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Method for the use of corner posts for the alignment of very fine pitch chip scale packages during placement on printed circuit boards

IP.com Disclosure Number: IPCOM000008379D
Publication Date: 2002-Jun-11
Document File: 6 page(s) / 126K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for the use of corner posts for the alignment of very fine pitch chip scale packages (CSPs) during placement on printed circuit boards (PCBs). Benefits include improved reliability and reduced defects.

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Method for the use of corner posts for the alignment of very fine pitch chip scale packages during placement on printed circuit boards

Disclosed is a method for the use of corner posts for the alignment of very fine pitch chip scale packages (CSPs) during placement on printed circuit boards (PCBs). Benefits include improved reliability and reduced defects.

Background

              For very fine pitch CSPs, such as those having the center-to-center solder ball distance of 0.75 mm or less, two problems occur during the placement process. The placement accuracy of conventional pick-and-place machines is insufficient to ensure that the CSPs are placed with the solder balls at 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 insufficient. The CSP may shift from its position during the rest of the placement process and subsequent board movement may occur before reflow soldering.

              If the CSPs are misplaced by less than half the land diameter, the self-alignment mechanism of the component ensures that it forms reliable solder joints. The self-alignment mechanism is driven by surface tension forces of the molten solder during the reflow soldering process.

              If the CSPs are misplaced by more than half the diameter of the PCB land off, the self-alignment mechanism does not work during reflow soldering and causes solder joint defects.

      The misplacement problem remains unsolved. More accurate placement machines are not under development. If developed, the increase in accuracy is expected to slow down the placement process, adversely impacting the through-put rate on the manufacturing lines.

      The avoidance of component shifting after placement is conventionally resolved by either avoiding the movement of the placement machine table on which the board sits or by slowing it considerably. Both these solutions slow the placement process, adversely impacting the through-put rate on the manufacturing lines.

      The CSP is conventionally placed on the PCB without the use of corner alignment posts (see Figure 1). The component is picked up by the placement head of a pick-and-place machine on a surface mount technology (SMT) assembly manufacturing line and placed by visual alignment of the solder balls to the PCB lands.

General description

      The disclosed method utilizes posts 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. T...