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Method of protecting Solder-joint interfaces without need for capillary underfill material

IP.com Disclosure Number: IPCOM000181961D
Original Publication Date: 2009-Apr-20
Included in the Prior Art Database: 2009-Apr-20
Document File: 3 page(s) / 473K

Publishing Venue

Motorola

Related People

Ben Tan Tin Chong: INVENTOR [+4]

Abstract

This submission is to introduce an innovative method to protect solder interconnect with epoxy fillet under PCB drop or contamination conditions. Currently, underfill or underfilm is widely used in PCBA (Printed Circuit Board Assembly) for protecting solder interconnect, however with higher investment cost for dispensing machine, maintenance and rework tooling that will result in higher conversion cost for factory. Cost, cycle time and quality-friendly epoxy flux could be the alternative solutions to replace underfill or underfilm. We utilized a reflow curable epoxy-flux with encapsulant properties for assembly of CSP (Chip-Scale Package)/BGA (Ball-Grid Array) packages. This material also provides fluxing functions for Pb-free solder joint formation. When cured, the flux residue forms a fillet that provides protection against mechanical stress. Therefore, it provides "underfill-like" protection to the BGA interconnects, but without need for capillary flow dispensing. This epoxy-flux is applied using dipping method from a doctor blade & then placed onto a Printed Circuit Board (PCB). After reflow, the solderjoint is formed & the epoxy-flux forms a fillet that protects the solderjoint to improve BGA package mechanical reliability.

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Method of protecting Solder-joint interfaces without need for capillary underfill material.

By Ben Tan Tin Chong, Lee Joong Thye, Leon Lin Tingyu, Leo Merilo

Motorola, Inc.

 

ABSTRACT

This submission is to introduce an innovative method to protect solder interconnect with epoxy fillet under PCB drop or contamination conditions. Currently, underfill or underfilm is widely used in PCBA (Printed Circuit Board Assembly) for protecting solder interconnect, however with higher investment cost for dispensing machine, maintenance and rework tooling that will result in higher conversion cost for factory. Cost, cycle time and quality-friendly epoxy flux could be the alternative solutions to replace underfill or underfilm. 

We utilized a reflow curable epoxy-flux with encapsulant properties for assembly of CSP (Chip-Scale Package)/BGA (Ball-Grid Array) packages. This material also provides fluxing functions for Pb-free solder joint formation. When cured, the flux residue forms a fillet that provides protection against mechanical stress. Therefore, it provides “underfill-like” protection to the BGA interconnects, but without need for capillary flow dispensing. This epoxy-flux is applied using dipping method from a doctor blade & then placed onto a Printed Circuit Board (PCB). After reflow, the solderjoint is formed & the epoxy-flux forms a fillet that protects the solderjoint to improve BGA package mechanical reliability. See Fig 1 for the schematic.

Fig 1. Schematic drawing of epoxy flux forming underfill-like fillet around solder interconnect

PROBLEM

Traditionally, BGA solderjoints need capillary underfill dispensing for added protection against impact or shock loading. Underfill process is not favoured from a Design for Manufacturability (DFM) perspective because this is a secondary process that needs extra capital investment (dispenser, oven cure). Underfilled components have low rework success rate, and any badly-reworked PCBA needs to be scrapped. This results in Factory personnel incurring unnecessary cost. 

 

SOLUTION

The following are proposed assembly steps to achieve mechanical protection at both interfaces of the BGA/CSP solderjoints.

For in-coming BGA, we can improve the component-side interface reliability threshold by the following innovative sequence:

(a)  Using standard pick & place equipment, pick the Component from reel à dip into flux plate containing epoxy flux à flip over the component (to allow flux to flow onto Component-side) à reflow to obtain fillet around each solderball è the BGA is now protected at the component-side interface. This i...