic-based high-density-interconnect substrates (HDI substrates) now presents an attractive alternative to ceramic materials for high-performance substrates. HDI substrates incorporate microvias for higher density. All substrates on array packages feature vias that carry the electron from the die to the PWB. Traditionally, the vias are mechanically drilled. Microvias are a dimension smaller than vias, and must be created by some method other than mechanical drilling. The drilling process not only produces larger holes, it also restricts having a path through the entire substrate. Microvias, on the other hand, are not only smaller, but can appear in partial distances within a multilayer substrate. Blind vias, for instance, begin at the edge of a substrate, while buried vias are contained within inner layers of the substrate. Creating Microvias Microvias can be created in a number of different ways: Lasers and photolithography are the most prevalent methods at this time, although new twists are being periodically introduced.
Lasers use collimated light to create via holes one at a time or sequentially. Photolithography methods of creating vias require masking steps and expensive tooling, similar to creating lines and spaces on a wafer. Lasers tend to be more cost effective when the number of desired holes is small. Alternatively, photolithography methods are more cost effective when the desired number of holes is larger. Plasma etching-using dry chemicals-is another technique to create vias. Finally, direct-write combines laser and photolithography techniques. Given the crisp focus of the direct light of a laser, the resulting vias are the cleanest holes. This is particularly important when the device will be operating at GHz speeds. Creating a laminate substrate that employs microvia techniques to achieve high density offers an advantage over ceramic, in that laminates provide improved second-level reliability. This improved reliability occurs because the CTE of the material is more closely matched to that of the PWB. As a result, underfill is not required between the packaged part and the PWB, as it sometimes is with ceramic substrates. However, because a CTE mismatch still exists between die and substrate, underfill is required between these two parts to prevent failure at this junction. The choice of underfill is important, as catastrophic events may occur otherwise, such as a cracked die. Isolating the underfill glue to within a single packaged part offers the advantage of not affecting the PWB or other parts already placed on the PWB in the event of a defective part. Even in today's economy, suppliers are reporting an upturn in the demand for substrates with microvias. Thus, when overall industry demand picks up, the demand for HDI substrates should expand very rapidly.
|
|||||||||||||||||||||
Copyright © 2001