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Draft IPC 9708

Originator: Mudasir Ahmad

Rev1.2

IPC-9708

Test Methods for Characterization of PCB Pad Cratering

1 SCOPE

This document provides test methods to evaluate the susceptibility of Printed Circuit Board (PCB) materials and

designs, to cohesive dielectric failure underneath surface mount (SMT) attach pads. The test methods can be used

to rank order and compare different PCB materials and design parameters, but do not define acceptance criteria.

1 .2 Performance Classification This specification recognizes that surface mount assemblies (SMAs) will be

subject to variations in performance requirements based on end use. While Performance Classes are defined in IPC-

6011, these performance classifications are not specific as to the required reliability. At this point in time, the

acceptance criteria need to be established by agreement between customer and supplier.

1.3 Definition of Terms The definition of all terms used herein shall be as specified in IPC-T-50, except as

otherwise specified in Section 3.

BGA

Ball Grid Array Package

PCB/PCB

Printed Circuit Board/Printed Circuit Board

Component

Packaged semiconductor device

Solder Joint/Ball

The solder interconnection between a component and PCB

Pad CrateringThe formation of a cohesive dielectric crack or fracture underneath the pad of a surface

mount component, most commonly BGA packages

1.4 Interpretation “Shall ” is used throughout this specification whenever a requirement is intended to express a

provision that is mandatory; deviation may be considered if sufficient data is supplied to justify the exception.

The words “should” and “may” are used whenever it is necessary to express non-mandatory provisions. “Will” is

used to express a declaration of purpose.

To assist the reader, the word “shall” is presented in bold characters.

2 APPLICABLE DOCUMENTS

The following documents are applicable and constitute a part of this specification to the extent specified herein. Sub-

sequent issues of, or amendments to, these documents will become a part of this specification. Documents are

grouped under categories as IPC, Joint Industry Standard, ITRI, EIA and others depending on the source.

2.1 IPC

1

IPC-T-50

Terms and Definitions for Interconnecting and Packaging Electronic Circuits

IPC-D-279

Design Guidelines for Reliable Surface Mount Technology Printed Board Assemblies

IPC-TM-650 Test Methods Manual

2

IPC-S-816 SMT Process Guideline and Checklist

IPC-9502 PCB Assembly Soldering Process Guidelines for Electronic Components

IPC-6012 Qualification and Performance Specification for Rigid Printed Boards

IPC-4101 Specification for Base Materials For Rigid and Multilayer Printed Boards

2.4.1 Electronic Industries Association

4

JESD22-B117A “BGA Ball Shear,” July 2006

Deleted:

1

0

JESD22-B115 Solder Ball Pull, May 2007

3 TERMS, DEFINITIONS AND CONCEPTS

3.1 General

Mechanical bend and shock tests are routinely performed on surface mount assemblies to ensure that they can

sustain anticipated production, handling and end use conditions. The strains and strain rates applied to SMT

assemblies during bend and shock testing can lead to a variety of failure modes in the vicinity of the solder joints.

Figure 1 shows the failure modes that are known to occur in BGA assemblies subjected to mechanical loading

conditions.

The prevalence and distribution of these failure modes depend on several factors, including the solder metallurgy

used, the package type, construction, component-to-PCB-pad size ratio and PCB materials. Usually, multiple failure

modes occur concurrently, at different strain and strain rate levels.

Package Substrate

Solder Ball

B

A

C

D

A

B

C

D

E

F

G

Legend

Package Pad Lift/Crater

Pkg Base Metal/IMC Interface Fracture

Pkg IMC/Solder Interface Fracture

Bulk Solder Fracture

PCB IMC/Solder Interface Fracture

PCB Solder pad/IMC Interface Fracture

PCB Pad Lift/Crater

PCB

E

F

G

Figure 1: Example Failure Modes Occurring in a BGA Printed Circuit Board Assembly

To mitigate the failure modes occurring at marginal strain levels, it is crucial to decouple the failure modes and identify

the weakest link in the assembly. Unfortunately, failure modes cannot be easily differentiated in high speed monotonic

bend tests, where the test duration is fairly short and multiple failures occur in rapid succession.

In shock testing, the entire assembly is subjected to incremental G-levels and gross failures are detected only if a

change in strain level or electrical resistance is detected. The onset of pad cratering (failure mode G) may not have an

electrical signature. An example of a pad cratering failure mode is shown in Figure 2. [1-6]

Several factors could play a role in the increased prevalence of this failure mode for Pb-free assemblies than in SnPb

assemblies.

a. Pb-free solders are generally stiffer than SnPb solders. Consequently, they can transfer more of the applied

global strain to the PCB.

b. Phenolic-cured PCB materials typically used in Pb-free assemblies are more brittle than conventional dicy-

cured FR4 materials.

c. The higher reflow temperatures which Pb-free assemblies are subjected to lead to higher strains in the

2