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


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On the question of whether to retain non-functional pads, it was MBDA’s practice to retain them. And thermal relief pads were used on copper planes with the objective of reducing heat-sinking effects.

MBDA had special clearance rules for selective soldering, with local spot masking an alternative if these clearances could not be maintained.

On the subject of microvia design, the defining standard was IPC-2221. MBDA’s maximum aspect ratio was 1:1, with minimum drill diameter 0.13 mm and pad diameter 0.35 mm. Staggered microvias were preferred; stacked microvias were only permitted in special circumstances. Copper filling was mandatory for surface microvias used for via-in-pad designs.

Cawthorne talked in detail about the benefits of via protection, with reference to IPC-4761 regarding the covering or filling of via holes with organic material, and described the characteristics of tenting, partial plugging and complete plugging methods. With all of these, coplanarity remained an issue, and planarisation was necessary as an aid to assembly. MBDA’s approach was to plug and copper-cap all plated-through vias, although it was preferred to use microvias for outer-layer conductors.

On controlled impedance, the defining standard was IPC-2221 Section 6.4, and although IPC-2221 gave equations for impedance calculation, MBDA worked closely with their PCB fabricators who had access to advanced simulation tools, to determine the effects of material properties and pressed dielectric thicknesses and adjust dimensions accordingly.

Ian_Fox .jpgIan Fox commented on the close parallel between the requirements of Rolls Royce and MBDA with regard to material specifications and design-rule conventions, and particularly concurred on the benefits of carefully selecting PCB fabricators and building close working relationships with them, as he introduced Part 2 of the webinar programme and reviewed the attributes of a range of surface finishes, going on to discuss testing and qualification of printed circuit boards and incoming inspection procedures.

Electroplated and fused tin-lead was the traditional PCB finish of the military and space communities and remained the preferred finish of the European Space Agency. As Fox said, “Nothing solders like solder!” Hot air solder leveling was still used as a method of selectively applying tin-lead and lead-free solder finishes, but there were issues with non-uniformity of thickness and non-flat surface profiles that made it unsuitable for fine-pitch surface mount assembly. 

Electroless nickel immersion gold (ENIG) was probably the most common surface finish in current use. The defining standard was IPC-4552, which specified a nickel thickness of 3-6 microns and a minimum gold thickness of 0.05 micron. No maximum thickness was stated but a typical process maximum was 0.15 micron.

Fox pointed out that the electroless nickel deposit was not pure metal but contained between 8─11% phosphorus as a by-product of sodium hypophosphite used as a reducing agent in the chemical formulation. The copper surface required to be sensitised, typically with palladium, to initiate the nickel-phosphorus deposition. The thin layer of gold prevented oxidation of the nickel surface and rendered it solderable.

Immersion gold was deposited by a galvanic displacement process which effectively involved a controlled corrosion of the nickel surface and tended to be self-limiting in thickness. The immersion gold chemistry was highly acidic and if the process was not properly controlled could cause a hyper-corrosion effect resulting in “black pad” defects.

ENIG offered the advantages of a flat solderable surface to aid fine-pitch surface-mount assembly and could be aluminium wire-bonded. Additionally, electroless nickel had been observed to increase the fatigue life of plated holes.

Electroless nickel electroless palladium immersion gold (ENEPIG), as defined by IPC-4556, offered improvements over ENIG. A layer of approximately 0.5 micron of palladium between the nickel and the gold eliminated black pad effects and rendered the finish wire-bondable with gold or aluminum.

Of the other finishes available, immersion silver—effectively an organo-metallic coating deposited directly on copper—had been associated with creep corrosion and silver migration under certain conditions; immersion tin was good for press-fit but had limited shelf life and was not suitable for multiple reflow operations; neither were the organic solderability protective finishes. So these alternative finishes tended not to be preferred options in hi-rel mil-aero applications.

Moving on from finishes to the final stages of the bare-board fabrication process, Fox described how each PCB was validated before delivery by non-functional electrical test in accordance with IPC-9252, to verify continuity, isolation and track resistance, using a test program generated by the fabricator using a net list derived from the supplied digital manufacturing data.

Rolls Royce specified a minimum test voltage of 40 volts, although 250 volts was preferred, and if testing was carried out below 250V then it was necessary to carry out an additional 250V “proof of design” test. They required a minimum of 10 megaohms isolation between unconnected circuit elements, and a maximum trace resistance of 5 ohms, except in the case of unusually long nets, where trace resistance should not be greater than 0.5 ohms per 25 mm. Adjacency settings for flying probe test equipment were specified as 1 mm on internal features and 2 mm on externals. And if heat sinks were fitted, a minimum insulation resistance of 10 megaohms at 500 volts to unconnected circuitry was required.

How did Rolls Royce qualify a PCB design in terms of structure and fabrication, assembly and reliability, and how and at what stage was the design sealed? These were the areas Fox focused on next.

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