Browse Prior Art Database

Method for package corner stress mitigation features and bend-line management

IP.com Disclosure Number: IPCOM000132645D
Publication Date: 2005-Dec-28
Document File: 6 page(s) / 513K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for package corner stress mitigation features and bend-line management. Benefits include improved functionality, improved performance, improved reliability, and improved cost effectiveness.

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Method for package corner stress mitigation features and bend-line management

Disclosed is a method for package corner stress mitigation features and bend-line management. Benefits include improved functionality, improved performance, improved reliability, and improved cost effectiveness.

Background

      Package corner solder joints are subject to mechanical stress. Lead-free solder increases the problem due to being stiffer than leaded solder and unable to absorb the stress induced during board bending from handling and shipping. Additionally, the high temperature in lead-free processing weakens the adhesion force in thicker intermetallic layers, which are more brittle.

              Conventionally, solder-joint failure is solved by the following techniques:

•             Larger metal pad diameter opening

•             Larger solder balls

•             Increased solder-joint standoff height

•             Sacrificial solder balls at the package corner

              These solutions do not meet the requirement for smaller packages with increased input/output (I/O) and finer ball pitch. As a result, an innovative solution is required to improve solder-joint reliability.

      The bend line is the line on which the board flexes during mechanical stress. Corners placed on the bend line incur the lowest displacement while corners away from the bend line undergo higher displacement. Conventionally, three solder joints at each package corner share the stress. For example, corners B and D have the highest displacement from bend line Z-Z’ during bending or shock. Corners A and C experience the highest fracture rate in flip-chip ball grid array (FCBGA) applications with thermal solutions (see Figure 1).

General description

      The disclosed method includes package corner stress mitigation features and bend-line management. The method lowers stress at package corners and shifts the bend line or makes the region rigid. As a result, solder-joint mechanical performance is improved.

              The key elements of the disclosed method include:

•             Additional solder joints along the package edges

•             Package orientation with two package edges parallel to the board bend line and two edges perpendicular to the bend line

•             Package placement with bend line support by the board connector or standoffs

•             U-notch board cut out

•             Stiffening bar or connector

•             Location of the U-notch or stiffener in a high-stress region to shelter packages/components from the bend line

Advantages

              The disclosed method provides advantages, including:
•             Improved functionality due to lessening stress at the package corners
•             Improved reliability due to improving mechanical performance by preventing solder joint failure

•             Improved cost effectiveness due to improving mechanical performance without requiring stiffening features

•             Improved cost effectiveness due to elim...