Surface-Mount Solder attachments: due to questionable test regimens and extravagant claims, IPC developed a standard to ensure proper solder joint reliability tests.

The ideal solder joint forms a reliable, electrically continuous and mechanically sound connection. Appropriate design for reliability (DFR) is needed to ensure proper performance. Solder joints created using DFR, when manufactured with good quality, can perform in the operational environment for which the product was designed--for the complete design life.

Accelerated Testing Issues

DFR aspects are reviewed in IPC-D-279, Design Guidelines for Reliable Surface-Mount Technology Printed Board Assemblies. However, in many cases, adequate reliability should be confirmed by accelerated testing. The guidelines for proper accelerated testing are given in IPC-SM-785, Guidelines for Accelerated Reliability Testing of Surface-Mount Solder Attachments. IPC-SM-785 is a guidelines document, not a standard, and proper accelerated testing requires considerable resources and time. With no standard in place, highly accelerated test regimens have appeared--regimens that do not meet IPC-SM-785 guidelines--in tandem with extravagant claims as to what test results mean in terms of product reliability.

The ever-decreasing size of components now requires that solder joint reliability be designed into components. An objective measure is needed to provide a way to compare reliability among competing products. For this reason, IPC-9701, Performance Test Methods and Qualification Requirements for Surface-Mount Solder Attachments, was developed.

Reliability Test Requirements

While JEDEC tests deal solely with components, the primary purpose of IPC-9701, released in January 2002, is to test solder joint reliability threatened by the thermal expansion mismatch that occurs between a component and the printed circuit board (PCB). Therefore, completely different physical parameters and damage mechanisms should be considered. Since the PCB is a constant in most cases (consider FR-4--thick enough to prevent strain relieve by PCB bending), the tests are designed to show suitability, or lack thereof, of a given component for various operating environments. For test purposes, the PCB and the surface finish should be standardized in a manner that will not influence test results.

These facts should not prevent the use of the methodologies outlined in IPC-9701 to comparatively evaluate different surface finishes, or any other variable, as long as the deviations from IPC-9701 are clearly stated. Any extrapolation to product operating conditions, such as conditions noted in Appendix A of the document, would not be valid.

Tables 1 and 2 provide test conditions and qualification requirements from IPC-9701, along with resulting test temperature ranges ([DELTA]T) and mean cyclic temperatures ([T.sub.SJ]). Pertinent remarks are also included, regarding the preferred test condition and qualification requirements, as well as temperature cycling ranges that violate the caveats of IPC-SM-785.

IPC-9701 standardizes the performance test methods on five test conditions, ranging from a benign TC1 reference cycle condition of 0[degrees] <-> 100[degrees]C to the severe TC4 condition of -55[degrees] <-> 125[degrees]C. The number of thermal cycles (NTC) for qualification requirements varies from NTC-A, which equals 200 cycles (easily achieved at any of the test conditions and essentially assuring only proper solder wetting), to NTC-E, which equals 6,000 cycles. The dwell time at the T(min) and T(max) temperature extremes is 10 minutes for all test conditions.

The relatively low-acceleration TC1 test condition is the preferred reference test condition for benchmarking purposes, as the test most closely mimics actual use conditions and has the least likelihood of being dominated by extraneous damage mechanisms. The preferred qualification requirement for test condition TC1 is NTC-E, which equals 6,000 cycles.

IPC-9701 also includes the proviso that the temperature cycling ranges for test conditions TC3, TC4 and TC5 are likely to have more than one damage mechanism. This fact would violate the caveats for proper accelerated testing for solder reliability stated in IPC-SM-785--a consequence of the time-, temperature- and stress-dependent material behavior of solder. Such confounding of results, by multiple damage mechanisms, would cause problems with the extrapolation of the accelerated test results to product reliability estimates. Nevertheless, acceleration factors are provided for these test conditions to provide relative guidelines.

Acceleration Factors and Equations

Two acceleration factors (AF) exist: 1) AF (cycles)--relates the cyclic fatigue life of solder joints obtained in tests concerning the life of a product in a given use environment and 2) AF (time)--relates the time to failure of solder joints obtained in tests concerning the life of a product in a given use environment.

The acceleration factor in terms of cycles to failure is:

AF(cycles) = N(field)/N(test).

Acceleration in terms of time is:

AF(time) = AF(cycles) X f(test)/f(field).

