IPC-4552.pdf - 第23页

IPC-4552 Proposal June 2001 0.0 5.0 10.0 15.0 20.0 25.0 0k 0100k 0200k 0300k 0400k 0500k 0600k 0700k 0800k 0900k 1000k 1100k 1200k 1300k 1400k 1500k 1600k 1700k 1800k number of cycl es ohms 2.0mm 2.5mm 3.0mm 3.5mm 4.0mm …

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IPC-4552 Proposal June 2001
measurement techniques).
One of the byproducts of such an extensive series of tests was the identification of the variability that exists between
ENIG suppliers and that while the deposit thickness values maybe different the performance from a base
solderability standpoint is the same. It is clear that, all involved in ENIG – supplier, board house and OEM – need
to quantify their individual processes and have the necessary data to demonstrate that the process is known and
understood from a statistical / chemical and solderability viewpoint.
Contact Resistance Testing:
One of the functions that ENIG provides as a surface finish is that of a contact resistance surface that typically
interfaces with a “soft touch” membrane keypad. The membrane is usually carbon impregnated but other metal
dome switches are also used for this type of application, (the group is still requesting performance data on this type
of switch). While the group had a lot of anecdotal evidence of its use, cell phones, keypads etc, nobody had data for
this type of switch that was not company specific and therefore not shareable with the IPC community. In an attempt
to obtain shareable data, the committee decided to test some different patterns commonly in use in the industry.
After much searching we came upon S/G Industries, a supplier of membrane overlays to the automotive industry
among others. With their help, three different designs were fabricated:
1) Interlocking Square contacts
2) Interlocking Round contacts
3) Half Moon contacts
Note: At the time of writing this paper, test data from only the Interlocking square pattern is available.
To increase the data set it was decided to have two of the ENIG chemical suppliers plate these patterns. The testing
would be run until failure or until the resistance change would be considered non-useable for the switch application.
From the data from the first pattern testing at the time of writing, it would appear that we will stop test at 2 million
cycles with no negative, i.e. increase in resistance, being evident. The committee feels that 2 million cycles is more
than adequate to prove the use of ENIG as a suitable surface for this type of application.
The test protocol was as follows:
1) Each PWB/keypad assembly consists of five (5) pads.
2) Each pad represents a different carbon contact and trace size ranging from 2.0mm to 4.0 mm.
3) A total of six assembly sets will be tested
4) There are two ENIG suppliers, Vendor’s C&E
5) Initial testing consists of applying a 1000 gram load to the entire keypad (1000 grams / 5 switches per
keypad = 200 grams per switch).
6) Initial resistance measurements are taken from each switch location with this force applied without
passing current through the switch (mechanical durability test only)
7) The PCB/keypad assembly is then actuated in 50,000 cycle intervals with resistance measurements
taken at each interval. The forces applied during resistance measurements are identical to those used
during the initial resistance measurements (200 grams per switch).
8) Reference of positions measured: Position 1: 2.0 mm carbon contact, Position 2: 2.5 mm carbon
contact, Position 3: 3.0 mm carbon contact, Position 4: 3.5 mm carbon contact, Position 5: 4.0 mm
carbon contact.
The results from 1.85 million cycles for the interlocking square pattern are detailed below.
As can be seen from this summary data set the general trend in the resistance data is down, the direction of choice.
In discussions with S/G Industries regarding these phenomena, they explained that the carbon particles that are
impregnated into the pill tend to be compacted as the switch is compressed over time. This therefore reduces the
contact resistance, which is clearly demonstrated in the above graphs. A visual
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IPC-4552 Proposal June 2001
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APPENDIX 1 Fig 14: Contact resistance data for Vendor C for
Interlocking Square contacts
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APPENDIX 2 Fig 14: Contact resistance data for Vendor C for
Interlocking Square contacts
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IPC-4552 Proposal June 2001
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vendor C vendor E
vendor
Means and Std Deviations
Level Number Mean Std Dev Std Err Mean
vendor C 60 3.85283 0.894992 0.11554
vendor E 60 2.00300 0.712604 0.09200
Fig 15: comparison of gold thickness by vendor for the Interlocking Square Contact test
Fig 16: Interlocking Square Contact Test Coupon
4.0 mm
3.5 mm
3.0 mm
2.5 mm
2 mm
Fig 16: Interlocking Square Contact Test Coupon
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