Browse Prior Art Database

OVERPRINTING PASTE FOR SELF-ALIGNING

IP.com Disclosure Number: IPCOM000007879D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2002-May-01
Document File: 4 page(s) / 167K

Publishing Venue

Motorola

Related People

Allen Hertz: AUTHOR [+3]

Abstract

The electronics industry is recognizing the requirements for better utilization of the area of printed circuit boards. One means to accomplish this would be to use Integrated Circuit packages which include interconnects which reside under- neath the carrier in an array format. One ofthe more popular formats is the ball grid array (BGA), which is where solder spheres of known volumes are preapplied to the underside of the carrier in a grid pattern.

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Technical Developments

OVERPRINTING PASTE FOR SELF-ALIGNING

by Allen Hertz, Hank Liebman and David Tribbey

BACKGROUND:

  The electronics industry is recognizing the requirements for better utilization of the area of printed circuit boards. One means to accomplish this would be to use Integrated Circuit packages which include interconnects which reside under- neath the carrier in an array format. One ofthe more popular formats is the ball grid array (BGA), which is where solder spheres of known volumes are preapplied to the underside of the carrier in a grid pattern.

  Because the interconnections are underneath the carrier, it is not realistic to inspect the placement of the component prior to reflow. The standard place- ment routine for BGA components was to dip them into flux, applying a thin film to the lower half of the solder spheres. Following the fluxing operation, the parts are placed, assisted by vision, onto the receiving substrate. Previous studies have shown the placement of a ,060" pitch device using a vision algo-

rithm which identifies the package center from the silhouette is less than six sigma (Cp c 1.8 were mea.+ ured). Several modifications to the process have to be developed to increase the Cp beyond two, or greater than six sigma without changing the com- ponent or increasing the area required on the board. CURRENT SITUATION:

  The first method was to enlarge the corner pads on the receiving substrate.1 The enlarged comer pads provided an increased placement window by utiliz- ing the surface tension of liquidous solder to self align the BGA carriers. This modification effected the resulting geometry of the associated solder spheres after reflow. The enlarged comer pads is effeo tive for parts assembled using the fluxing process. The assembly equipment which is capable to dip the carrier into flux is not standard equipment on the assembly line. The cycle time to include the flux dip process was also a consideration.

Standard Grid Array Pattern

Enlarged Metal Comer Pads

'US Patent 5,381,307, issued January 10.1995; Hertz, Tribbey, Thompson

0 Motorola. 1°C. ,996 83

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MO7VROLA Technical Developments

  To accommodate a more standard surface mount assembly line, the process was modified removing the fluxing process and adding apertures to the sten- cil to apply solder paste for tact the part prior to reflow and clean the metal to create reliable solder joints.

  The use of solder paste is known to create voids in the solder/component interface due to the entrapped outgassing of the flux. The voids, varia- tions in solder volumes, and geometry of the result- ant joints have brought some field reliability risks into the system, where the comer pads eventually open.

  The standard pad configuration does not pro- vide a six sigma assembly placement process. The use of a flux dip process requires additional capital, which until recently2 has not been avai...