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Method for a stacked solder-paste printing technology

IP.com Disclosure Number: IPCOM000007692D
Publication Date: 2002-Apr-15
Document File: 6 page(s) / 163K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a stacked solder-paste printing technology. Benefits include improved yield and improved reliability.

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Method for a stacked solder-paste printing technology

Disclosed is a method for a stacked solder-paste printing technology. Benefits include improved yield and improved reliability.

Background

              Open solder joints can occur with on-cartridge voltage regulator (OCVR) packages. Differing reflow peak temperatures and times above liquidus are tried in order to close the gaps to the OCVR packages. The attempts have failed, and yield loss hovers in the range of ~38-42%.

              Resistor-Pack (R-pack) joints fatigue and crack after approximately 1000 temperature cycles due to a coefficient of temperature expansion (CTE) mismatch. The conventional solution adjusts the plated area of the lead and the locations. However, board assemblies vary the thermal mass (copper layers) of the printed circuit boards. Solder fatigue/cracking changes correspondently.

              Open joints to BGA sockets occur because of thermal warpage during reflow soldering. The conventional solution is to adjust the reflow profiling using higher soaking time. However, loss fluctuates at ~1-3%.

General description

      The disclosed method is a stacked solder-paste printing technology. A selective chemical-etching process etches down stencil’s metal sheet and maintains areas that require high solder-paste thickness. These areas occur only in the particular areas that need the thickness, such as the location of OCVR packages, BGA packages and sockets, and R-pack joints.

              A step-up area is located at the back of the flat stencil. The height of the step-up area is in the range of ~1.3x-2x where x is the thickness of the stencil sheet, typically 10 mils (able to be etched to 5 mils). Contact with the printed circuit board (PCB) occurs during the printing process. The creation of the step-up area has negligible impact to squeegee movement or paste scooping effects. This design increases the solder thickness and volume only to packages that have thermal warpage issues. Solder contact is improved during reflow soldering. The issues of open solder joints are solved.

              The use of a higher stencil sheet increases the solder thickness. However, a problem is created in bridging to oth...