Electrical Failures Electrical Failures

IPC / SMTA Cleaning Workshop

November 16, 2010

IPC / SMTA Cleaning Workshop

November 16, 2010

Content Content

• Technology Innovation

• Device Interactions

• Tin Whiskers

• Soil Effects

• Complexities

Rapid Technology InnovationRapid Technology Innovation

• More performance in smaller platforms

– High Speed Processors

– Lead-Free Soldering

– Reliability critical driver

ReliabilityReliability

• The capacity of a device to perform as

designed

– For specific interval under stated conditions

– Minimal failures

• The Goal: Performs as promised every time

Device InteractionsDevice Interactions

Device Interactions Device Interactions

Surface Mount

TechnologySemiconductor

Fabrication

Advanced

Packaging

-Photoresists

-Plating

chemicals

-Solder pastes

-Fluxes

-Coatings

-Wafer Bumping

-Flip Chip

-Package on Package

Mostly IPC

standardsMostly SEMI

standards

JEDEC

Standards

Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.

Increase in Hardware FailuresIncrease in Hardware Failures

• Gaseous contamination– Chemical effects

• Creep Corrosion

– Mechanical effects• Compressive Stress

• Tin Whiskers

• Optical signal interference

• Friction

– Electrical effects• Circuit impedance

• Dendritic growth

• Arcing

ASHRAE (2008). Gaseous and Particulate Contamination Guidelines for Data Centers.

ReliabilityReliability

• Manufacturers are in a constant struggle to

– Maintain the reliability of their hardware

– Every Shrinking feature sizes

– Decreased distance signals travel

White residue flux migration

MiniaturizationMiniaturization

• Heat dissipation

– Air flow exposes circuits to gaseous contamination

• Non-hermetic sealed packages

– Moisture entrapment

• Decreased spacing

– Voltage differences increase the risk of ion migration

– Compressive stresses

• Corrosion

– Smaller component features reduces the distance corrosion needs to travel

ASHRAE (2008). Gaseous and Particulate Contamination Guidelines for Data Centers.

Co-Planar Board FinishesCo-Planar Board Finishes

• Coplanar board finishes are especially susceptible

– Immersion silver plating (ImAg) over copper

– Organically Coated Copper (OSP)

• Silver and copper are highly reactive to sulfur

– Levels as low as 3 ppm is enough to induce corrosion

• Silver is more noble metal

than copper

– In the presence of atmospheric

water, forms an electrochemical

cell

Sulfur Bearing Gases Sulfur Bearing Gases

• Even in absence of moisture can

– Attack copper and silver

–Migrate as silver and copper sulfide

– Breeches exposes

underlying board

finishes

– Corrosion grows

and leads to

electrical opens

e-

Electron / Metal Atom Flow

e-

e-

e-

e-e-

e-

e-

e-

e-

e-

e- M

Anode

CopperCathode

SilverM

- +

Galvanic Cell Galvanic Cell

• Galvanic cell potential from copper and silver metal

reduction

– Copper, being more active metal, represents the anode when

electrochemical reactions with silver take place

– Copper corrodes faster than other based oxidizing

environment.

Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.

Tin WhiskersTin Whiskers

Tin Whiskers Tin Whiskers

• Lead-Free = High tin alloys

• Whiskers bridge the gap between conductors

• Cause shorts

• Carry enough current to stop the circuit from

functioning correctly

+_

Copper Trace

Lead-free Solder

Whisker

Howell et al. (2010) Effect of Soldering Method and Flux Type on Tin Whisker Growth in

SAC 305. SMTA IWLPC, Toronto, CA.

Whiskers grow to relieve compressive stress in tin

Compressive Stress

Whiskers grow by atomic diffusion in a crystalline manner

Whisker GrowthWhisker Growth

Howell et al. (2010) Effect of Soldering Method and Flux Type on Tin Whisker Growth in

SAC 305. SMTA IWLPC, Toronto, CA.

Greatest Incidence of whiskers on the edges of the traces

This is probably because

• Greatest concentration of flux residue

• Exposed copper / thin tin coating

Whisker Location Whisker Location

Howell et al. (2010) Effect of Soldering Method and Flux Type on Tin Whisker Growth in

SAC 305. SMTA IWLPC, Toronto, CA.

Soil Effects

Assembly Contamination Assembly Contamination

• Stencil cleaning– Wet solder paste – Chip bonder adhesives

• Bare Board– Oxides – Ionic contaminants

• Flux residues – Organic acids – Rosin– Resin structures – Polymers – Functional additives

Flux Residues – A Common Enemy Flux Residues – A Common Enemy

Stencil

Printing

UnderfillSoldering FLUX

Conformal

Coating

Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.

Soil EffectsSoil Effects

• Electromigration: “Electrochemical Migration” (ECM)

– Surface Mount Technology:

– Movement of ions under a potential gradient–

• Closely associated with SIR (Surface Insulation Resistance)

• BETWEEN adjacent metal conductors

• Electromigration (EM)

– Semiconductor / Packaging:

– Movement of atoms caused by electrons

flowing through a metal

– WITHIN a metal conductor

Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.

Electrochemical Migration

No-clean, Halogen-free FluxesElectrochemical Migration

No-clean, Halogen-free Fluxes

Mackie, A. (2009, Oct). Electromigration – Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.

Tin Salt Migration Tin Salt Migration

• Partially removed flux residue under component

– Dendritic growth

– Creep corrosion

Flux Residue Migration under

Conformal Coating

Flux Residue Migration under

Conformal Coating

Flux Left under Components Flux Left under Components

• Feature size reduction

– Increase failure risks

Ion Migration in an Electric Field Ion Migration in an Electric Field

• Propagated by

– The charge balance at the interface

– Current density entering and leaving the device

– Electrolyte from moisture causes ions to split and

form dendrites

AnodeCathode

High VoltageHigh Voltage

Source: FET1_Drain_Joint_Failure_18018797_3_jpg.htm

Shorting / Sparking / Failure Shorting / Sparking / Failure

Electrochemical Migration

Key FactorsElectrochemical Migration

Key Factors

• Contamination– Soils

– Gaseous Moisture (%RH)

• Adjacent metallic conductors

• Electric field (potential gradient)

• Ionizable metals

• Hydrophilic / mildly-hydrophilic continuous phase

Failure Region Failure Region

ConclusionsConclusions

• Reliability of hardware is more prone to risk

due to ever shrinking feature sizes

• Move to Lead-Free increases risk factors

– Co-Planar Board Surfaces

– Creep Corrosion

– Tin Whiskers

• Flux residues bridge conductions

– Electromigration risks

• Removal and mitigation of contamination

sources is increasingly important to assure

reliable devices that perform as promised

QuestionsQuestions

Author Author

Mike Bixenman

Kyzen Corporation

mikeb@kyzen.com

615-983-4530 Desk

615-584-9089 Cell