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Lead-free bright tin plating for barrel and rack applications for connectors.

The WEEE Directive from the EU requires electronic products to be lead-free with effect from July 2006. As an alternative to tin-lead plating of connectors with demanding specifications, this article describes a lead-free pure tin electrolyte based on an organo-sulfonate system from which whisker-resistant deposits with good solderability can be achieved, even on copper substrates. The tin deposits are bright and comparable in appearance to bright tin-lead deposits.

Introduction

The most reliable coating material for connectors today is still hard gold or palladium-nickel with a thin layer of hard gold. For less demanding connector functionality (i.e. where connectors are only required for a few insertion cycles) tin or tin-lead coatings are used whenever possible to minimise production costs. Tin-lead was preferred in the past because of the whisker freedom and improved solderability mentioned above. Moreover, bright coatings are often preferred as these are harder and less subject to finger marks, and thus less inclined to abrasion and cold welding. The draft European WEEE (Waste Electrical and Electronic Equipment) Directive legislation proposes the elimination of lead from electronic components by July 2006. Most Japanese electronics companies have approved lead-free production by 2004/2005 and the main Japanese producers of consumer electronics today already offer completely leadfree products. European electroplating companies are increasingly involved with this development. The trend towards lead-free products means that a suitable substitute for tin-lead must also be sought for low-speed plating.

The phenomenon of whisker formation by tin in lead-free electroplated tin and tin alloy coatings has been comprehensively documented and remains a cause for concern. Figure 1a shows a control card with tinned connector pins. The parts were coated using a conventional tin electrolyte based on sulfate. The component was then used for several months in an electroplating shop until it failed as a result of whisker formation as shown in Figure 1b. Some whiskers grew to lengths up to 5mm and caused a short circuit in the rear part of the connector pins (Figure 1c). The development of practical methods to reduce and eliminate tin whiskers is a basic requirement to ensure reliable manufacture of lead-free electronic components.

Requirements for Coating

Tin-lead continues to be the only universal plating alloy which remains whisker-free and corrosion-resistant after stringent ageing tests. Much progress has been made in diffusion and characterisation of lead-free solder alloys as a replacement for traditional tin-lead solders, but no final and satisfactory alternatives have been found to date. In bright tin plating the problem is compounded, because lead on the one hand improves solderability by reducing the melting point, and on the other hand functions as a brightener component in the electrolyte. If brightness is required this difficulty can only be corrected by increasing the organic brightener additives. Unfortunately this can also have a negative effect on solderability, ductility and whiskering. Particular attention must be paid to this when selecting electrolyte systems and electro-organic components.

[FIGURE 1a OMITTED]

[FIGURE 1b OMITTED]

[FIGURE 1c OMITTED]

New Bright Pure Tin Electrolyte for Barrel and Rack Applications

Responding to these demands, Shipley has developed a new electrolyte on an organo-sulfonate basis. The deposits correspond to the theory proposed by Shipley researchers for producing deposits with minimum whisker risk [3]. In summary, this theory says that greater angles between crystallographic network planes leads to more open spaces and thus provides a greater number of paths for the tin to self-diffuse. As a result, such coatings have much less difficulty absorbing the stress in the coating created through copper diffusion and formation of the inter-metallic stage C[u.sub.6]S[n.sub.5]. The new Solderon BT-250 electrolyte was developed based on this know-how, and has a minimal number of critical angles. In spite of brighter qualities the coatings are extremely whisker resistant and even after artificial ageing in dry heat are still highly solderable and ductile, even with only a 5mm coating on copper alloys.

Investigations

The following bright tin processes were compared:

Solderon BR - (bright tin-lead on organo-sulfonate basis)

Solderon BT-250 - (bright tin on organo-sulfonate basis)

Tin Gleam - (bright tin on a sulfate basis)

Whisker Tests

To investigate whisker formation, three different layer combinations and coating conditions were selected. The deposits were plated on a laboratory scale using a mini barrel, with a 5 mm coating over the basic brass material, with and without an underplate (1-2 [micro]m nickel sulfamate or 1-2 mm bright acid copper) After plating, a proportion of the samples from each combination was annealed at 150[degrees]C for one hour. Subsequently the parts were stored for a minimum of three months at 52[degrees]C dry heat. In order to judge whisker formation the samples were examined for whiskers monthly, using a scanning electron microscope (SEM). The whiskers identified in SEM investigations were then measured using a Shipley internal classification system which takes account of the length and frequency of whiskers [2]:

* Class 0 - no observable whisker growth

* Class 1 - infrequent, short length (<5 [micro]m)

* Class 2 - infrequent, moderate length (5-25 [micro]m)

* Class 3 - more frequent, short or moderate length (<25 [micro]m)

* Class 4 - long (>25[micro]m), classic whisker shape, 3-4 [micro]m diameter.

