Long-term Reliability of Lead-free Soldered Assemblies...2 A Lead-free Case Study ... free assembly,...

Long-term Reliability of Lead-free Soldered Assemblies

SBS Technologies 08.02.2005

Introduction

The EU directive known as RoHS will become effective July 1, 2006.

Thereafter, substances defined in the directive such as lead, mercury,

chromium VI and some common flame retardants such as PBB and PBDE,

may no longer be used in new electrical and electronic equipment. The

replacement of lead in lead/tin solder creates some challenges and

uncertainties, including questions about the long-term reliability of assemblies.

Through research on long-term reliability, SBS Technologies has been able to

answer several important questions about the impending change to lead-free

solder assembly.

© 2005 SBS Technologies, Incorporated, All rights reserved. 2

A Lead-free Case Study

Confronted with an EU legal mandate and its consequent uncertainties, the European subsidiary

of SBS Technologies, which is located in Augsburg, Germany, began research into lead-free

issues in 2003. The German facility manufactures robust and reliable high-performance

computers for industrial, telecommunications and aerospace applications. It is certified according

to the appropriate quality (ISO 9001:2000) and environmental (ISO 14001) standards, and also

by numerous customer audits. In fact, a major client desired to receive prototypes of a fully lead-

free assembly, in their normal build grade, by the second quarter of 2005 at the latest. This

request became the basis for a thorough investigation.

The final assembly and in-house testing would take place at SBS in Augsburg, while the main

serial production would be outsourced to the contract manufacturer. The parties involved got

together and launched a pilot project in 2003, in which a lead-free soldered assembly already

being supplied to the customer was subjected to accelerated ageing and finally tested. To ensure

the independence of the results, the research was carried out in the department of manufacturing

automation and production systems at the University of Erlangen, Germany. The results were

submitted early in 2004.

Long-term reliability of electronic assemblies

Before products made with new technology or materials can be introduced, their long-term

performance requirements must be fulfilled. The increasing use of electronic assemblies in

safety-sensitive systems in medical and automotive technology (in the latter instance, under

sometimes extreme external influences) can only mean the significance of lifetimes and reliability

will further increase. For reasons of thermal, mechanical and climatic stresses, alterations and

damage can result to the circuit boards, the components mounted on the assemblies, or the

solder connections, with detrimental consequences ranging up to complete equipment failure.

Thus, the reliability of electronic assemblies largely depends on the lifetime of the solder

connections. The significant factors influencing the lifetime of a solder connection are represented

below in Figure 1.


Figure 1: Lifetime of solder connections - requirements and influencing factors

Long-term Reliability Testing Strategies

Probably the most significant type of damage to solder connections is fatigue crack formation

through temperature changes, so this has decisive importance in testing. A multitude of various

test standards exists for the determination of the reliability of electronic assemblies. Based on

these tests, it is possible to make decisions regarding the suitability of various materials in a

reasonable amount of time, and to qualify new procedures. In principle, there are two standard

procedures for accelerated thermal-mechanical ageing analyses:

▪ rapid temperature changes in the two-chamber test system, and

▪ slow temperature changes at controlled rate

Both procedures have their advantages and disadvantages. SBS Technologies used both

procedures to enable the most comprehensive assessment possible of the long-term reliability of

lead-free assemblies.

Lifecycle duration of soft solder joints

Micro structural changes Building/growth of

Inter-metallic phases

Characteristics of material bonding

Environmental oxidation

Definition of failure mechanisms

Test and field conditions

Micro structural Initial conditions

Solder joint geometry


Rapid temperature changes in two-chamber test system

Rapid temperature changes in the two-chamber test system produce high mechanical tensions,

and enable fast testing progress on account of the short transition time between warm and cold

chambers. The assemblies are subjected to a defined number of cycles at temperatures of 125°C

and -40°C. The total number of cycles in this procedur e is usually 1500, with mechanical

investigation after 750 cycles.

The strength of contacts is associated with the security of their electrical conduction, making it a

further decisive factor in the reliability of an electronic assembly. Therefore, contact strength

values obtained with new component types such as solder paste are qualified through analysis

with surface-mount components in a comprehensive temperature shock test series. These

investigations are mostly performed with test procedures based on the force needed to shear off

two-pole components. The breaking point usually occurs, given sufficient strength of the substrate

metallization, at the contact point with the component, as it is here that the smallest load-bearing

surface cross-section exists, and friability arises in this region through the formation of

intermetallic phases. Figure 2 shows the test set-up for measuring shearing force.

