Research Papers

Effects of Corner/Edge Bonding and Underfill Properties on the Thermal Cycling Performance of Lead Free Ball Grid Array Assemblies

[+] Author and Article Information
P. Borgesen

Department of Systems Science and Industrial Engineering,  Binghamton University, P.O. Box 6000, Binghamton, NY 13902-6000

D. Blass1

 Universal Instruments Corporation, Conklin, NY 13904

M. Meilunas

 Universal Instruments Corporation, Conklin, NY 13904


Current address: Lockheed Martin, Owego, NY, 13827.

J. Electron. Packag 134(1), 011010 (Mar 19, 2012) (8 pages) doi:10.1115/1.4005904 History: Received September 02, 2011; Revised November 29, 2011; Published March 07, 2012; Online March 19, 2012

Underfilling will almost certainly improve the performance of an area array assembly in drop, vibration, etc. However, depending on the selection of materials, the thermal fatigue life may easily end up worse than without an underfill. This is even more true for lead free than for eutectic SnPb soldered assemblies. If reworkability is required, the bonding of the corners or a larger part of the component edges to the printed circuit board (PCB), without making contact with the solder joints, may offer a more attractive materials selection. A 30 mm flip chip ball grid array (FCBGA) component with SAC305 solder balls was attached to a PCB and tested in thermal cycling with underfills and corner/edge bonding reinforcements. Two corner bond materials and six reworkable and nonreworkable underfills with a variety of mechanical properties were considered. All of the present underfills reduced the thermal cycling performance, while edge bonding improved it by up to 50%. One set of the FCBGAs was assembled with a SnPb paste and underfilled with a soft reworkable underfill. Surprisingly, this improved the thermal cycling performance slightly beyond that of the nonunderfilled assemblies, providing up to three times better life than for those assembled with a SAC305 paste.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 1

Cross-section of an encapsulant A edge fillet

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Figure 2

Weibull plot of thermal cycle results for corner/edge bonded SAC305 BGAs

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Figure 3

SAC305 solder joint cracks after thermal cycling of BGAs reinforced with encapsulant A fillets

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Figure 4

Cracking of encapsulant B corner fillets after 100 drops and 1061 thermal cycles

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Figure 5

Cracking of encapsulant B full fillets after 100 drops and 1433 thermal cycles

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Figure 6

SAC305 solder joint cracked near the component pad, encapsulant B full fillet, 100 drops, and 1433 thermal cycles

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Figure 7

Cracking of underfill H near the solder mask in the gap between two BGA joints

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Figure 8

Cracking of underfill H next to a solder joint that propagates into the component

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Figure 9

Cracks and voids in underfill G between two solder joints

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Figure 10

Deformed SAC305 joint encapsulated by underfill G after thermal cycling

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Figure 11

Solder cracks found mostly near the PCB pad for SAC305 BGAs underfilled with underfill C

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Figure 12

Cross-section showing voids in underfill C near the component side of the underfill layer

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Figure 13

Flux residues from BGA ball attach in an underfill C underfilled SAC305 BGA

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Figure 14

Solder cracks observed in SAC305 BGA assemblies underfilled with underfill D

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Figure 15

Solder joint crack in SAC305 BGA underfilled with underfill F

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Figure 16

Cracking near IMC layer in a SAC305 BGA underfilled with underfill E

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Figure 17

Crack in underfill E propagated into both component and PCB

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Figure 18

Weibull plot of thermal cycle results for underfill C

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Figure 19

Solder joint crack from a mixed alloy BGA underfilled with underfill C




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