Browse Prior Art Database

Method for a structure to add alignment posts to very fine pitch chip scale packages on printed circuit boards

IP.com Disclosure Number: IPCOM000008378D
Publication Date: 2002-Jun-11
Document File: 9 page(s) / 135K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a structure to add alignment posts to very fine pitch chip scale packages (CSPs) on printed circuit boards (PCBs). Benefits include improved reliability and reduced defects.

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Method for a structure to add alignment posts to very fine pitch chip scale packages on printed circuit boards

Disclosed is a method for a structure to add alignment posts to very fine pitch chip scale packages (CSPs) on printed circuit boards (PCBs). Benefits include improved reliability and reduced defects.

Background

              Very fine pitch CSPs (see Figure 1) have a center-to-center solder ball distance of 0.75 mm or less. Two problems can occur during the placement process (see Figure 2). First, the placement accuracy of present pick-and-placement machines is not sufficient to ensure that CSP misalignment is limited to less than half the land diameter. Second, because the amount of solder paste printed on the lands decreases with the CSP pitch, the tackiness provided by the solder paste to hold the CSP in its placed position is not sufficient. This condition causes the CSP to shift from its position after placement, during the rest of the placement process, and with subsequent board movement before reflow soldering.

              Conventionally, the misplacement problem remains unsolved. More accurate placement machines have not yet been developed. When available, the increase in accuracy is expected to slow down the placement process and adversely impact the throughput rate of board assembly lines. The avoidance of component shift after placement is conventionally solved by either avoiding the movement of the placement machine table on which the board sits during placement or by slowing it down considerably. Both these solutions, as mentioned above, slow down the placement process, and thereby adversely impact the throughput rate on the board assembly lines. In addition to board handling during the pick and placement process, board handling by automated conveyors between placement and reflow may also induce CSP shift. Handling can also be slowed down to reduce shift, but again it impacts line throughput.

              Alignment posts are used and work well on a variety of other SMT component types (notably fine pitch connectors) that are prone to shift between placement and reflow. Conventionally, no method adds alignment posts to CSPs.

              An alignment post structure must be added using higher precision placement equipment rather than board assembly equipment. CSP assembly processes are more accurate than board assembly processes and provide adequate precision to perform the addition of an alignment structure.

General description

              The disclosed method adds structure to chip scale packages (CSPs) to provide through-hole alignment features. In conjunction with holes drilled in the printed circuit board at the appropriate locations, this structure enables the CSP package to be placed accurately enough to avoid solder joint defects that may be induced between placement and reflow soldering,

              The structure adds tapered alignment posts to two or more corners of the CSP in a secondary operation after the assembly of the CSP package. This alignment method can be applied selectively to...