EIPC Summer Conference 2023: Day 1 Review

Estimated reading time: 14 minutes

He reviewed the manufacturing process for copper foil, indicating the nature of crystal growth in the electrodeposited metal from the smooth surface of the drum to the rougher surface at the electrolyte side, and explained how this surface is subjected to a nodular treatment to increase its roughness and create a mechanical anchor to increase subsequent adhesion of prepreg resin. 

At high frequencies, the “skin effect” causes the signal to be carried close to the surface of the conductor, and a rough surface results in significant signal loss. 

To find a compromise between adhesion and loss, it is necessary to fundamentally understand the influence of roughness on the loss and to develop a way of modelling it. 

Wittmann described the Hall-Huray model, which has been widely used in the PCB and copper foil industry, to analyse the electromagnetic field effects but has the limitation of only considering the roughness in terms of spherical nodules, whereas real copper-foil nodular treatments tend to have more complex shapes. So the objective is to develop a new model applicable, whatever the roughness shape.

Atomic force microscopy is used to measure the geometry of a progressively finer series of foil treatments before finite element analysis is used to predict how the copper-to-prepreg interface reacts to high frequency signals. It is necessary to employ an adaptive mesh to keep good accuracy, with a very fine mesh at the treatment surface, a fine mesh on the thickness corresponding to the skin depth, and a coarse mesh corresponding to the bulk material. Wittman showed animated computer graphics which clearly demonstrate the modelling operation and the differences in insertion loss between four foil treatment examples. 

He confirmed that this new approach allows copper roughness loss to be calculated with consistent and repeatable results and without any assumption of surface geometry. It was also confirmed that the new very-fine-nodular treatment developed by Circuit Foil is almost equivalent to a foil with no treatment in terms of insertion loss.

The theme of the afternoon session was special material solutions and it was moderated by EIPC treasurer Emma Hudson.

E-mobility Applications

The challenges faced by copper-clad laminates in e-mobility applications were discussed in a joint presentation by Andreas Folge from NanYa Plastics and Volker Klafki from Technolam.

Folge clarified the definition of e-mobility as the principle of using electric propulsion for a wide range of transportation types to take goods and individuals by land, air, and sea from A to B. Transportation types include cars, buses, trucks, off-road vehicles, trains, bikes, golf carts, rickshaws, pedelecs, scooters, ships, ferries, and other sea going vessels, as well as drones and aircraft. 

He listed the current challenges of e-mobility: administrative obstacles such as delays in granting licences and permits by public administrations, the infrastructure network such as number and kind of charging points, the source of energy that powers the batteries, as well as the price, volume, and range of the batteries and how they are recycled.

He reviewed some facts and figures: Worldwide, between 2018 and 2021, there had been a tripling of the number of electric light-duty vehicles. Europe has increased by 458%, China by 355% and the U.S. by 182% as consequences of city planning, infrastructure, fleets, and incentives.  

Looking at electric buses as an interesting example, he referred to the situation in Taiwan as an illustration, commenting that of about 17,000 city buses currently in Taiwan, only about 10% are electric. The government is making substantial funds available to accelerate the electrification of public buses, with the objective of phasing-out at least 14,000 diesel engine city buses and electrifying 30,000 long distance buses between 2025 and 2030.

He predicted that the car of tomorrow will require advanced, highly integrated logic PCBs and power PCBs, and the electronics will be subject to new external and internal loads in terms of temperature, humidity, and duty cycle. The resulting requirements will involve material adoptions and special concepts for automotive electronics, together with very good technical skills and understanding. 

Laminates will need to withstand increased temperature loads and temperature cycle loads, together with higher operating voltages with reduced isolation distances in smaller box volumes. 

Klafki suggested that the challenge of high temperature will be addressed using fillers added to the resin system to equalise the thermal expansion coefficient in the z-axis between copper and resin, but UL and IPC specifications define limits on how much filler can be used. Also, fillers increase the viscosity of the resin varnish at the pre-preg manufacturing stage and complicate the impregnation process. High voltages are present in many of the components of the electric vehicle. Therefore, extra consideration must be given to clearance and creepage distances and the comparative tracking index of the material. Precautions are also needed against the progressive effect of partial discharge. Klafki used a video cartoon of Woody Woodpecker as an illustration. 

Folge summarised the specialist laminate products developed by NanYa in response to the emerging requirements of the full spectrum of the electronics industry, and emphasised the benefits of vertical integration in NanYa’s supply chain. 

Measuring Thermal Parameters

Thermal management is an increasingly important consideration in electronics applications and a wide range of insulated metal PCB substrates is available. But how reliably are thermal parameters measured and specified? Robert Art, global account manager with Ventec International Group, discussed thermal measurement methods for insulated metal substrates. 

Besides glass transition temperature and maximum operating temperature, the significant data sheet value for an insulated metal substrate defines its ability to transmit heat, and may be expressed as thermal resistor Rth, thermal impedance Zth, or thermal conductivity TC.

Art stressed that while Rth and Zth values are measured directly, thermal conductivity is calculated from the Rth measurement. He pointed out that different test methods give different results. For example, for a given material ISO 22007-2 gave a thermal conductivity value of 5 W/mK, ASTM E1461 gave 3.3 W/mK, ASTM D5470 gave 3.0 W/mK, whereas the Bruggeman Model gave 12 W/mK.

He outlined the features of the test methods: ISO 22007-2 is a fast, highly accurate procedure that measures the true bulk properties of the material without transfer losses, using a probe that needs to be at least 3 mm thick. ASTM E1461 is a contactless method used for high temperature testing. ASTM D5470 is easily influenced by the environment and requires a long test time. It is not suitable for low Rth values but is used as a main standard on the market. 

But Art believes that IPC TM-650 2.4.54, a new standard recently published by IPC, will replace ASTM D5470 and finally give designers and thermal engineers a way to compare multiple technical data sheets side-by-side. He listed details of what features had been improved or fixed to improve the accuracy of measurement. These includes defined parameters, defined probe location, and tighter thickness tolerances. 

He advises taking a closer look at the datasheet, checking what test method have been used, being aware of measurement tolerances, and being wary of remarks such as “modified” or “version x,” which could be an indication of the test having been manipulated to give a desired result.Page 2 of 3

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