solder reliability
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Contents:
Reliabilityof Solder interconnections-Generalaspects
Literature:
K. Puttlitz, Handbook of Lead-Free Solder Technology forMicroelectronic Assemblies
D. Frear, The Mechanics of Solder Alloy Interconnects
J. Lau, Thermal Stress and Strain in Microelectronic Packaging
K. Puttlitz, Area array interconnection handbook
Design for Reliability 20.11.2012
Solder Joint Reliability
As stated before, electronic assembly designs have incorporatedvarious types of technology configurations to formmechanical,electrical and thermal interconnections.
These configurations have developed from through-holetechnologies (single-sides, double-sided and PTH architectures)
to SMT technology SMT technology includes:
Standard, fine pitch and very fine pitch devices
Leaded and leadless components
Ball and column grid arrays
Max. input/output-to component size ratio without excessive use of boardacreage
Durable, long-lasting and inexpensive mass production methods
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Solder Joint Reliability is the ability of solder joints to remain inconformance to their visual/mechanical, thermal and electrical
specifications over a given period of time, under a specified setof operating conditions.
Component-level solder joint reliability within the packagestructure itself
Board-level solder joint reliability deals with the reliability ofthe solder joints of a package after it has been mounted on aboard or substrate, encompassing both the solder-to-packageand solder-to-board interfaces.
board-level reliability testing is more difficult to implement.
Solder Joint Reliability
Board level vs. die level
Mechanisms that determine the reliability (fatigue, creep, corrosion etc.)are same in board and die level, but the differences in material/processcharacteristics have to be taken into consideration
Crack propagation distance is much larger in bigger board level solderjoints
Strain levels can be considerably higher in board level because of CTEmismatch
The influence of design, process, materials (thermomechanical)properties and environment has to be addressed on both levels in orderto fully understand the reliability especially under cyclic stresses(fatigue)
Board-Level Area Array Interconnect Reliability
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The solder properties are largely dependent on both the mechanical andphysical properties (which are also dependent on chemicalcomposition)
The bulk solder and substrate compositions together with the thermo-mechanical history define the condition, state and properties of the joint(microstructure)
These factors greatly contribute to joint properties
Creep and fatigue strengths
Ductility
Electrical and thermal conductivity
Diffusivity CTE
Resistance to corrosion and other environmental effects
Solder Joint Reliability
CTE Mismatch and area array package solder joint fatigue
PCB expands about 6 times more than typical ceramic package
Board-Level Area Array Interconnect Reliability
Temperature excursions in electronic systems
It is to be noted that while a device may be turnedon and off thousands of times a second the solderjoints only experience the average device-powerdissipation
Since the IC is the heat source it has a highertemperature than the chip carrier which in turn hashigher temperature than the solder joint
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Product reliability is an important factor especially in portableelectronics, because these increasingly powerful and morecomplex electronic equipment experience different kinds ofelectrical, thermal, mechanical, and thermo-mechanicalstrains and stresses in their service environments.
The importance of solder interconnection reliability is increasedmainly due to two reasons:
Firstly, higher interconnection densities
Secondly, the employment of lead-free solders, component under bumpor lead metallizations, and PWB protective coatings add to the
complexity of the interconnection metallurgies
Solder Joint Reliability
Reliability testing
Lead-free technologymore complex reactionsmore complex microstructures
Testing even more important than before
Better understanding of failure mechanisms underdifferent loading conditions is needed
Different combinations of various tests: Thermal cycling Drop-testing
Power cycling
Vibrational testing
Corrosive environment
Thermal annealing
etc.
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There are three major mechanisms of solder joint failure,although these often interplay with each other simultaneously.
1) tensile rupture or fracture due to mechanical overloading
2) creep failure, or damage caused by a long-lasting permanent load orstress
3) fatigue, or damage caused by cyclical loads or stresses.
