Scope of UL PCB silver task group

Brief history

At the IPC Orlando 2001 meeting, the UL/IPC task group was initiated with the designation 3-11g, "Metal Finishes Data Acquisition." The group of circuit board manufacturers and chemical process suppliers discussed the increasing use of silver as a PCB surface finish, and the emerging use of silver in lead-free solders. It was noted that without clarification or adjustments to UL documentation, there would be violation and misuse of the existing UL specifications.

Over the next three years, the task group performed background experiments, devised proposed revisions to UL 796, the Standard for Safety of Printed Wiring Boards, and presented the proposals at UL's STP meetings. The following modifications to UL 796 silver migration test have been adopted based on this work:

• For silver-containing PCB materials which require migration testing, the Withstand Voltage test is 1.6 kV/mm (40V/mil) (1000V max) based on the silver conductor spacings
• For electrochemical migration testing, the temperature and humidity conditions are 35°C +/- 2°C (95°F +/- 5.0°F) at 87.5% +/- 2.5% RH

With respect to migration and dendrite formation, the task group conducted additional research testing to demonstrate the immersion silver surface finish on a PCB is no more likely to migrate than other standard surface finishes. The tests used for the research included the silver migration test described in UL 796, Section 23 and the Electrochemical Migration Resistance test described in the IPC test method manual, TM-650 2.6.14.1. The surface finishes subjected to the research testing were bare copper (as the control), electroless nickel immersion gold (ENIG), electrolytic silver, and immersion silver. It was determined the research confirmed that immersion silver is no more likely to migrate than standard surface finishes. In addition, research performed by NEMI on the SAC alloys was used to show the SAC alloys are also no more likely to migrate than standard surface finishes. Therefore, silver migration testing on immersion silver and tin-silver-copper (SAC) alloys is not required.

Future revisions to UL 796 should include use of an insulation resistance test. The task group plans to continue work on other surface finishes and other electronic materials and products containing silver.

Ag migration update memo

October 1, 2004

The immersion silver exception for silver migration testing has been published. The UL 796 revisions are dated September 22, 2004  (paragraphs 7.2.1 and 7.11.1) and the UL 796F revisions are dated September 27, 2004 (paragraphs 2.3.25 and 5.12.1.1).

Immersion silver may be added to the PWB manufacturing process without silver migration testing. Addition of immersion silver may be a paperwork-only addition if the board has already been evaluated for metal plating and additional manufacturing process steps with temperatures above 100 C are not necessary. See paragraph 9.1.6 regarding retesting.

A special FUS inspection is required by the UL FUS field representative to verify the process is Immersion Silver before boards with immersion silver may be shipped. A verification checklist has been developed for the field representatives to use during their visit. The special inspection should be performed on a RUSH basis unless the client has requested a specific date for the inspection to occur.

April 8, 2005

The UL/IPC task group provided samples to UL so that UL can perform round-robin testing at their various labs according to the electrochemical migration resistance test described in the IPC test method manual, TM-650 2.6.14.1 (the Telcordia GR-78 Migration Test Method). The goal is to replace the current UL 796 Section 23 Silver Migration Test Method with the Telcordia Test Method.

Silver migration test method sample construction guidelines

• Each sample shall be manufactured using each step of the most severe manufacturing process with regard to temperature and time duration of any given step.
• Double-sided samples shall represent single-sided samples.
• For double-sided boards, the conductor pattern shall be included on both sides of the sample as a mirror image.
• The board minimum Recognized build-up thickness shall be used.
• The board minimum Recognized copper thickness shall be used, if applicable. The copper may be plated up to 1 ounce for test purposes.
• Samples shall be submitted with standard conductor width of 0.3mm (12 mils).
• Samples shall be submitted with A) minimum spacing anticipated for a given voltage, B) secondary spacings (to be tested in the event the minimum spacing fails), C) secondary spacings (to be tested in the event the minimum spacing fails), and D) standard spacing 0.30 mm (12 mils), as shown in Figure 30.1.An additional set of samples shall be provided if silver plugged-through holes are intended in production and not capped by copper.
• Uncoated sample represents a coated sample. When a permanent coating is used to retard silver migration, a set of samples incorporating each different coating shall be submitted. When production always employs a permanent coating (such as solder resist) over the silver, the test samples shall also be coated with the same material.
• Samples shall be submitted for testing with 610 mm insulated test leads attached to permit adjacent conductors to be energized. Plated-thru-holes may be used to attachment the lead wires to the coupon to ensure the attached wires will not compromise the test spacing.
• Insulated wire test leads should be soldered to the test sample using unactivated or mildly activated rosin flux (ROL0 or ROL1 per J-STD-004, "Requirements for Soldering Fluxes"). Care should be taken to avoid getting solder or flux on the test pattern; this can be achieved by covering the test pattern with clean aluminum foil. Teflon-insulated wires should be used to prevent possible contamination of the test chamber by hydrolysis of the wire insulation.