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White paper: Bumping BGAs using solder paste printing process for RFI shields packaging

Technical Articles

by Dr. Gerald Pham-Van-Diep, Srinivasa R. Aravamudhan and Joe Belmonte1 of Speedline Technologies and Dr. Benlih Huang, Indium Corporation

One manufacturing process used to attach RFI shields for medical applications/cellular phone circuit board assemblies consists of "snapping" the shell-like shields onto solder spheres that are soldered to printed circuit board [PCB] pads [1]. The snapped on RFI shields do not require soldering onto the printed circuit board and can be removed if necessary. In order to create a large enough solder balls for the shell to snap onto, a large quantity of material must be available.

Currently this process is a two-step process. The first process step requires solder paste to be printed onto the printed circuit board pads using standard solder paste printing process. The second step is to print or place solder spheres onto the printed solder paste deposits. The printed circuit board with solder paste and solder spheres are then reflowed to achieve a ball height sufficient for the RFI shield to snap onto. This two-step process requires either the use of a component placement machine (pick and place) or the use of specially designed print head that is available on only limited printing equipment. The goal of this experimentation is to develop a process that can achieve the ball height requirement using only standard solder paste printing technique and virtually any solder paste printing equipment.

Recently, work has been completed to identify a stencil design and solder paste formulation that will allow sufficient solder paste to be printed and form a large solder sphere after reflow to permit RFI shield attachment. This ‘SnapShot’ shield attach process eliminates the requirement to purchase solder balls and the additional equipments need to place or print the solder balls. This paper will present the results from formal studies that have been completed to verify the performance of the solder paste printing process for the ‘SnapShot’ shield attach process.

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