EIPC Summer Conference 2022: Day 2 Review

Estimated reading time: 11 minutes

Dr. Marko Pudas senior engineer and project manager with Picosun in Finland, discussed the technique of atomic layer deposition as a method of applying thin barrier films to PCB assemblies, which had been the subject of a successful project funded by the European Space Agency for mitigating tin whiskering. 

Pudas described the principle of atomic layer deposition (ALD) as two or more chemical vapours or gaseous precursors reacting sequentially on the substrate surface, producing a solid thin film. The process cycle comprised the introduction of molecules containing element A, the adsorption of the molecules on the surface, the introduction of molecules containing element B and reaction with element A on the surface, and the completion of one monolayer of compound AB. The cycle was repeated until the desired film thickness was reached. 

ALD was a batch-coating vacuum deposition process that had been shown to cover the most demanding 3D structures, down to pore sizes of less than 100 nanometres. It was capable of depositing a wide range of materials; aluminium oxide and titanium dioxide were named as examples, and was extremely repeatable in thickness and quality. Adhesion was chemical and the coating would not peel off. Typical coating thicknesses were in the range of 100 nanometres to 0.5 microns.

Pudas showed several examples illustrating the benefits of ALD in space applications, not only tin whisker mitigation, but corrosion protection, oxidation protection, even coating the insides of gas containers to prevent leakage. In high-reliability electronics it enabled very effective conformal coating of PCB assemblies, and he referenced many performance and qualification test results. 

An iNEMI project comparing various test environments for conformal coating evaluation had demonstrated the effectiveness of ALD coatings on silver and copper in extremely corrosive environments. And ALD had been proposed as an alternative to polymer solder mask in critical applications.

Session 5, focused on manufacturing technologies for 5G and beyond, was moderated by Stig Källman from Ericsson. 

Dr. Sebastien Depaifve, technical service engineer with Circuit Foil, discussed developments toward the next generation of ultra-flat ED-copper foils for high speed digital and radio frequency applications. He described the progression of improvements in hyper-very-low-profile foils over the last decade to the ultra-low-profile foils of the present day, explaining that in high frequency applications the signal is carried mainly on the edges of the conductor, and surface roughness contributes to significant signal loss.

He reviewed the factors influencing insertion loss: the type and content of silane bonding treatments, the metallic content of nodular or nodule-free foil treatments, the size and shape of nodules if present, the roughness on both side of the base foil and the grain size of the copper.

The electrodeposition process had been progressively improved, with optimisation of the drum surface preparation and the use of specific organic levellers. Grain-refining additives had been used to optimise grain size to one micron, which gave maximum electrical conductivity, and a model had been developed by Circuit Foil to study the influence of nodular treatment shapes on insertion loss. Lower profile copper foil resulted in lower transmission loss, especially at high frequencies, and a metallic-free passivation process had been developed.

With the decrease in roughness, it was necessary to compensate for loss of mechanical adhesion by silane or other chemical adhesion promoters, and it had been confirmed that silane content had no significant influence on insertion loss.

Depaifve concluded by reiterating that optimisation of all the different parameters was required for next generation materials development, and that the optimisation must be conducted in co-development with resin and glass suppliers.

In a logical follow-on to Depaifve’s paper, Jim Francey, Isola Group’s RF business development director in Europe, presented an update on reliability and loss properties of copper foil for 5G applications on behalf of the iNEMI 5G Copper Foil Project. The iNEMI team set out to characterise various copper surface treatments with the objective of mitigating signal loss while still maintaining good adhesion and, hence, durability of the PCB. In addition, the team undertook a comparative study to determine signal loss from copper roughening by oxide-alternative chemical bonding treatments during PCB fabrication.

Traditional measurement methods for copper adhesion such as peel strength were not always indicative of the durability and reliability of printed circuits, and contact profilometry was inadequate for ultra-low-profile copper foils. Noncontact 3-D tools as defined in IPC TM 650 2.2.22, such as white light interferometry and laser scanning microscopy gave more accurate 3D height profiles.

Certain reliability issues associated with ultra-low-profile copper foils had been identified, particularly copper-to-dielectric delamination either on the clad laminate or oxide-alternative side, or dielectric-to-dielectric where prepreg was bonded to an etched laminate surface. This interface was also a site for possible bond-line CAF growth.

The iNEMI project would benefit the industry by formulating a compendium of material performance criteria and cross referencing to a suite of existing standards, enabling users to specify a category of material that would meet electrical performance and reliability requirements whilst minimising excessive costs through over specification. Additional cost and time savings might also be realised by reducing the need for design reiteration and requalification to meet OEM specifications. 

Completion of the project would enable commonality in specifying the topology of copper foil and bonding treatments, and provide better assurance for meeting PCB electrical performance characteristics. It would also provide predictability of durability and reliability of copper foil adhesion with respect to signal loss for various copper surface finishes, as well as reducing product qualification costs and associated timescales.Page 2 of 3

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