Having a nickel underlayer contact finish helps prevent zinc (Zn) migration on the surface, which can help reduce PC board soldering temperatures. Nickel underlayer or a tin-nickel finish helps ensure a good surface contact usually free of rust or oxidization. A solid nickel contact finish would be better as it would provide increased contact performance but similar to a solid nickel contact, would require increased soldering temperatures along with a higher price.
The exact cause of tin whiskers is still unclear. Research is still being carried out on this subject. However, there are certain measures that can be implemented to help prevent whisker growth. Gold plating over a palladium nickel underplate has been proven to minimize whisker growth. If a tin contact is used, only a thin layer of tin should be used over a nickel base which is the standard plating for Harwin tin-plated connectors.
Latest research has shown that having an undercoat (usually nickel) below the tin plating has a very significant impact on the likely growth of tin whiskers. In numerous tests it has either eliminated or made insignificant any previous differences shown in matte and bright tin, studies of which were done without this undercoat plating. This is due to the elimination of compressive stress around the formation of tin-copper intermetallics (IMC).
References:
https://www.digikey.com/en/product-highlight/t/te-connectivity-amp/faston-250
https://www.harwin.com/support/articles/pure-tin-plating-considerations/
See also:
Soldering Stainless Steel with Electronics Solder
Solder Sleeve Solderability?
Tin Whiskers in Pure Tin Surface
Summary
36-3547-ND
WM19887-ND
3M157218-ND
60705-1-ND
36-3545-ND
36-3546-ND
298-16696-ND
298-16708-ND
A122466-ND
298-14414-ND
A111992-ND
298-11517-ND
298-16676-ND
298-10016-ND
A114155-ND,2266-63771-1-ND
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2266-726388-3-ND,726388-3-ND
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298-10015-ND
2266-1742583-1TR-ND,1742583-1-ND
298-19711-ND
2266-1969447-1-ND,A107715-ND
2266-160855-2TR-ND,160855-2-ND
3547
192030379
HT-73F-250-32
60705-1
3545
3546
EDV14-250M-L
EDV18-250M-Q
61960-1
DV10-250M-L
1217002-1
DNF14-250FIMB-D
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DV18-250MB-MY
42559-1
5-160432-3
62050-2
170330-2
2-160304-4
DNF14-250FIM-2K
170324-2
521087-1
DNF14-250M-C
EDV10-250M-Q
EDNF10250FIMB-Q
63301-1
DV18-250MB-CY
63757-2
1-160773-0
2825084-1
1217006-1
63604-1
DPF18-250FIM-2K
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D14-250MB-M
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192030390
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2238106-1
2238105-1
DNF14-250M-M
D10-250M-D
41531-1
63696-1
63688-1
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62336-2
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2178301-1
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2178300-1
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2238178-1
DNF10-250FIMB-K
4-1377174-6
1742626-1
DNF14-250FIMB-K
160389-3
3572
60621-1
42579-1
1217102-1
2238143-1
63963-1
2238177-1
6-160432-3
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42404-1
2178438-1
282349-1
160807-2
2-160759-9
ED14-250MB-L
ED14-250MB-Q
726388-3
DV14-250MB-M
D14-250MB-C
DPF14-250FIM-L
ED10-250M-Q
DNF14-250M-E
42373-3
42563-9
ED18-250MB-Q
ED18-250MB-L
63892-1
D18-250MB-C
1742583-1
DNF14-250FIM-E
1969447-1
160855-2