PCB Design, Fabrication and Use from the Mil-Aero End-User Perspective

Estimated reading time: 13 minutes

As part of the qualification procedure, Rolls Royce would initially visually assess external features and quality in accordance with the basic requirements of IPC-A-600, then microsection, primarily to assess the build, core thicknesses, dielectric separations and copper weights of internal layers, drill quality and copper thickness of plated-through vias, but also include an assessment of key features such as blind and buried vias. And finally, to cross-check to ensure that their measurements agreed with the lab report supplied by the fabricator.

The integrity and structural robustness of the fabrication would subsequently be assessed by solder float testing in accordance with IPC-TM-650, 2.6.8E, Condition A, ensuring that the test piece contained key structural features, through-holes, blind and buried vias, followed by further microsection examination. Where appropriate, multiple solder float tests would be carried out to observe at what point the structure began to break down.

A number of complete PCBs would then be subject to assembly simulation and reliability testing by multiple reflow operations using a representative oven profile, followed by extended thermal cycling, visually assessed for evidence of bow and twist, and again microsectioned to examine key features. A typical accelerated thermal cycling regime used at Rolls Royce was three cycles per hour between -55°C and +125°C. Successful completion of this testing would assure that the design was robust.

In parallel with bare-board reliability testing a fully assembled board would be produced by the standard production route with an appropriate solder reflow, followed by functional testing and Rolls Royce’s standard screening procedure. Successful electrical validation after reflow followed by completion of the screening tests, would validate the printed circuit design and build and allow the design to be sealed, and the design document updated to include all materials, measurements and test results. Should an alternative fabricator be required at some point in the future, this approach would enable the design to be moved with a high degree of confidence that its performance would be unaffected.

Having described the details of the qualification procedure, Fox discussed Rolls Royce’s incoming inspection and quality standards for production PCBs. Samples from a delivered batch were visually inspected at 4X magnification and examined for significant defects such as scratches that could result in open circuit tracks, etch defects that could result in opens or shorts, flaking or crazed solder mask and stained or discoloured solder pads that could affect solderability.

If microsections were supplied with the delivered batch of PCBs, they would be examined, and measurements and comments compared with the submitted laboratory report, paying attention to drilled hole quality and plated copper thickness. Also, innerlayer copper foil thicknesses and electroless nickel thickness where checked if the finish was ENIG or ENEPIG.

Rolls Royce used IPC reference standards as follows: IPC-6011 Generic Performance Specification for Printed Boards; IPC-6012 Qualification and Performance Specification for Rigid Printed Boards; IPC-6013 Qualification and Performance Specification for Flexible Printed Boards; IPC-6016 Qualification and Performance Specification for High-Density Interconnect (HDI) Layers or Boards; and IPC-A-600 Acceptability of Printed Boards. Boards were classified according to end use: Class 1 – general electronic components; Class 2 – dedicated service electronics products; Class 3 – high reliability electronic products; and Class 3/A – supplementary requirements to Class 3 for space and military avionics electronic products. Inspection and quality requirements for each class of PCB were detailed in the individual IPC specifications.

Fox went on to discuss Rolls Royce’s handling and storage procedures for PCBs upon receipt. All boards delivered were required to be securely packaged to prevent mechanical damage in transit, with packaging suitable for use in an ESD protected area. Dissipative packaging was acceptable, but full moisture-barrier ESD bags were preferred, and if multiple PCBs were contained within a single bag they required to be individually wrapped in sulphur-free paper. Once removed from their packaging all PCBs required to be handled wearing either ESD-safe gloves or finger cots, unless they had been transferred into assembly pallets. Any boards incorrectly handled were required be washed, dried and re-baked prior to use.

PCBs delivered in dissipative packaging were immediately transferred to low-humidity storage cabinets and stored until required, although boards delivered in full moisture bags could be stored in normal-humidity environments for up to 14 days prior to being used. If the 14-day period was exceeded, the boards were required to be baked prior to use, at 120°C for a minimum of four and a maximum of eight hours.

Fox commented that PCBs fabricated in FR-4 material and soldered using a tin-lead profile were extremely robust and that delamination during soldering was rare for assemblies stored in normal-humidity conditions for up to seven days, although this might not be true for lead-free soldering. Polyimide-based rigid, flexible and flex-rigid PCBs were not as robust, and assemblies stored under normal humidity conditions for more than 24 hours required to be re-baked prior to soldering.

In general terms, the procedures described for Rolls Royce were directly comparable with those described for MBDA, and the presenters were unanimous in re-emphasising the importance of working closely with their chosen PCB fabricators at all levels and all stages of design, qualification and production of their circuit boards.

From a personal point of view, it was a great privilege to listen to two leading expert PCB technologists from the military and aerospace industry sectors sharing their experiences and describing the procedures and precautions taken by their respective companies to ensure the quality and reliability of PCBs supplied to them by their approved suppliers. Many thanks are due to Charles Cawthorne and Ian Fox for generously providing the knowledge, to Bob Willis for his professional and seamless management of the webinar, and to the European chapter of SMTA for delivering another interesting and informative learning experience.

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