solder reliability

7/28/2019 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.


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


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


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.


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


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.



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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


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


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 reliability

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