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The technological development and characteristics of an innovative process and composition for immersion plating and fusing of a solderable tin/lead deposit over copper are discussed. The process offers a viable alternative to hot air solder levelling, electrodeposition/selective stripping, or inhibitor coatings for maintaining solderability of printed wiring boards. A flat, uniform solderable tin/lead coating on all feature surfaces and edges is achieved. A number of important benefits are derived. The ability to coat any copper surface uniformly, including fine pitch features, is substantially enhanced. Solderability is improved because of a thick, flat, co‐planar and uniform tin/lead deposit on all copper surfaces. Typical thickness and composition of the fused alloy are 150 to 300 microinches (4 to 8 microns) and 65 to 75% tin.

The importance of the quality of the plated‐through‐hole copper barrel in double‐sided and multilayer PCBs is considered with regard to the problem of voids and blowholes in the solder fillet. The thickness of the copper, and its integrity and adherence to the drilled surface define its ability to withstand the pressure burst of gas from the outgassing laminate during the few seconds of the thermal spike induced by the molten solder prior to solidification. The ability of copper electroplate to bridge over areas devoid of electroless copper and produce a barrel free of pinholes is shown to be crucial to this problem. In addition, the use of a nickel layer is shown to enhance greatly the impermeability of the barrel to the evolving gases.

The drilling and preparation of a hole in FR‐4 laminate prior to the deposition of electroless copper is considered in relation to the quality of soldering achieved on the finished printed circuit board. Data pertaining to the drill speed, drill feed, and stack position are presented and the effect of drilling temperature is demonstrated. The variability of laminate is discussed in relation to outgassing during soldering. Finally the importance of the post‐drilling treatment of the hole‐wall is shown. The relative effects of baking after drilling, ultrasonic cleaning and chemical treatments such as alkaline potassium permanganate are illustrated.

Data from laboratory and production scale tests are given that show that the efficiency of catalyst adsorption controls the coverage by the electroless copper deposit in plated‐through‐holes in FR‐4 laminate, and that this, in turn, governs the outgassing performance of the finished board. The nature of electroless copper nucleation and growth is discussed and the reasons for the formation of voids in the deposit are identified.

During the past several years, technical requirements of the printed circuit board industry have increased greatly, due to the need for greater processing latitude, higher density devices and higher reliability in the finished parts. This need has resulted, in many cases, in the emergence and/or recognition of numerous problem areas which have created greater demands on the desmear and PTH processes, specifically. Problems such as hole‐wall pullaway, resin recession, blistering, gross voiding, and blow holes have grown in importance and severity as a result of today's needs. The development of a reliable and practical permanganate desmear process has proven to solve numerous of these technical problems. The characteristics, operation, and advantages of this three‐step permanganate process will be discussed in some detail, with particular emphasis on demonstrated improvement in PCB production results.

With the ever increasing demands for high performance electronic devices there is a need for circuit board laminates that have enhanced properties when compared to conventional materials such as the widely used epoxide‐based FR4 laminates. Equipment manufacturers require boards with better mechanical stability and improved electrical characteristics. At the same time, new environmental legislation is set to drive electronics assembly temperatures much higher as manufacturers start to use lead‐free soldering processes. The legislation is also raising questions about the long‐term viability of brominated resins as the basis for imparting flame retardancy to laminates. Fortunately, laminate manufacturers have responded to these challenges by developing and introducing a wide range of new laminates that address these issues. This paper describes some of these challenges and gives an introduction to the new high performance laminates that are finding increasing use. It also highlights the need for chemical processes used in the manufacture of interconnects with laminates to be specifically optimised for the chosen substrate material.