AN2036 APPLICATION NOTE
Solderability Tests
Lead-free devices, from STMicroelectronics, are warranted to pass the solderability tests, and to form a reliable solder joint with the base material of the circuit board, using Leaded (Sn-Pb) solder or Lead-free (Sn-Ag-Cu) solder. This document describes the context of this assertion. ST Lead-free devices conform to the ECOPACK 7191395 specification from ST. This, in turn, complies with the European directive on Restrictions on Hazardous Substances (RoHS) 2002/95/EU, and with the IPC/JEDEC 020 standard for resistance to soldering heat and soldering process. These devices have to be solderable with both Leaded materials and processes, and with Lead-free materials and processes. With a composition close to the ternary eutectic composition (Table 1.), the TinSilver-Copper (Sn-Ag-Cu) alloy is considered as the standard lead-free material. Only Tin-Bismuth coated devices are not tested in Tin-Lead solder. This is because the Tin-Bismuth coating is not recommended for mixing with Tin-Lead solder, for board mounting, due to a reliability risk on the solder joints. Table 1. Solder Composition for Solderability Tests
Solder Composition Tin Lead-free Silver Copper Tin SnPb Lead 36 to 40 Range (% weight) 94.6 to 96.6 3.0 to 4.3 0.4 to 1.1 60 to 64
There are two kinds of tests that are applied to evaluate such ability of a surface to be wetted:
Dip and Look Wetting Balance
Both tests are well known, and applied, worldwide. However, different methods are often used (such as MIL standard 883-2003, JEDEC J-STD-002, JESD22-B102 and IEC 60749-21). The ones chosen by ST are intended to cover the ones that are in most widespread use by our customers. In these tests, the packages are "aged", in order to test the solderability under the worst conditions and to warrant at least 2 years of storage with no significant degradation of solderability.
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DIP AND LOOK TEST
After appropriate ageing, the connections are dipped inside a non-activated flux, then into molten solder. Then, after cooling and optional cleaning in alcohol, the connections are inspected to verify the critical area (the one that is to be soldered to the board) of the connection surfaces is covered by fresh solder (Figure 1.). The test is considered passed if 95% of the surface is covered with fresh solder. Figure 1. Terminals after Dip and Look Test
The mechanism for forming the solder joint depends on the coating and solder compositions as well as on the test conditions (as listed in Table 2.).
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Table 2. Test Conditions
8 hours steam ageing or ageing in humidity chamber at 85C and 85%rH (Humidity chamber used for pre-plated components) Dry ageing at 150C: 8 hours for NiPdAu Pre-plated units and 16 hours for solder coated units (Ageing is non cumulative = to be done on different samples) Sn-Ag-Cu melted at 245C Flux dipping Lead-free Solder Solder dipping 10 seconds 0.5 s for Preplated NiPd(Au) connections Surface Mount Devices Sn-Pb melted at 220C Flux dipping Leaded Solder Solder dipping 10 seconds 0.5s for Preplated NiPd(Au) connections Device that can be soldered by wave soldering Sn-Pb or Sn-Ag-Cu melted at 260C Solder Wave Resistance to Dissolution Flux dipping Solder dipping 5 to 10 seconds 10 seconds 0.5s 5 to 10 seconds 5 seconds 0.5s for Tin based connection coatings 5 to 10 seconds 5 seconds 0.5s for Tin based connection coatings
8 hours steam ageing or ageing in humidity chamber at 85C and 85%rH Sn-Ag-Cu melted at 245C Insertion Packages or Through Hole Devices Lead-free Solder Flux dipping Solder dipping Sn-Pb melted at 245C Leaded Solder Flux dipping Solder dipping
Note: Non activated flux to be 25% by weight of colophany diluted in isopropanol as per IPC J-STD004.
5 to 10 seconds 5 seconds 1s
5 to 10 seconds 5 seconds 1s
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WETTING BALANCE TEST
Connections are dipped inside a non-activated flux. Then the package is loaded on the wetting balance arm, and the connections are dipped into a molten solder. The force exerted on the balance arm is plotted as a function of time. This test cannot be applied for all the packages. It can, though, be adjusted to fit special cases. Typical forces versus time curves are shown in Figure 2. Figure 2. Wetting Force Plotted against Time, for Three Cases
No Forced Ageing
Wetting force mN/mm
Ageing: 16 hours, 150C
Ageing: 8 hours, 85C and 85% Humidity
Immersion: 10 seconds
The shape of the curves can be explained as follows: At the beginning, the buoyancy forces (due to the interfacial tension) are predominant, and so the wetting force is negative. Gradually, the solder begins to wet the terminals, and the wetting force increases with time, overcoming the buoyancy force. Eventually, it reaches a flat (plateau) zone.
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When the measured wetting force is zero, the two forces are balanced. This point is called the "zero cross time" (t0), and indicates the transition from no-wetting (overall force less than zero) to wetting (overall force greater than zero). Note: No aging is applied, before the wetting balance test, during process control of production. Aging is applied for data collection purpose only. Criteria: the test is considered as passed if: The wetting force reaches 0.1mN/mm within 5 seconds of immersion The zero cross time (t0) is less than 3 seconds.
Note: these criteria are applied for small-signal devices. Power devices can require different criteria to take into account the thermal mass of their connections. Table 3. Tests Conditions
Solder Flux dipping Sn-Ag-Cu melted at 245C (5C) Solder dipping Flux dipping Sn-Pb melted at 235C (5C) Solder dipping 10 seconds 10 seconds 5 to 10 seconds 5 to 10 seconds
Note: The wetting force, as measured by the balance, is divided by the total perimeter of the connections at initial immersion. In the case of leads that are connected to a thermal mass, such as to a dissipater, the test must be redefined to include preheating.
CONCLUSIONS
Through two standard tests ("Dip and Look" and "Wetting Balance"), ST Lead-free devices are warranted to pass solderability tests using Sn-Pb solder or Sn-Ag-Cu solder, and so to meet all the requirements for electronics soldering. In fact the solderability tests provide customers with an additional warranty that the components can be soldered without affecting the common reliability performance.
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REVISION HISTORY
Table 4. Document Revision History
Date 08-Nov-2004 Version 1.0 First Issue Revision Details
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Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners 2004 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America www.st.com
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