Figure 2: Test set-up for measuring shearing force

In the manufacture of assemblies, the surface tension of the fluid solder is reduced by using

suitable fluxes, while at the same time, cleansing the solid surfaces increases their surface

tension. Limitation of the boundary surface tension is obtained through enrichment of alloy

elements and higher temperatures. A qualitative representation of wetting processes in the

boundary surface between solder and component can be obtained for example through contact

angle measurements or wetting force measurements.

shearing force Fs

Chisel

Sheared off CR0805

chisel

Component CR 1206

Force measuring tool


Results after 750 temperature shock cycles

The study provided for the parallel investigation of identical assemblies with lead-free and lead-

containing solder for their long-term endurance under rapid temperature changes, with defect

analysis after 750 and 1500 cycles. In addition to optical inspection of the contacts, destructive

testing was also performed to enable quantification of the results. For the purpose of the analysis,

the highly-integrated assemblies offered an adequate number of two-pole components, small-

outline components (SO), quad-flat-pack components (QFP) and area-array housings such as

ball grid arrays (BGA). As shown in figure 3, special areas were designated for analysis after 750

cycles, which were destructively tested to assess their final condition.

Figure 3: Designated areas on the circuit boards for destructive testing


Two-pole components

Two-pole components are stressed by temperature shock mainly over the length axis. The

induced force can result in the formation of cracks along the intermetallic phase during the

lifetime. Generally the failure is detectable on the circuit board metallization side, as this is where

the largest forces arise. By shearing off the components, quantification is obtained. As shown in

table 1, after 750 cycles a sufficient number of component contacts had been investigated by

shearing force measurement. The same component shape was always designated. The failure

analysis shows the solder break lies in nearly all cases on the circuit board side. This evidences

the sufficient strength of the metallization. The measured values for lead-free and lead-containing

tests showed no reduced strength. In terms of average value, strengths about 35N resulted. The

standard deviation of the measured values with lead-free tests was significantly higher than with

lead-containing. Conclusions could only be finally drawn however after 1500 cycles.

MEASURED VALUES IN N

AVG

STANDARD DEVIATION

28.0 13.8 19.0 11.6 12.4 18.2 17.4 18.8 15.4 17.6 18.2 14.2 21.0 19.2 18.8 16.4 15.2 28.6 31.8 16.2 25.0 21.8 24.0 19.8

PWA 1 Lead Free 17.2 21.4 26.6 17.4 28.2 20.0 24.4 20.6 14.6 20.4 17.0 31.6

20.05

5.18

14.0 17.2 19.6 19.8 21.2 21.0 18.8 19.2 36.2 28.6 20.4 18.6 24.2 31.2 22.6 29.4 21.6 13.0 30.6 18.4 22.8 20.6 20.6 17.2

PWA 3 Leaded

19.2 25.8 14.8 23.0 21.4 24.8 21.8 18.4 28.6 13.0 21.8 23.6

21.75

5.18

20.0 19.6 16.8 18.8 26.0 23.0 16.8 21.2 27.8 21.2 20.0 17.6 19.4 14.8 19.6 16.2 12.4 21.8 17.4 19.6 21.4 22.0 17.8 18.2

PWA 2 Lead Free 22.4 20.4 21.0 18.6 20.2 19.6 17.0 19.6 17.0 19.0 20.8 15.0

19.44

2.94

Table 1: Measured shearing strength after 750 cycles

Small-outline components

Metallographic microsectioning was applied to the two-pole SO components before measuring

the contact strengths. Independently of the solder type, the microsections revealed clear cracks.

The crack formation extends from the heel of the joint along the intermetallic phase throughout

the whole connection. Thus there was complete failure. Figure 4 shows this was complemented

by the presence of a large pore under the component lead. In the assessment of the texture

however, no negative effects could be detected to explain the failure of the solder connection.

The intermetallic phase is somewhat thinner and fine-grained, and a darker stripe is clearly

recognizable, indicating an oxide layer. This could be a reason for the weakness of the

connection. Figure 5 shows the microsection of a contact with lead-containing solder.


SBS was aware that problems could arise at this point. The expansion coefficient of the Alloy42

Leadframe of the SO component (TSOP housing) is significantly higher than that of the circuit

board and the solder. Given the temperature difference of 165ºC and the rapid change time in the

2-chamber system, great mechanical stresses are created that can lead to tearing of the solder

connection. IBM already described this in an article in 1993. SBS did not, however, refrain from

this long-term stress because it was also desirable to investigate the other components. In the

normal course of soldering or operation, such an extreme temperature change does not occur. In

the second test, no cracks whatever arose, as the temperature change was not as extreme.

