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issue: December 2004 APPLIANCE Magazine

Engineering - Relays
Impact of Lead-Free Soldering on the Reliability of Relays


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by Dr. Werner Johler, Tyco Electronics AXICOM, Au, Switzerland

In recent years, environmental issues have become more and more important. Many materials and substances are forbidden for use in products and in manufacturing because they are toxic, ozone-killing, or problematical in some other respect.

Figure 1. Comparison of temperature profiles for leaded and lead-free reflow solder processes.

One of the critical elements that is still used is lead, which is also used in tin-lead solders. Leaded solders have predominated in electronics assembly as they are relatively inexpensive and perform reliably under a variety of operating conditions. They also have unique characteristics, such as a low melting point, high-strength ductility, high-fatigue resistance, and high conductivity, all of which result in highly reliable soldered joints. In recent years, however, there has been an industry-wide push to convert to lead-free solders. Currently, there are no globally valid rules or legislation available which control the use of or ban on certain materials. However, there are some legislative activities occurring in certain parts of the world: Europe and Japan have rules in place that will go into affect in July 2006, and China is currently in talks about similar legislation.

In addition to the logistical challenge, the transition from tin-lead to lead-free solder will pose some technological challenges. Possible elements to be substituted for lead are silver, copper, bismuth, indium, zinc, and antimony. The alternatives should have a low melting temperature and should also be low cost, readily available, and non-toxic. Today, SnAg3.5Cu0.7 for SMD soldering and SnCu0.7 for wave soldering are considered to be the most promising candidates to be substituted for tin-lead. Using lead-free solder (SnAg3.5Cu0.7 – melting temperature = 217°C) will result in an increase of the peak temperatures during soldering, and at the same time, an increase in the duration of the process must be expected. Most critical will be SMD soldering, as the thermal stress for electromechanical devices is already very high, even when tin-lead is used as solder. In order to not destroy the components by overheating, smaller process windows, and even changes of equipment might be necessary.

Reflow Soldering - Besides the higher peak temperature, the following points have to be considered:

  • The lower the difference between the temperature on the soldered joint and the maximum air temperature, the better the process and the less the components are stressed.

  • Oxidation must be prevented by flux in order to allow proper formation of the soldered joints. In addition, an inert atmosphere (N2) will prevent oxidation and improve process quality.

    Typical temperature profiles are given in Figure 1.

    Wave Soldering - For these relays, no relevant changes will be implemented for lead-free wave soldering. Unchanged process temperatures can be expected and, therefore, the same resistance to soldering heat must be provided by the relays.


    Profile
    SnPb Eutectic Assembly Pb-FreeAssembly
    Feature
    Large Body Small Body Large Body Small Body
    Ramp up rate (TL to Tp)
    3°C/s max. 3°C/s max.
    Preheat
       
    -Temperature Min (Tsmin)
    100°C 150°C
    -Temperature Max (Tsmax)
    150°C 200°C
    -Time (min to max) (ts)
    60–120 s 60–180 s
    Tsmax to TL (Ramp up Rate)
      3°C/s max.
    Time maintained above:
    183°C 217°C
    Temperature (TL)/Time (tL)
    60–150 s 60–150 s
    Peak Temperature Tp[°C]
    225+0/-5 225+0/-5 225+0/-5 225+0/-5
    Time within 5°C of actual
       
    Peak Temperature (tp)
    10–30 s 10–30 s 10–30 s 20–40 s
    Ramp down Rate
    6°C/s max. 6°C/s max.
    Time 25°C to Peak Temp
    6 minutes max. 6 minutes max.
    Figure 2. Description of reflow soldering profiles for tin-lead and lead-free assemblies.
    Small bodies: package thickness < 2.5 mm and volume < 350 mm3
    Large bodies: package thickness less-than or equal to 2.5 mm and volume less-than or equal to 350 mm3
  • Standardization

    Standards for lead-free soldering processes have been developed by the International Electrotechnical Commission (IEC 60068-2-58) and IPC/JEDEC (J-STD-20B and J-STD-20C). The IEC60068-2-58 standard describes the test methods to be applied for both tin-lead and lead-free solders. Test methods for solderability, resistance to dissolution, and the soldering heat for surface mounted devices are described.

