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Method for the use of larger corner solder balls with PCB alignment holes for the alignment of very fine pitch chip scale packages during placement

IP.com Disclosure Number: IPCOM000007944D
Publication Date: 2002-May-07
Document File: 5 page(s) / 168K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for the use of larger corner solder balls with PCB alignment holes for the alignment of very fine pitch chip scale packages during placement. Benefits include reduction of defects and improved throughput.

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Method for the use of larger corner solder balls with PCB alignment holes for the alignment of very fine pitch chip scale packages during placement

Disclosed is a method for the use of larger corner solder balls with PCB alignment holes for the alignment of very fine pitch chip scale packages during placement. Benefits include reduction of defects and improved throughput.

Background

              Conventional chip scale packages (CSPs) are picked up by the placement head of a pick-and-placement machine on an SMT assembly manufacturing line and placed by visual alignment of the solder balls to the PCB lands (see Figure 1). Misalignment can occur during the placement process of the components on the printed circuit board (PCB) prior reflow soldering.

              For very fine pitch CSPs (that is, those having the center-to-center solder ball distance of 0.75 mm or less), two problems occur during the placement process, placement and tackiness. The placement accuracy of present pick-and-place machines is not sufficient to ensure that the CSPs are placed with the solder balls less than half the land diameter.

              Because the solder paste amounts printed on the lands decrease with the CSP pitch, the tackiness provided by the solder paste to hold the CSP in place is not sufficient. As a result, the CSP can shift from its position after placement during the placement process and subsequent board movement before reflow soldering (see Figure 2).

              If the CSPs are placed less than half the land diameter off, the self-alignment mechanism of the component, which is driven by surface tension forces of the molten solder during the reflow soldering process, ensures that the component overcomes the misalignment during placement, self-aligns and forms good, reliable solder joints. However, if the CSPs are placed off more than half the diameter of the PCB land, the self alignment mechanism does not work during reflow soldering and causes solder joint defects (see Figure 3).

              Conventionally, the problem described above is solved by either avoiding the movement of the placement machine table on which the board sits or by slowing it down considerably. Both these solutions also slow down the placement process and adversely impact the throughput rate of the manufacturing lines.

General description

              The disclosed method includes the use of large solder balls (.030-inch diameter, typically) on the corner of the chip scale package (CSP) which, in conjunction with through holes drilled in the printed circuit board at the appropriate locations can enable the CSP package to be placed accurately enough to avoid solder joint defects after reflow soldering. These large .030-inch diameter corner balls replace the four smaller corner balls that are typically .012-inch diameter for a 0.5-mm pitch CSP.

              The key elements are the large .030-inch diameter corner balls on the CSP and the through holes drilled in the printed circuit board. The large .030-inch diameter corner balls are round and facilitate...