Initially, it was thought the conformal coating would require masking of board through-holes to ensure good electrical contacts when connectors and other components are installed. Alstom’s RFQs to board vendors included a specification for the coating material and through-hole masking. However, one of the vendors, Harting of North America suggested an alternative: no masking, plus components and connectors with press-fit pins designed to cut through the conformal coating to make electrical contact with the printed circuit board metal traces.
This may not sound as robust as, for example, press fitting or inserting and reflow soldering components before coating. However, “press-fit after coating” technology has proven to be highly reliable in military and telecommunications equipment used under severe conditions.
There is no question that conformal coating improves a PCB’s resistance to environmental conditions. Applying it before component insertion protects PCB traces while allowing easier troubleshooting and repair if a component fails. However, Alstom’s foremost concern is equipment reliability and train safety, followed closely by designs that allow cost-effective manufacturing.
When Harting received Alstom’s RFQ for the backplane board, its engineers saw opportunities to improve the board’s layout, reliability and cost structure. Based on Harting’s experience supplying similar boards in a wide range of high-reliability applications, its engineers proposed the use of connectors and other components with press-fit pins that allow conformal coating before insertion. These pins have a cross-section with sharp corners that not only penetrate conformal coatings, but also cut through the PCB metallization oxide layer to make reliable electrical contacts.
Alstom’s specs called for a urethane coating material that has excellent adhesion properties to provide a high level of protection for the PCB solder mask. This was no problem for the pin design suggested by Harting, which can cut through both urethane and acrylic coatings.
Besides creating a low resistance contact, this type of pin has tremendous reliability benefits. After the coating is penetrated by these pins, it reseals around them, and thereby increases the environmental protection for all those component connections. Essentially, the board is completely sealed.
Another benefit of the specifications proposed by Harting was the opportunity to create a PCB with narrower metal traces. This allows higher component density and a smaller board size. Ultimately, this has a favorable impact on the end equipment design and its cost structure.
Final Step – Testing
To be cost-effective, complex backplanes must be assembled with automated component insertion equipment. This requires equally complex test equipment. Regardless of board design and component installation technology, a vendor must supply a 100 percent tested, ready-to-run assembly. Board tests should include:
- Automated optical inspection (AOI) for proper component insertion, etc.
- Electrical continuity and isolation
- Resistance, capacitance and inductance measurements
- High frequency signal integrity analysis
- Electromagnetic compatibility (EMC)
- Mechanical integrity of connectors and other assembly components
- Climatic and environmental tests, including vibration and shock
- Functional testing/simulation
Ultimately, Alstom’s selection of its backplane vendor was strongly influenced by the press-fit assembly and testing capabilities at Harting’s Elgin, Ill., value added business (VAB) facility. Its automatic presses are a speedy way to insert components, and every insertion stroke is measured and recorded for proper pressure and pin insertion depth.
The backbone for the company’s test capabilities is its RoBAT automated backplane tester. This test system combines electrical, mechanical and optical measurements in a single automated machine that covers the entire backplane fault spectrum at the assembly stage.
The RoBAT S1 is the only fixtureless backplane tester Harting found to have commercially realistic test times. In addition, fixturing costs are a tiny fraction of the bed-of-nails design that has long been a staple of the industry. Besides comprehensive electrical testing, it has both AOI and co-ordinate measurement (CMM) capabilities, which are essential since more than 50 percent of all faults found on assembled backplanes are non–electrical. These capabilities make sure all components are in the proper places, pins are inserted far enough into the board, and there are no bent pins.