Figure 4: Microsection through a contact with lead-free solder

Figure 5: Microsection through a contact with lead-containing solder


Ball Grid Arrays

In the analysis of BGAs no cracks could be detected. On the other hand, voids were

conspicuous, which could be detected in nearly all contacts. The voids are not enlarged by the

temperature cycles, implying gas-tight metallization. For this reason, moreover, they could not be

explained solely by micro-vias (Figure 6).

Figure 6: X-ray pictures of BGA connections (0 cycles/750 cycles; lead-free)

The polished section analysis shows a good, fine-grained texture for both solder types. Voids

could be found on both the component and the circuit board sides. The intermetallic phase

formation is adequate and, as with the SO components, very fine-grained.

Results after 1500 temperature shock cycles The analyses conducted after 750 cycles showed clear failures on a particular component, which

was to be expected. Nevertheless, for the final assessment, the long-term reliability of the entire

system was assessed after 1500 cycles. For this test, as after 750 cycles, optical, X-ray-optical

and destructive testing were performed. The selected components are marked in Figure 7.


Figure 7: Areas analyzed after 1500 temperature shock cycles

Components used for microsection

Area with 0805 components used for shearing force measurement


Two-pole components

With the two-pole components, a proportion of cavities could almost always be established in the

X-ray analysis, independently of the testing. This varied with the number of cycles, and

represented a fundamental weakening of the solder connections. With all sheared-off

components, the solder break could be recognized on the component side (Figure 8).

Figure 8: Optical inspection of the breaking point of a sheared-off two-pole component

Thus, weakening of the circuit board metallisation can be excluded. The shear force

measurement confirmed the optimal formation of the solder connections with the two-pole

components. As the solder connection can be broken with similar force to other two-pole

components, dependence on a particular component type is not in question (Table 2).


MEASURED VALUES IN N AVG. VALUE

STANDARD DEVIATION

15.4 8.8 13.8 20.2 14.0 17.2 18.8 17.8 16.8 21.8 13.0 23.4 13.2 12.0 18.0 13.4 16.0 20.2 19.6 20.6 14.8 13.4 20.6 14.6 10.4 13.8 18.6 8.8 13.2 12.8 15.2 14.8 15.6 19.2 17.0 16.4

PWA 1 Lead Free Msmt 1 15.0 16.2

15.91

3.43

16.7 16.2 11.3 19.9 18.3 19.1 14.0 16.5 20.2 14.6 10.6 14.9 15.6 16.4 11.4 16.8 15.4 17.2 25.5 15.9 19.9 19.8 13.9 14.9 18.9 20.1 16.0 21.1 14.6 15.3 16.7 17.4 20.1 16.1 11.5 16.2

PWA 1 Lead Free Msmt 2 17.0 17.6 19.5 16.0 17.0 16.3 18.4 16.1 15.7

16.72

2.82

15.0 12.2 23.0 10.2 15.4 16.8 15.6 11.6 9.8 14.8 16.4 13.6 11.0 13.2 14.4 21.2 11.4 16.4 11.6 24.4 15.6 16.8 23.8 26.6 11.6 19.4 15.4 19.4 17.6 16.2 10.2 18.2 19.6 15.0 17.4 18.4

PWA 2 Lead Free Msmt 1 15.2 9.8

15.90

4.21

17.2 19.8 12.0 20.6 18.8 20.2 12.6 17.0 21.4 15.2 11.2 12.6 16.2 17.0 15.0 17.4 13.0 17.8 26.2 16.4 20.6 20.4 14.6 15.4 19.6 20.8 16.6 21.8 15.2 13.0 17.2 18.0 21.0 18.8 12.2 16.8

PWA 2 Lead Free Msmt 2 17.6 18.2 20.6 16.8 17.6 17.0 19.0 16.8 14.5

17.28

3.10

Table 2: Measured shearing strengths after 1500 temperature shock cycles

Small-outline components

In the analysis of SO components after 750 cycles, the clear cracks in the solder connections

could be traced back to the nickel/iron alloy of the component leads. For this reason, after 1500

cycles a SO component with copper leadframe was analysed. The poor qualities of the nickel/iron

alloys were confirmed by the microsections. But no crack formation was established with the

copper-leadframe components. The electrical function was intact and the mechanical strength

was adequate for the end-of-lifetime requirement. This was true both for the lead-free and lead-

containing solder connections. The texture with both connection media is fine-grained, the

intermetallic phase can clearly be recognized and meets the requirements. In the X-ray analysis,

voids at the critical connection heels were still detectable – these could possibly support crack

formation. From the external optical appearance, no damage could be established, the solder

connections are good and the meniscuses correspondingly developed.

