Embedded passive patents.

In response to the article by Jim Howard ["An Open Architecture Approach to Buried Passive Components," December 2003]: Some of the information presented may mislead a reader to believe that it is permissible to manufacture PCBs with thin (0.000-0.004" thick) capacitive layers without consideration of relevant patents or licensing requirements.

Howard spends a good portion of the article discussing the Leary patent (4,494,172), concluding that "capacitive planes described in Leary, are available to manufacturers who use simple capacitive planes to create inexpensive capacitive PCBs." while this statement is true (an expired patent can be practiced), Figure 1 of Howard's article showing a two-metal core is not an accurate representation of the structure of the Leary capacitor. The Leary patent clearly defines a capacitive structure having a minimum of three metal layers with the two outboard layers tied together by vias. In addition, Mr. Howard talks about the use of "thin dielectrics." The dielectric thicknesses described in Leary are 0.005-0.009". Logically, then, it should follow that Howard's use of the term "thin dielectrics" should be interpreted to mean dielectrics with a thickness in the range of 0.005-0.009" as taught in Leary. In the industry however, the use of the term "thin dielectrics" is usually considered to be a dielectric with a thickness of less than 0.003".

Furthermore, an aspect of Leary that Howard fails to mention in his article is that in order to increase capacitance Leary adds a fourth metal layer to his structure with yet another 0.005-0.009" dielectric. Thus, Leary teaches away from using thin dielectrics to increase capacitance.

Finally, Table 1 may also be misleading. While tables are often used to get a large amount of data across to a reader in short form, it should be understood by the reader that this table is Howard's interpretation of what he determines are the relevant patents. Howard's characterization of some of the patents, their dielectric thicknesses and what he calls "unique element" should be viewed as his opinions and certainly not as an all-encompassing analysis of the ideas these patents actually cover. For example, the Howard patent 5,079,069 discloses dielectric thicknesses in the range of less than 0.0005" up to 0.002", not equal to 0.002" as stated in the table.

Anyone wishing to practice "thin dielectrics" in the range of 0.005-0.009" may find the support to do so in the Leafy patent, but I would recommend first seeking competent legal advice from a registered patent attorney. Anyone wishing to practice what I will now term "thinner" dielectrics, i.e., dielectrics with a thickness in the range of 0.00-0.004", will want to first consider the unexpired patents that are in this arena.

George Dudnikov

Chief Technology Officer,

PCB Fabrication Operations

Sanmina-SCI

Jim Howard responds:

The primary question raised by individuals not familiar with patents is why a patent like "Leary" (1985) precludes others from obtaining a patent for the same basic idea using different dimensions. As an example, many companies use a power ground layer to provide distributed capacitance to a large number of surface devices to suppress potential voltage spikes and eliminate the need for surface bypass capacitors. Leary explains the use of an epoxy layer having a preferred thickness of 0.005-0.009" between the copper foil power ground plates to provide a "large distributed capacitance" for mounted electronic components. Additional description of the Leary concept explains that the epoxy layers are "thin enough" to form a "large distributed capacitance" of at least 0.02 microfarads between the voltage plate and ground plates. This "inhibits switching signals front causing voltage spikes in the power lines" and eliminates the requirement of including "many discrete isolation capacitors."

All inventors and patent attorneys understand the reason that a later inventor cannot claim the same basic invention. A doctrine of equivalence exists that prevents the patenting of the same basic invention by making changes that anyone skilled in the art can achieve. Part of the law regarding what is patentable is explained by U.S. Code 35 section 103(a):

   A patent may not be obtained though
   the invention is not identically disclosed
   or described as set forth in section 102
   of this title, if the differences between
   the subject matter sought to be patented
   and the prior art are such that the
   subject matter as a whole would have
   been obvious at the time the invention
   was made to a person having ordinary
   skill in the art to which said subject matter
   pertains, Patentability shall not be
   negatived by the manner in which the
   invention was made.

It is clear that if the type of change made in a power ground layer is (as an example) from 0.005-0.001" in dielectric thickness for the same purpose that it was originally designed, the change does not constitute a new invention. If the power ground layers are mounted opposite each other to provide this distributed capacitance (which is a known amount), the order of the layers and numbers of layers may be chosen by the designer for the proper effect and adds nothing new to the concept. In summary, a new idea must be formulated to apply for a new patent. Many patents in the embedded capacitance field may have been examined by the patent office without Leary or other references as prior art; however my points have only been addressed to the application of a simple capacitive power ground like Leary in a printed circuit design.