One way to analyze solder joint reliability is to perform solderjoint modeling, or analysis of solder joint strengths andweaknesses using computer models.
Solder Joint Reliability
Ref:C Bailey, Universityof Greenwich
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Ref:C Bailey, Universityof Greenwich
Numerous studies have been made of the effect of geometry on thereliability
The most accurate models are finite element representations which considerplastic flow properties
In the most sophisticated cases also the time- dependent processes
Creep deformation
Fatigue crack initiation and propagation
These suffer from the lack of parametric generality
Other models, which are analytic and parametric in nature, are weakened bygross approximations in the solder behaviour and failure criteria
SMT-Joint Geometry and Design
dVdtW
V
VWW
e
e
ee
,
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Stress distribution (von Mises) FC-joint
Elastic analysis(also solder):
Heating: 0 100 C
Max stress =170 MPa
Time dependent deformation:
Heating: 0 100 C
Max stress =11 MPa
40 - 50 MPa
Life-time according tom Darveaux
Crack nucleation [cycles]
Crack propagation [m/cycle]
Cysles to fracture
210C
f WCN
43C
WCdN
da
dNda
aNN ff 0
a
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The joint design, including lead shape and height, and volume (shapeand size) of the solder, has great effect on the long-term performanceof a joint.
An extremely simplified equation for estimating the shear stress () in the jointis:
=CTE difference,
T=temperature change,
Dnp=distance from the neutral point (component centre) and
t=joint heightThe flexible component leads decrease the stress affecting the solder joint in
leaded SMT-components
SMT-Joint Geometry and Design
DT np
DT np
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DT np
DT np
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Coefficient of Thermal Expansion(CTE)
Global, local and internal effects resultfrom the CTE difference between
Component and PWB
Solder and metallizations
Different phases in solder etc.
Heating and cooling operations duringsoldering processes can result inextremely largeTs and temperaturegradients
Also power dissipation during use cancause problems
Complicated states of stress and strainmay result
SMT-Joint Geometry and DesignDT np
Aside from modeling, solder joint reliability is also assessedthrough reliability testing.
Reliability testing consists of subjecting representative samplesbearing the solder joint of interest to industry-standard
reliability tests so that:
1) factors that cause or accelerate the various solder joint failuremechanisms will be uncovered and understood
2) actual reliability data may be generated for further analysis.
Solder Joint Reliability
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Solder volume associated with a joint affects the stress distribution andcan also affect crack propagation rates once crack has been initiated
Poorly formed joints can have built-in stress concentration sites that providepremature crack initiation
Large solder volumes in leadless chip carriers have demonstrated better fatigueresistance than smaller- volume joints
Larger volumes distribute the applied stress over larger cross-sectional area
Also large solder volume provides additional area for the crack to propagatethrough
The main factors determining reliable solder joint are:
Uniform properties
Chemical composition
Microstructure
Joint shape
SMT-Joint Geometry and Design
Through-hole joint configurations refer to package types inwhich the component leads are inserted and soldered intopredrilled holes in PWB
Solder Joint Reliability- Through Hole Components
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As PTH-joints are generally very reliable especially from mechanicalpoint of view, most problems arise from the quality of coatings andmanufacturing process.
Some rules of thumb:
A gap of 150-200m between lead and hole wall is normally specified
The protrusion of lead should be kept small (0.8-2.0 mm) to minimize drainageof the solder fillet
The through-hole pad should be round and approx. three times the leaddiameter
To maintain appropriate fillet formation, the minimum height of the lead
should equal the pad width
Solder Joint Reliability- Through Hole Components
Process problems
Cold joints
Temperature of the surfaces are not high enough
Dissolution is slowed down/ prevented
Wetting problems
Macroscopic movement during cooling Voids/cavities
Crack nucleation sites
Solder bridging
Bath contamination (Zn, Cd)
icicling
Bridging (also wave pressure affects)
Solder Joint Reliability- Through Hole Components
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