BGA Reliability TestingFundamentals

Nikolai Mäntyoja 06.08.2014

Contents

• Introduction

• Vibration Testing

• Crack Initiation

• Thermal Cycling

• Recrystallization/recovery

• Combined Testing

• Compressive Load

• References

Introduction

• Dominant failure mode for solder joints is low cycle thermal fatigue which is caused by CTE mismatch during thermal cycling but for portable devices the shock impact is often more important

• The largest strain often occurs at the edge joint (depending on the die size), which is a probable place to crack or recrystallization occur

• The failure is usually defined as a change in electrical resistance. The same definition is used in HEVI-3 tests

• The drawback of event detector is the count of one event when there can be multiple fails in one measuring window

• Components at different locations experience different stress levels• Solder properties and strain rate are a function of the temperature!

Introduction• Simulations demonstrate that the maximum stress/strain is in

the edge/corner joints

• For individual solder joint, the maximum stress/strain is in the corner regions

• Both assumptions have been confirmed by cross-section samples of HEVI-3 project

Vibration Testing

• Vibration induced damage (stress) is proportional to board strain and that strain is proportional to board curvature

• One example for vibration testing time: 2237 hours, until 40% is failures

• Usually vibration is random vibration from 50 Hz to 1000HzWith a 0.1 G^2 / Hz PSD (high) to 0.045 G^2/Hz PSD (low)

• Cracks usually start near the interface and propagate through the solder transgranularly (confirmed by HEVI-3)

Vibration Testing

• Common opinion: low amplitude vibration causes only elastic material response

• In reality: viscoplasticity and creep should not be ignored• Vibration response at RT is close to elastic and plastic response is

obvious in elevated temperatures• Vibration has significant effect on the inelastic behavior of solder

joint, especially when coupled with thermal cycling• The first mode has the most influence on solder joint fatigue

Crack Initiation

• Crack propagation consists of three stages– Crack initiation (primary crack)– Stable propagation until 60% width is reached– Accelerated propagation (secondary crack)

• During vibration, crack initiation accounts for about 15% of total fatigue life and majority of the time is spent in a stable growth stage.

• Combined tests cause cracks on both sides of the solder (has been proven by HEVI-3)

Thermal Cycling

• TC is usually from -50-55C to 125-150C• With a dwell time of 12-15 minutes (enough to stress relaxation)• And ramp time of 20-24 minutes (enough to avoid thermal shock)• TC tests normally last several thousand cycles before failures /

consistently recrystallization occur• Those recrystallized regions provide favorable sites for cracks to

nucleate and propagate intergranularly = recrystallization-assisted cracking

• With enough cycles, most of the cracks should initiate intergranularly

Thermal Cycling

• The fatigue life decreases with a larger temperature range. The temperature range for HEVI-3 tests is from -35C to 95C, which is relatively short. This can cause longer testing periods compared to literature

• Damage of solder interconnections is a result of the accumulation of internal energy in the form of dislocations in the plastically deformed regions

• Some cracks can propagate without recrystallization but not through the whole solder (confirmed by HEVI-3)

Recrystallization/recovery

• High thermomechanical stresses lead to plastic deformation. A fraction of the energy from the plastic deformation is stored in the metal in the form of dislocations. The energy is released during recovery, recrystallization and grain growth.

• Recovery and recrystallization are two competing processes but recovery has a lower activation energy.

• Cracks propagate intergranularly through the recrystallized areas

Recrystallization/recovery

• Zigzag shape of crack might be an indication of recrystallization• When recrystallized grain size is large, cracks can propagate also

transgranularly or on mixed mode• Recovery is annihilation of lattice defects by their movement to the

grain boundaries where they disappear• Efficiency of recovery is diminished by impurities, second phase

particles and tangling of dislocations• Formation and rotation of subgrains by recovery

Combined Testing

• Earlier BGA solder joint failures with combined loading than with seperate TC or vibration load (HEVI-3 supports this)

• Traditional linear superposition overestimates the lifetime since it doesn't take into account the interaction between different loadings

• → Combined testing damage accumulation is not linear• An example: Outermost solder ball’s lifetime goes to 1/9th with

combined testing compared to TC only

Combined Testing

• Too high vibration can neglect the effect of temperature swings– The vibration level should be adjusted low enough for combined effects– With high vibration ductile crack propagation and no observable microstructural

changes

• None of the cracks initiate completely through the IMC (confirmed by HEVI-3) since IMC cracks appear mostly under mechanical shock or high amplitude vibration, where high stress and strain rate take place

• With vibration only, the acceleration factor can be up to 41 or 88. With combined testing, acceleration factors up to several hundreds can be achieved

• With combined testing both transgranular and intergranular cracks can be found

Compressive Load

• A heatsink on the chip causes compressive load. This might cause more creep collapses and bridging of the solder joints.

• Simulations indicate that compressive loading creates more damage in the solder bumps(1st level interconnections) but less damage on solder balls (board level interconnections)

• Failures are usually in the packages outmost corner• Pad/solder interface crack dominant failure mode with compressive

loading

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