    The IPC/JEDEC J-STD-20B and J-STD 20C, respectively, describe the moisture/reflow sensitivity classification for non-hermetic solid state surfaces. Both standards describe the solder profiles that have to be applied to determine the characteristics of components (see Figure 2). Most of them are also applicable to electromechanical relays.

    Electromechanical SMD relays must be tested for the following characteristics in order to qualify them for lead-free soldering processes: wetting, resistance to soldering heat, and determination of moisture sensitive level.

    During the transition time, it is suggested that lead-free components be soldered with tin-lead solder, or tin-leaded components mounted with lead-free solder.

    Figure 3 (top). Process diagram for the reflow wetting tests and resistance to soldering heat tests.

    Figure 4 (bottom). Internal pressure increase is dependent on the process temperature and materials used and moisture in the plastics.

    Electromechanical Relays

    More than 2/3 of the telecom and signal relays are built for surface mounted assembly (SMT). Even relays for through-hole assembly have to be surface-mount compatible (SMC). The higher temperature (255°C) and the longer process duration for lead-free assemblies result in significant thermal-mechanical stress to the precision electromechanical system of the relay.

    This thermal stress must not lead to a change in the functional characteristics or a negative impact on the reliability of the relays in service.

    Unlike other electronic components, relay housings are filled with gas and are sealed—in some cases, even gas-tight. During the soldering process, the seal must not be destroyed; otherwise, the inert gas would be lost. In housings that are not properly sealed, reduced clearance and creepage distances can no longer be applied, and cracked housings would result in reduced reliability.

    Experimental Tests

    For the purpose of this study, all tests were performed on IM relays. These are signal/telecom relays with two changeover contacts, a maximum switching power of
    60 W/65.5 V A, a rated voltage of 250 V, and rated current of 2 A. The body volume is 339 mm3, and the package thickness
    is < 2.5 mm.

    Wetting Test

    Two different methods according to IEC60068-2-58 were applied. Wetting was tested with the solder bath and the reflow solder method. As mentioned above, mixed assemblies must be expected for a certain period. Table 1 shows the wetting tests
    performed.

    Figure 3 shows the process diagram for the reflow wetting tests and resistance to soldering heat tests. The same profile was applied for the moisture sensitive level tests using different SMD solder processes. Temperature was measured on both the relay terminals and on top of the package.

    Moisture-Sensitive Level Tests

    In order to determine the moisture-sensitive level, relays were stored at 30°C at 60-percent relative humidity (RH) for 192 and 696 hours. After storage, the relays were subjected to the soldering heat and the internal pressure was measured inside the relay; theoretically, the pressure increase would be 720 mbar at 235°C and 780 mbar at 255°C. When poor materials or wet components are soldered, the pressure increase is much higher. At a certain pressure, the relay housing bursts (see Figure 4).

    Resistance to Soldering Heat Tests

    In order to test the resistance to soldering heat and sealing of the relays, a soldering process was applied three times with a peak temperature of 255°C. The change in the functional values was measured, and the seal was checked with a helium leakage tester before and after the SMD soldering process.

    In order to judge the reliability of the relays after the triple soldering, electrical endurance tests were performed on relays processed under the same SMD soldering conditions:

  • Dry circuit (30 mV/less than or equal to 10 mA) – 2.5 x 106 operations

  • Cable load open end 48 V/10 m – 1.6 x 106 operations

    Climatic Tests

    Relays with lead-free terminals were soldered using both tin-lead and lead-free soldering processes. To judge the solderability of all possible combinations of lead-free components and PCBs with tin-lead alloy surfaces, the combinations shown in Table 2 were further investigated. After SMD soldering, the soldered joints were visually inspected and cross-sections made. The quality of soldering was judged.

    The climatic performance was also checked using relays soldered on PCBs. Based on the parameters listed in Table 2, the following tests were performed:

  • Whisker generation 55°C/56 days

  • Dry heat 110°C/56 days

  • Damp heat 55°C, 95-percent RH/56 days

  • Thermal shock -40°C /+85°C – 200 cycles

    After storage, the following parameters were judged:

  • Contact resistance and functional values

  • Sealing

  • Conditions of soldered connections

  • Experimental Results


    Figure 5 (top). Operate and release voltage as a function of the maximum soldering temperature and duration of the soldering process. Minimum, average, and maximum values are given before and after the soldering process.

