The current technology includes transmission of between optical units (typically modulated laser sender and receiver units) and fiber optic cables or flexible kapton fiber optic cables with which we are all relatively familiar. Methods for forming and connecting fiber optic systems are well developed. The primary new developments in this area may be in modulation and multiplexing techniques and connection methods. At present the optical and digital devices may be mounted on opposite sides of the PCB for optimum design and all connections are on the surface of the PCB. The limitations for this technology for on board transmission of signals are size,surface space requirements,and limited channels available. As can be seen in Figure 1,no decided advantage may be realized in the inclusion of embedded passives in this type of architecture beyond those advantages seen in any multilayer structure.

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Several factors are driving the need for buried passive components in printed circuit boards and chip carriers. Increasing frequency increases the difficulty in quieting noise by the use of surface mounted discrete capacitors and resistors. Decreasing voltage makes transmission lines more sensitive and signal integrity more difficult to maintain. Increasing power consumption by silicon devices drives the need for lower inductance,and higher capacitance power distribution in close proximity to the to the silicon device. A larger number of active silicon devices per unit of surface area decrease available space for passives.

The increased need for smaller,faster,and cheaper electronics has led the microelectronics industry to explore a number of new enabling technologies. Embedding passive components into multi-layer printed circuit boards offers the potential to deliver a number of benefits,including saving valuable board surface area,increasing performance,reducing manufacturing costs,improving reliability,and providing opportunities for less expensive substrate materials. As PCB manufacturers embrace this technology,and as requirements for tighter tolerances become more necessary,laser trimming for these components will also become necessary,prompting manufacturers to add embedded passives trimming equipment to their current manufacturing process. Technology for trimming embedded resistors has been recently demonstrated for production applications1,and the industry is beginning to look more closely at cost models. Cost models are currently available,2,3 presenting a general overview of the costs,and allowing for comparisons to alternative surface mount technologies. This paper will discuss the process of laser trimming embedded resistors in a production environment,and will investigate the process cost of ownership.

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There are many board finishes in use today and the EMS provider must learn to use many,if not all of them. However,all board finishes do not perform the same during assembly and test operations,which can impact assembly yield and solder joint reliability. One of the main characteristics the EMS provider wants is consistency. We want consistency in surface topography,appearance,wetting,solder joint formation,surface contact,etc. Consistency permits the EMS provider to tune their assembly process so that they can provide high yields with consistent solder joint formations. In order for a new board surface finish to be accepted for general use,it must be approved by the OEM as well as the EMS provider. The supplier must approach potential EMS and OEM users to determine their interest in the new finish. The EMS providers must run some trials to determine the advantages and disadvantages of the finish,compared to existing finishes. The OEM must determine if the finish is suitable for and compatible with the particular product application. This assessment must consider all of the assembly operations,test,and final field application. The resulting interconnections must provide a long term,reliable product for the OEM and end customer.

Over the last five to eight years the use of HASL alternatives by the OEM’s and CM’s has increased dramatically,particularly in response to increasing board density and the need for a solderable planar surface that increases their yields at assembly. The chemical industry has responded to meet the needs of the OEM’s but unfortunately some of the product offerings and the manner in which they were installed were less than stellar. This paper outlines for the OEM / CM engineer/auditor some of the areas that should be examined in more detail than might otherwise happen. The information outlined should also be used for surface finish engineers at PWB fab houses that might be new to a finish and may possibly save them from some very late nights on a line trying to understand a problem.