IPC-9701 utilizes the Engelmaier-Wild solder fatigue model, featured in Appendix A of IPC-D-279, to estimate acceleration factors. Of course, other models exist that can be utilized for this purpose, but, since most of these models are based on empirical information from accelerated reliability tests of solder joints, the resulting acceleration factors should not differ significantly for models derived from solder fatigue data. The IPC committee anticipates that future revisions of the document will include acceleration factors from other models when they become available.

Assembly Standards Update

The following industry consensus standards and guidelines have been published this quarter:

* IPC-SA-61A, Post Solder Semiaqueous Cleaning Handbook, has been updated and covers aspects such as cleaning chemicals, equipment, processes, process control and environmental considerations.

* IPC-HDBK-838, Guidelines for Design, Selection and Application of Conformal Coatings, assists designers and users of conformal coatings in understanding the characteristics of various coating types, as well as factors that can modify properties when coatings are applied.

* IPC-2511B, Generic Requirements for Implementation of Product Manufacturing Description Data and Transfer XML Schema Methodology, identifies the generic requirements for implementation of product manufacturing description data and transfer. This version uses XML schema to facilitate Web-based data transfer.

* IPC-4202. Flexible Base Dielectrics for Use in Flexible Printed Circuitry; IPC-4203, Adhesive Coated Dielectric Films for Use as Cover Sheets for Flexible Printed Circuitry and Flexible Bonding Films; and IPC-4204, Flexible Metal-Clad Dielectrics for Use in Fabrication of Flexible Printed Circuitry, cover requirements for materials appropriate for flexible circuitry needs--from base films through adhesive-coated and metal-clad dielectrics.

* IPC-SMEMA-9850, Surface-Mount Placement Equipment Characterization, establishes the procedures to characterize (evaluate) machine placement capability for speed and accuracy.

The following standards are being balloted for industry acceptance:

* J-STD-0028, Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires

* J-STD-003A, Solderability Tests for Printed Boards

* J-STD-004A, Requirements for Soldering Fluxes

* IPC-HDBK-005, Soldering Paste Handbook, to support J-STD-005, Requirements for Soldering Pastes

* J-STD4)20B, Moisture/Reflow Sensitivity Classification for Non-Hermetic Solid State Surface-Mount Devices

* J-STD-033A, Standard for Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface-Mount Devices

* IPC-0040, Optoelectronic Assembly and Packaging Technology

* IPC-D-356. Bare Board Electrical Test Information in Digital Form

* IPC-2577, Sectional Requirements for Supply Chain Communication of Manufacturing Quality Assessment--Product Data exchange (PDX)

* IPC-4412, Specification for Finished Fabric Woven from "E" Glass for Printed Boards

* IPC-4552, Specification for Electroless Nickel/Immersion Gold (ENIG) Plating for Printed Circuit Boards.

Also, IPC-A-610, Acceptability of Electronic Assemblies, was recently adopted by the U.S. Department of Defense.

TABLE 1: Test conditions prescribed in IPC-9701.

Test Condition       T(min)          T(max)         [DELTA]T

TC1              0[degrees]C     +100[degrees]C   100[degrees]c
TC2              -25[degrees]C   +100[degrees]C   125[degrees]C
TC3              -40[degrees]C   +125[degrees]C   165[degrees]C
TC4              -55[degrees]C   +125[degrees]C   180[degrees]C
TC5              -55[degrees]C   +100[degrees]C   155[degrees]C

Test Condition       TSJ               Remarks

TC1              50[degrees]C     Preferred reference
TC2              37.5[degrees]C
TC3              42.5[degrees]C   Violates IPC-SM-785
TC4              35[degrees]C     Violates IPC-SM-785
TC5              22.5[degrees]C   Violates IPC-SM-785

TABLE 2: Qualification requirements prescribed in IPC-9701.

Qualification Requirement   Cycles   Remarks

NTC-A                       200
NTC-B                       500
NTC-C                       1,000    Preferred for TC2, TO & TC4
NTC-D                       3,000
NTC-E                       6,000    Preferred with reference TO

Jack Crawford is director of assembly, standards and technology with IPC, Northbrook, IL; [phone]; e-mail: [e-mail]

Werner Engelmaier is president of Engelmaier ASSOCiAtes, L.C. Ormond Beach, FL; [phone]; e-mail: [e-mail].