    Figure 6 (bottom). Minimum, average, and maximum contact resistance during the entire electrical endurance tests of normally open contacts (left bar) and normally closed contacts (right bar) as a function of the maximum soldering temperature.

    Wetting Tests

    When the solder bath method is applied, the terminals must be 95-percent covered with new tin. This was the case for all parameters tested.

    When the wetting test is judged after reflow soldering, the visual appearance and mechanical properties of the soldered joint were judged. All wetting tests showed sufficient wetting for lead-free relays. Nevertheless, lead in the solder or on the terminals improves the wetting characteristics significantly. The appearance of lead-free soldered joints, which was grained, was different from the soldered joints containing lead, which were smooth and shiny.

    Moisture-Sensitive Level Tests

    The moisture-sensitive level of electromechanical relays is determined by the moisture absorbed by the plastics—the peak temperature during the soldering process and the maximum pressure the relay housing can withstand before cracking. The tested IM relays were able to withstand an internal pressure increase during soldering of ?3,000 mbar before cracking. After the storage of the relays at 30°C/60-percent RH for 696 or 192 hours, respectively, the pressure increase was measured at a peak soldering temperature of 235°C and 255°C. As shown in Table 3, the pressure increase after a storage time of 192 hours was 830 mbar for a peak temperature of 255°C and 750 mbar for 235°C. After a storage time of 696 hours, the pressure increase is close to the permitted limit. Therefore, the moisture sensitive level (MLS) of the relays was defined as 3.

    Resistance to Soldering Heat Tests

    The functional values of the relays were measured initially and than after being soldered three times. As shown in Figure 5, no relevant changes in the operate and release voltage were measured. No dependence on the temperature was detected.

    After the triple soldering, typical electrical endurance tests for telecom relays were performed—contact application 0 and cable load open end—in order to check a possible negative impact of the higher peak soldering temperature and the longer process duration. Figure 6 shows that even during these most critical electrical endurance tests, no negative influence of the higher soldering temperature was found.

    Climatic Tests

    The results of the climatic tests are summarized in Table 4 and Figure 7. The performance during all climatic tests performed was not affected by the soldering conditions applied.

    Whisker growth - Whisker generation is an effect based on stress in the tin surface. To minimize this effect under the tin coating, a nickel layer was plated. On the bent terminals of the test sample, absolutely no whisker growth was observed. The combination of a nickel and pure tin coating reliably prevents whisker growth.

    Dry heat 110°C/56 days - Contact resistance was measured immediately after cooling the relays to room temperature. Normally closed contacts were measured before actuating the coil; normally open contacts were measured after the first actuation. The contact resistance values measured after temperature storage at 110°C for 56 days are not dependent on the soldering conditions. No changes in functional values and leakage rate were observed. Visual inspection of the soldered joints and the cross sections showed the soldered connections to be reliable for all parameter combinations.

    Damp heat 55°C-95-percent RH/56 days - The same procedure was applied for measuring the contact resistance. No changes were found and no dependence on different soldering conditions, as the measured resistance values are very stable. No changes in functional values and leakage rate were observed. Visual inspection of the soldered joints and the cross sections showed the soldered connections to be reliable for all parameter combinations.

    Thermal shock 40°C /+85°C – 200 cycles - The thermal shock test is the most stringent test for evaluating gas tightness and the integrity of the soldered joints. After 200 thermal shocks, the contact resistance showed no dependence on the soldering conditions. No changes in functional values or leakage rate were observed. Visual inspection of the soldered joints and the cross sections showed the soldered connections to be reliable for all parameter combinations.

    Terminal Surface
    Solder
    Temperature
    Flux/solder paste
    Method
    Sn 100
    SnAgCu 3.5/0.7
    245°C
    Colophony
    bath
    Sn 100
    SnPb 63/37
    235°C
    Colophony
    bath
    Sn 100
    SnAgCu 3.5/0.7
    245°C
    SenjuM31-GR N360-K1MK
    reflow
    Sn 100
    SnPb 63/37
    235°C
    SenjuM31-GR N360-K1MK
    reflow
    Table 1. Tested combinations for wetting of the terminals. The tests were performed according to IEC 60068-2-58 [30].