Results

To facilitate comparison between the various process/substrate combinations a whisker index was created from the whisker classification results. This index combines the severity of the whiskering and the rate of whisker growth into a single value.

Index = (as is*1) + (1 month*0.75) + (2 months*0.5) + (3 months*0.25)

An index of "0" is whisker free over the 3-month test. An index of "10" indicates severe whiskering from the beginning. As a general guide an index of <1 may be considered acceptable.

Table 2 shows results within the relevant whisker indices.

The results show that in the case of the sample electrolyte on a sulfate basis, whisker formation can only be suppressed by counter-measures such as nickel underplate or heat treatment after deposition. However, the new electrolyte on an organo-sulfonate basis has similar results to conventional tin-lead electrolytes.

A driving force for whisker formation is stress in the tin layer which can be caused by the diffusion process. Three main diffusion mechanisms can be distinguished; diffusion through the crystal matrix, along material dislocation lines and grain intersection diffusion. The cause of whisker formation is seen to be copper diffusion along granular intersection from the substrate into the coating material. As a result, stress builds up through formation of an intermetallic phase (Cu6Sn5) in the tin layer. The generally good values obtained with underplating result because nickel is a very good diffusion barrier, and prevents diffusion of copper into the coating material. Figures 2a and 2b show Auger maps of a tinned copper substrate after artificial ageing, with and without nickel underplating. In Figure 2b it is clear that the copper of the substrate diffuses along granular intersections into the tin, although with nickel underplate (Figure 2a) no granular diffusion can be observed.

Heat treatment for one hour at 150[degrees]C gives results which are similar to those using a nickel underplate. This leads to a thin homogeneous intermetallic phase (Cu6Sn5), which forms evenly in the crystal matrix under the coating material unlike granular diffusion. Through this treatment, no additional stress is applied to the coating material, and the intermetallic phase itself acts as a kind of diffusion barrier.

[FIGURE 2a OMITTED]

[FIGURE 2b OMITTED]

Solderability

To test solderability, comparative tests were carried out using a wetting balance. A layer of 5 mm was deposited onto a copper substrate using the tin lead, tin sulfate or tin organo-sulfonate systems. Subsequently wetting was measured immediately after tinning and after artificial ageing (dry heat 155[degrees]C, 16h). Picture 3a shows wetting speed to the zero cross time before and after ageing. The sum of the forces acting on the component at this point is 0. Figure 3b shows the wetting force as a function of the wetting angle after 2 seconds. The smaller this angle, the quicker the wetting of the component.

The following conclusions were drawn for bright electrolyte in barrel and rack applications:

[FIGURE 3a OMITTED]

[FIGURE 3b OMITTED]

1. The wetting speed of 90/10 tin-lead is comparable to pure tin and tends to be better after ageing. Nevertheless the wetting speed of less than 1 second can be described as good.

2. Without ageing, wetting angles are comparable and are less than 30[degrees]. In general, good solderability is assumed when the wetting angle is less than 30[degrees].

3. As is to be expected, pure tin shows poorer results than tin-lead after ageing. This is because the corrosion resistance of tin-lead is significantly higher than that of pure tin. Nevertheless all three layer systems are in a position to fulfil normal functional requirements for solderability.

Hardness and Cracking formation

To determine hardness and cracking performance, prenickelled brass strips were coated with 50 mm tin or tin-lead and a Vickers hardness test carried out. To investigate cracking, a round rod bend test was carried out, in which brass strips coated with acid copper were tinned with 4-6 [micro]m, bent to an angle of 110[degrees] over a round rod of 10mm diameter, and subsequently inspected using an SEM for cracking. Figure 4 shows the results of the hardness test. Figures 5a-c show cracking after the bend test.

[FIGURE 4 OMITTED]

The suitability of tin as a contact metal can only be judged indirectly using hardness and cracking tests. Under normal climatic conditions, tin-lead and tin layers form thin oxide films which lead to increased contact resistance. Because, however, the coating material--as shown in the hardness test--is somewhat softer, the oxide film can be easily broken up and contact made. The low hardness of the coating material means that these layers require a higher connection force and lead to stronger abrasion. These coatings are therefore only suitable for a small number of connection cycles. As the results show, 90/10 tin-lead behaves similarly to pure tin on an organo-sulfonate basis across the whole current density range, but the hardness of pure tin coatings from a sulfate electrolyte declines with higher current densities. Experience shows that bright tin coatings are relatively brittle compared to matt tin coatings.

As the pictures show, all three coating types show some cracking, but this is less significant with pure tin coatings than with tin-lead. If the connection area is outside the bending area, as is the case with crimp connectors, no problems are to be expected from cracking.

