ST AN2036 APPLICATION NOTE

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 Tin- Silver-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	Range (% weight)
		Tin	94.6 to 96.6
	Lead-free	Silver	3.0 to 4.3
		Copper	0.4 to 1.1
	SnPb	Tin	60 to 64
		Lead	36 to 40

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.

November 2004

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AN2036 - APPLICATION NOTE

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 85°C and 85%rH (Humidity
		chamber used for pre-plated components)
		– Dry ageing at 150°C: 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 245°C
				Flux dipping		5 to 10 seconds
		Lead-free
						5 seconds ±0.5s for Tin
		Solder
						based connection coatings
				Solder dipping
						10 seconds ±0.5 s for Pre-
Surface Mount						plated NiPd(Au) connections
				Sn-Pb melted at 220°C
Devices
				Flux dipping		5 to 10 seconds
		Leaded Solder				5 seconds ±0.5s for Tin
				Solder dipping		based connection coatings
						10 seconds ±0.5s for Pre-
						plated NiPd(Au) connections
				Device that can be soldered by wave soldering
				Sn-Pb or Sn-Ag-Cu melted at 260°C
		Solder Wave Resistance to Dissolution
				Flux dipping		5 to 10 seconds
				Solder dipping		10 seconds ±0.5s
		8 hours steam ageing or ageing in humidity chamber at 85°C and 85%rH
				Sn-Ag-Cu melted at 245°C
Insertion Packages or		Lead-free Solder		Flux dipping		5 to 10 seconds
Through Hole				Solder dipping		5 seconds ±1s
Devices
				Sn-Pb melted at 245°C
		Leaded Solder		Flux dipping		5 to 10 seconds
				Solder dipping		5 seconds ±1s

Note: Non activated flux to be 25% by weight of colophany diluted in isopropanol as per IPC J-STD004.

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