     
    Parameter Combination
    Max. Temperature [°C]
    Surface Pins
    Surface PCB
    Solder Paste
    1
    255
    Sn 100
    Sn 100
    SnAgCu 3.5/0.7
    2
    235
    Sn 100
    SnPb 63/37
    SnPb 63/37

    Table 2. Parameters for climatic tests.


     
    Level
    Floor life
    Tests conditions
    Pressure increase [mbar]
    Function
    235°C
    255°C
    2a
    4 weeks
    30°C/60% RH
    696 hours
    2,300
    2,300
    2,500
    2,500
    o.k.
    o.k.
    3
    1 week
    30°C/60% RH
    192 hours
    750
    830
    o.k.
    Table 3. Pressure increase during determination of the moisture sensitive level (MSL) of relays [JEDEC].

    Test/Parameter Combination
    Contact Resistance [mOhm]
    Functional Values
    Leakage Rate [mbar,1/s]
    Solder Connection Visual Plus Cross Section
    Dry heat/1
    less than/equal to 100
    o.k.
    less than/equal to 3.10-8
    o.k.
    Dry heat/2
    less than/equal to 100
    o.k.
    less than/equal to 3.10-8
    o.k.
    Damp heat/1
    less than/equal to 100
    o.k.
    less than/equal to 3.10-8
    o.k.
    Damp heat/2
    less than/equal to 100
    o.k.
    less than/equal to 3.10-8
    o.k.
    Thermal shock/1
    less than/equal to 100
    o.k.
    less than/equal to 3.10-8
    o.k.
    Thermal shock/2
    less than/equal to 100
    o.k.
    less than/equal to 3.10-8
    o.k.
    Table 4. Overview of the climatic test results.

     

    Figure 7. Minimum, average, and maximum contact resistance values measured after temperature storage of normally open contacts (left bar) and normally closed contacts (right bar) soldered with different solder profiles and after the climatic tests.

    Conclusion

    Various legislation in regions such as Europe, Japan, and China means that lead-free soldering can be expected in the near future. Producing electromechanical relays for lead-free soldering was a challenge, as lead-free soldering stresses the precision mechanics, which are at the heart of ultra-miniature relays. With the increase in peak process temperatures up to 255°C and the increase in process duration, the thermal stress goes to the performance limits of available engineering plastics.

    In order to make signal relays available for lead-free soldering processes, the following requirements must be fulfilled:

    • The design must be able to handle high temperatures during the soldering process. Possible mechanical changes must not have an impact on the functional values of the relay.

    • Materials with the lowest possible outgassing and water absorption rate must be used in combination with good mechanical properties at high temperatures.

    • Special care must be taken to keep the relays dry. Dry packing must be applied to ensure no cracking of the relays during the soldering process.

    In addition, taking account of the following basic requirements will enable manufacturers to produce relays suitable for lead-free soldering processes with significantly higher peak temperatures that meet the following standards:

    • Relays made from proper materials and manufacturing processes can withstand a lead-free soldering process. No cracking or “popcorning” of the housings was observed.

    • The IM relay is able to handle soldering temperatures up to 255°C without changes in the functional values.

    • Signal relays designed for lead-free soldering processes have the same performance during electrical endurance tests. No negative impact was observed.

    • The reliability of electromechanical relays soldered with lead-free processes can achieve the same level as standard SMD relays.

    • The appearance and surface of standard leaded soldered joints are totally different from lead-free ones. While leaded soldered connections are regular and shiny, lead-free connections are rough and grained.

    • Whisker growth on pure tin surfaces of terminals can be efficiently prevented by a nickel coating underneath the tin surface

    • The results obtained from mixed assemblies showed similar performance during all climatic tests.

    This is an edited version of a paper presented at the 52nd Annual International Relay Conference, held in April 2004.


    About the Author
    Dr. Werner Johler is technology manager at Tyco Electronics in Au, Switzerland. He received his Ph.D. degree in electrical engineering from the Technical University (Vienna, Austria) in 1988 and his MBA in 2003. From 1984 to 1988, Dr. Johler was a scientific staff member at the Institute of Switchgear at the Technical University of Vienna. In 1988, he joined Tyco Electronics AXICOM. He has been chairman of the Technical Committee TC94 “All or nothing relays” within CENELEC since 1999.
     

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