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Research Papers

Experimental Investigation by Speckle Interferometry of Solder Joint Failure Under Thermomechanical Load Aggravated by Boundary Conditions at Board Level

[+] Author and Article Information
D. N. Borza

e-mail: borza@insa-rouen.fr

I. T. Nistea

INSA Rouen,
Mechanics Laboratory LMR,
B. P. 8, avenue de l'Université,
76801 Saint-Etienne du Rouvray, France

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the Journal of Electronic Packaging. Manuscript received December 29, 2011; final manuscript received September 12, 2012; published online November 16, 2012. Assoc. Editor: Bongtae Han.

J. Electron. Packag 134(4), 041007 (Nov 26, 2012) (8 pages) doi:10.1115/1.4007812 History: Received December 29, 2011; Revised September 12, 2012

Reliability of electronic assemblies at board level and solder joint integrity depend upon the stress applied to the assembly. The stress is often of thermomechanical or of vibrational nature. In both cases, the behavior of the assembly is strongly influenced by the mechanical boundary conditions created by the printed circuit board (PCB) to casing fasteners. In many previously published papers, the conditions imposed to the fasteners are mostly aiming at an increase of the fundamental frequency and a decrease of static or dynamic displacement values characterizing the deformation. These conditions aim at reducing the fatigue in different parts of these assemblies. In the photomechanics laboratory of INSA Rouen, the origins of solder joint failure have been investigated by means of full-field measurements of the flexure deformation induced by vibrations or by forced thermal convection. The measurements were done both at a global level for the whole printed circuit board assembly (PCBA) and at a local level at the solder joints where failure was reported. The experimental technique used was phase-stepped laser speckle interferometry. This technique has a submicrometer sensitivity with respect to out-of-plane deformations induced by bending and its use is completely nonintrusive. Some of the results were comforted by comparison with a numerical finite elements model. The experimental results are presented either as time-average holographic fringe patterns, as in the case of vibrations, or as wrapped phase patterns, as in the case of deformation under thermomechanical stress. Both types of fringe patterns may be processed so as to obtain the explicit out-of-plane static deformation (or vibration amplitude) maps. Experimental results show that the direct cause of solder joint failure may be a high local PCB curvature produced by a supplementary fastening screw intended to reduce displacements and increase fundamental frequency. The curvature is directly responsible for tensile stress appearing in the leads of a large quad flat pack (QFP) component and for shear in the corresponding solder joints. The general principle of increasing the fundamental frequency and decreasing the static or dynamic displacement values has to be checked against the consequences on the PCB curvature near large electronic devices having high stiffness.

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References

Figures

Grahic Jump Location
Fig. 1

Layout of an out-of-plane speckle interferometer

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Fig. 2

Sketch of the printed circuit board assembly. IC—integrated circuit; 1,…,5—PCB holes for screw mounting; CON—connector.

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Fig. 3

Time-average fringe patterns of first two modes: (a) 166 Hz and (b) 273 Hz

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Fig. 4

Time-average fringe patterns of first two resonance frequencies: (a) 232 Hz; (b) 261 Hz; and (c) fringe pattern produced by FE simulation—243 Hz.

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Fig. 5

Amplitude distributions across the PCB correponding to the interferographic time-average fringe patterns: (a) and (b)—273 Hz, four screws and (c) and (d)—261 Hz, five screws

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Fig. 6

Lateral derivatives of the out-of-plane displacement field (five screw fixation) and vertical profiles showing the influence of the central screw; (a) ∂2w(x,y)/∂x2; (b) ∂2w(x,y)/∂y2; (c) and (d) profiles across (a) and (b)

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Fig. 7

Three local vibration modes of the QFP package: (a) at 2789 Hz; (b) at 6656 Hz; and (c) at 10785 Hz

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Fig. 8

Thermally induced global flexure of PCBA fastened with four screws

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Fig. 9

Thermally induced global flexure of PCBA fastened with five screws

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Fig. 10

Displacement maps (a) of the PCBA with four screws; (c) of the PCBA with five screws; profiles along a vertical line in the displacement maps of the PCBA (b) with four screws, and (d) with five screws

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Fig. 11

PCB and QFP device: (a) without PCB flexure; (b) in presence of PCB flexure and strong curvature due to the fifth screw; and (c) in presence of PCB flexure and slight curvature, only four fastening screws

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Fig. 12

Out-of-plane displacement of the RISC component and of the PCBA around it: wrapped phase map and unwrapped 3D surface

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