[FIGURE 5a OMITTED]

[FIGUER 5b OMITTED]

[FIGURE 5C OMITTED]

Operating and Deposit Data

Solderon BT-250 has a wide operating window, creates highly bright coating areas of 0.5 - 5 A/dm2 and is highly suitable for rack and barrel applications. By using antioxidant, tin (IV) formation and associated sludge formation can be effectively prevented. This allows economical preparation with low brightener consumption and good solderability. Within the recommended working range (Table 3), very constant deposit characteristics can be achieved.

Conclusion

The new bright tin electrolyte Solderon BT-250 enables the production of lead-free coatings which meet the requirements of the WEEE Directive [4]. Solderon BT-250 is ideally suited for replacing tin-lead in connector plating. The coatings are whisker-resistant even with copper substrates, and the bright characteristics (comparable to tin-lead) make the coating less subject to finger marking and also reduce wear on assembly and bending tools.

Literature

[1] Jordan M.: Die galvanische Abscheidung von Zinn und Zinnlegierungen, Seite 351 ff, Eugen G. Leuze Verlag, 1993

[2] Keith Whitlaw and Jeff Crosby, Shipley Europe Ltd.: An Empirical Study Into Whisker-Growth in Tin and Tin Alloy Electrodeposits

[3] Andre Egli, Michael Toben, Shipley EIF: Where Crystal Planes Meet: Contribution to the Understanding of the Tin Whisker Growth Process, IPC Works 2002

[4] Official Journal of the European Union--Directive 2002/96/EC of the European Parliament and of the Council of 27 January 2003 on waste electrical and electronic equipment (WEEE)
           As is  1month  2month  3month  Index

Example 1  0      0       0       0       0
Example 2  4      4       4       4       10
Example 3  0      0       1       2       1
Example 4  0      1       2       3       2.5

Layer Combination       Substrate

Tin-Lead (90/10)           Brass
Pure Tin (Sulfonate)       Brass
Pure Tin (Sulfate)         Brass
Tin Lead (90/10)      Br / 2 [micro]m Cu
Pure Tin (Sulfonate)  Br / 2 [micro]m Cu
Pure Tin (Sulfate)    Br / 2 [micro]m Cu
Tin Lead (90/10)      Br / 2 [micro]m Cu
Pure Tin (Sulfonate)  Br / 2 [micro]m Cu
Pure Tin (Sulfate)    Br / 2 [micro]m Cu
Tin Lead (90/10)      Br / 2 [micro]m Ni
Pure Tin (Sulfonate)  Br / 2 [micro]m Ni
Pure Tin (Sulfate)    Br / 2 [micro]m Ni

Layer Combination                      Storage

Tin-Lead (90/10)                     52[degrees]C
Pure Tin (Sulfonate)                 52[degrees]C
Pure Tin (Sulfate)                   52[degrees]C
Tin Lead (90/10)                     52[degrees]C
Pure Tin (Sulfonate)                 52[degrees]C
Pure Tin (Sulfate)                   52[degrees]C
Tin Lead (90/10)      150[degrees]C/1h [right arrow] 52[degrees]C
Pure Tin (Sulfonate)  150[degrees]C/1h [right arrow] 52[degrees]C
Pure Tin (Sulfate)    150[degrees]C/1h [right arrow] 52[degrees]C
Tin Lead (90/10)                     52[degrees]C
Pure Tin (Sulfonate)                 52[degrees]C
Pure Tin (Sulfate)                   52[degrees]C

Layer Combination      Coating   Index
                      Thickness

Tin-Lead (90/10)      5 [micro]m    0
Pure Tin (Sulfonate)  5 [micro]m    1
Pure Tin (Sulfate)    5 [micro]m    3
Tin Lead (90/10)      5 [micro]m    0
Pure Tin (Sulfonate)  5 [micro]m    0
Pure Tin (Sulfate)    5 [micro]m    7
Tin Lead (90/10)      5 [micro]m    0
Pure Tin (Sulfonate)  5 [micro]m    0
Pure Tin (Sulfate)    5 [micro]m    0
Tin Lead (90/10)      5 [micro]m    0
Pure Tin (Sulfonate)  5 [micro]m    0
Pure Tin (Sulfate)    5 [micro]m    0

       Parameter                  Range           Recommended

        Tin(II)                15 - 25 g/l            20 g/l

    Solderon Acid HC         170 - 210 ml/l          190 ml/l

      Temperature           15 - 25[degrees]C     20 [degrees]C

 Current Density (Rack)    1 - 4 A/d[m.sup.2]     2 A/d[m.sup.2]

Current Density (Barrel)  0.25 - 1 A/d[m.sup.2]  0.5 A/d[m.sup.2]

       Agitation                        1-3 m/min

    Deposition rate       Approx. 1 [micro]m per minute at 2
                                    A/d[m.sup.2]


by David Williams and Andreas Stutz, Shipley Europe Limited
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Title Annotation:Technical Update
Author:Williams, David; Stutz, Andreas
Publication:Finishing
Date:Nov 1, 2003
Words:2393
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