The B-24 is a single sided coupon with only 4 comb patterns 20 mil spaced (16 mil) line, while the B25A has 2 of the B-24 comb and one Y pattern and one 12 mil line and space (Pattern B from the original B25 test coupon). I have attached a comparison of the different test coupons used at Foresite called Comparison of Test Vehicles.

Thanks for your question. The answer is derived from Appendix C of IPC-9201 Surface Insulation Resistance Handbook. This Appendix discusses SIR test vehicles specifically, but these test coupons are also used for other kinds of tests. You'll see that the B-25A has more test patterns than the B-24.

The use and configuration of IPC standard test boards are the province of the various IPC task groups. As an example, the IPC-B-24 is controlled by the 5-24a Flux Specification Task Group. The IPC-B-25A is the province of the 5-33b Solder Mask Task Group.

History of IPC-B-24:

The IPC-B-24 board was originally designed in the early 1990s as a test vehicle that could be used to test the interactions between laminate, surface metallization, and fluxes, primarily in a wave solder application. The test board was first used in the IPC CFC replacement experiments to evaluate various fluxes as alternatives to high solids rosins. IPC-TP-1043 and -1044 contain more historical data. The choice of 0.016 in lines and 0.020 in spaces was chosen by the researchers as a spacing that could be consistently wave soldered in the face down orientation without solder bridging from the wave.

The board worked well for these experiments. In addition, the line widths and spacings of the boards were found to work well with stencil printed solder pastes. As many of the researchers of that time were involved in both the MIL-F-14256 and J-STD-004 committees, the inexpensive B-24 board became the default test vehicle for SIR for these standards.

The Telcordia GR-78-CORE, and its precursor NWT-TR-000078, used frequently in the telecommunications industry, allows the use of the IPC-B-24 comb patterns for SIR testing, but with an adjustment of pass/fail criteria based on the difference in the number of ‘‘squares.’’ The GR-78-CORE document does NOT allow the use of the B-24 board to test for Resistance to Electromigration. The only pattern which can be used for that specific test is the IPC-B-25 B/E pattern or IPC-B-25A pattern D (0.0125 in lines/spaces).

B-24 Design Variations

The B-24 board has only had one minor revision (Rev A). The earliest versions of the B-24 board had no guard trace emanating from Pin 21, and so possible electrical leakage could be experienced in the SIR measurements for Pattern B. Therefore, a grounded guard trace was added to Pin 21 in the revision.

History of IPC-B-25A:

The IPC-B-25A board was designed by the IPC Solder Mask Performance Task Group as part of the IPC-SM-840 Revision C effort. Prior to the C revision, the qualification vehicle was the IPC-B-25 standard test board, outlined earlier. The B-25 board was fairly large and expensive, and only a small portion was used for SIR testing with solder masks. The B-25 board had no guard banding for SIR testing and several flaws in the test board had been identified.

For the IPC-B-25A test board, two comb patterns from the IPC-B-24 test board were included and are designated patterns E and F. Pattern D was included from the older IPC-B-25 board, identical to pattern B/E. This pattern was included such that Bellcore (Telcordia) could use this test vehicle for materials qualification to NWT-TR-000078 or GR-78-CORE. Pattern C, the military Y pattern, was used to link to extensive qualification databases in the military.

If you are testing liquid or paste flux residues, you will need to use the IPC-B-24 test coupon.

B-24 is a Surface Insulation Resistance test board, and the IPC test method for this board is IPC-TM-2.6.6.3

B-25a is a multi-purpose test board used to evaluate the affects of moisture and insulation resistance of solder masks.

The IPC test method for the B-25 coupon is 2.6.3.1

As a side note, the B-24 test board has undergone an update, and is now called B-24B

The board was cleaned up a bit, and a few design issues corrected, but either will work fine.

Boards can be purchased through many board suppliers, and/or IPC direct.

I've dug out the test coupons for you along with their test methods. Hopefully this will clarify your questions.

Test method 2.6.3.3 Surface Insulation Resistance, Fluxes

IPC-B-24 Test coupon

1 Scope This test message is to characterize fluxes by determining the degredation of electrical insulation resistance of rigid printed wiring board specimens after exposure to the specified flux. This test is carried out at high humidity and heat conditions.

Test method 2.6.3.5 Bare Board Cleanliness by Surface Insulation Resistance

IPC-B-50 Test coupons,

1 Scope This test method is used to characterize the cleanliness of printed wiring board fabrication processes by determining the degradation of electrical insulation resistance under conditions of high temperature and humidity.

This test method examines the cleanliness of a test substrate prior to solder mask application, after solder mask application, and after any final metalization and/or surface finish operation (e.g., HASL or OSP), and may be used to demonstrate the cleanliness of internal layers of a multilayer board prior to lamination.

Test method 2.6.3.4 (Supersedes 2.6.3.4 and 2.6.3.1C for Conformal Coating Test) Moisture and Insulation Resistance – Conformal Coating

IPC-B-25A, Test Coupon

1 Scope This test method is used to determine the moisture and insulation resistance of applied conformal coating under prescribed conditions of temperature and humidity. Raw material qualification testing is performed on designated comb patterns.