Ball Grid Arrays

Also for the BGAs, the texture of the solder connections after 1500 temperature cycles could be

characterised as good. Fine texture with few grain boundaries and clear phase formation

underlined the quality of the connections. Further X-ray analyses again showed many voids,

which nevertheless are mostly classified as uncritical. The optical analysis showed the difference

in surface appearance between lead-free and lead-containing solder connections, with the

formation ideal from quality standpoints.


Gradual temperature change at controlled rate

In order to adapt the gradual temperature change at controlled rate to the actual field

requirements, the acceleration factor was calculated with a slightly modified Coffin-Manson

formula (IPC-SM-785). An important factor influencing the test parameters of temperature range

and cycle length is the operational requirements set by the manufacturer. These were specified in

the project according to ETS 300-019-1-3 as follows: room temperature +5ºC to +40ºC; long-term

average temperature +25ºC; temperature change 0.5ºC/minute; temperature cycle 1 cycle with

maximal 15ºC/24 hours; relative humidity 5 to 85%.

With this accelerated ageing, a real lifetime of ten years use according to telecommunications

specifications was simulated. The calculation resulted in 375.9 cycles. The effects of moisture

were not taken into account. The test lasted 376 cycles. Functional tests and failure analysis

where carried out after 200 cycles and 376 cycles. The same functional and failure analysis tests

were used as were used with 750 and 1500 quick temperature cycles.These analyses

impressively confirmed the positive results of the analyses after the rapid temperature changes

(Figures 9 to 12).

Figure 9: Optical analysis of the solder formation on an SO component

Leadfree connection, PWA 1 Leaded connection, PWA 2


Figure 10: Microsection analysis of the contacts on a BGA

Figure 11: X-ray pictures with voids

leadfree

leadfree

leaded

leaded


Figure 12: Optical analysis of solder formation on a BGA

Summary

In this study, surface-mount electronic assemblies were investigated for their long-term reliability.

Assemblies directly from manufacture were compared with assemblies that had been subjected

to an accelerated ageing process. To implement accelerated ageing, the assemblies were

exposed to temperature changes with defined parameters, corresponding to a real duty period of

ten years according to telecommunications specifications.

In addition to the destructive test procedures of shearing force measurement for testing the

strength of the solder connections, and the metallographic microsection investigations for

examining the texture, the electrical functions were previously tested and X-ray pictures of the

area-array components were taken for the detection of defects.

Conclusion

In conclusion, it was established that, in regard to long-term reliability, the tested assemblies

showed satisfactory results. After the artificial ageing, neither a visible growth of intermetallic

phases, nor defects in the texture, such as cracks, could be found, which was confirmed by the

shearing forces remaining stable in comparison to the initial state.

Leadfree connection, PWA 1 Leaded connection, PWA 2


The cracks in some SO components could be traced back to the poor suitability of the nickel/iron

alloy (Alloy42 Leadframe), and rectified. The X-ray pictures, which indeed showed a high

proportion of voids in the solder, and the in-house function tests carried out by SBS

Technologies, confirmed the integrity of the assembly. This signifies that the lead-free assemblies

completely fulfilled the requirements for long-term reliability, and suggested the conclusion that

SBS Technologies will attain its goal of basing the manufacture of complete surface-mount

assemblies on lead-free solder by the end of 2005.


© 2005 SBS Technologies, Inc. All rights reserved. SBS Technologies and the SBS logo are trademarks of SBS Technologies, Inc. All other brand names and product names contained herein are trademarks, registered trademarks, or trade names of their respective holders. This document contains information, opinions and conclusions that the author believed to be accurate as of the date of its writing, but SBS does not guarantee the accuracy of any portion of this document. This document and its contents are provided as is, with no warranties of any kind, whether express or implied, including warranties of design, merchantability, and fitness for a particular purpose, or arising from any course of dealing, usage, or trade practice. If you reproduce any parts of this document, you must reproduce and include all SBS copyright notices and any other proprietary rights notices. In no event will SBS be liable for any lost revenue or profits or other special, indirect, incidental and consequential damage, even if SBS has been advised of the possibility of such damages, as a result of the usage of this document and the software that this document describes. RESTRICTED RIGHTS LEGEND Use, duplication, reproduction, release, performance, display or disclosure by the Government is subject to restrictions set forth in subparagraph (b)(3) of the Rights in Technical Data and Computer Software clause at 48 CFR 252.227-7013. SBS Technologies, Inc., 7401 Snaproll NE, Albuquerque, NM 87109

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