ICT Seminar 2023 Review: Energy, Thermals, and Assistive Technology

Estimated reading time: 9 minutes

‘Things to Consider When It Comes to Thermals’
Robert Art, global account manager, IMS, with Ventec International Group, who gave valuable, practical advice on the choice and specification of materials for thermal management applications, explaining the terminology, the true significance of datasheet values, and the reasons why different test methods give different results. What does the OEM really want? What are the significant parameters? Does the designer understand what he is specifying? Is the fabricator instructed to use a more expensive material than necessary to fulfil the performance requirement? Art provided authoritative answers.

He began by reviewing the nature and basic construction of insulated metal substrates and their important datasheet parameters: thermal resistor, thermal impedance, thermal conductivity, glass transition temperature, maximal operating temperature, breakdown voltage and hi-pot withstand.

The first and most important datasheet value he considered was thermal resistor or thermal impedance, denoted Rth or Zth, which was measured directly and from which thermal conductivity was calculated. Different test methods gave different values; in the example he used, ISO 22007-2 gave 5W/mK, ASTM E1461 gave 3.3W/mK, ASTM D5470 gave 3.0W/mK, and Bruggeman Modelling gave 12W/mK. Art commented that the ASTM D5470 test was most commonly used; it gave the lowest value because it measured the complete stackup, including the losses in the transfer areas. 

“Don’t accept thermal conductivity values,” he advised, and demonstrated the sizeable tolerance bands associated with different test methods, implying that a low-cost competitor can quote almost any value and still be in tolerance, depending on the method. 

Art commented that Ventec is a member of an IPC working group that includes other suppliers and ZFR (Zentrum fuer Waermemanagement) in Stuttgart, to improve the accuracy of the measurement method by tightening thickness tolerances and clearly defining test parameters and probe locations.

He asked, “When should you take a deeper look to the datasheet?” and then listed some key points. These include checking which test method has been used for dielectrics with claimed thermal conductivity of 4W/mK and higher, checking in detail how Rth has been measured, and with what tolerance. For values below 0.4, it’s checking the dielectric thickness that has been used as a probe for an ASTM test, questioning why different thermal measurement methods have been used for closely related values quoted on the datasheet in order to improve their appearance, and being suspicious of remarks like “modified” in reference to test methods.

Having briefly discussed maximal operating temperature, breakdown voltage and hi-pot withstand, Art mentioned the significance of glass transition temperature in IMS dielectrics compared with rigid laminates, where a lower Tg gave a more resilient product and in combination with a low-expansion aluminium alloy helped to mitigate solder joint cracking.

But his over-riding message in specifying an IMS material was, “Be careful, and make sure you understand how it is measured.”

The final presentation of the seminar came from Chris Wall, technical director of Electra Polymers, who succeeded in making the subject of chemical legislation and restrictions on solder masks both interesting and enlightening. He explained that the European Union REACH regulation (Registration, Evaluation, Authorisation, and Restriction of Chemicals), which originally came into force in 2007, requires manufacturers and importers of chemicals to register the substances they produce or place on the market in Europe with the European Chemicals Agency (ECHA). The REACH regulation establishes a process for identifying and controlling substances of very high concern (SVHCs). When the UK withdrew from the European Union, the UK government created UK REACH, which adopted all the provisions of REACH (as they were BREXIT at the time) and regulates chemicals placed on the market in the UK. The UK REACH and the EU REACH regulations operate independently of each other and suppliers must ensure that they comply with both regulations where necessary. Clearly, this raises potential issues for global suppliers.

Wall pointed out that, currently, the specific relevance to solder mask formulation is to the widely-used photoinitiators “907” (2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one) and “369” (2-benzyl-2-dimethylamino-4'-morpholinobutyrophenone). Both materials have recently been prioritised for inclusion in the Authorisation List of substances that can only be used under specific conditions, such that the risks are adequately controlled. Certain other components of solder masks are subject to, or likely to become subject to, usage restrictions.

The timeline for photoinitiators “907” and “369” is an Authorisation Period of 18, 21, or 24 months from the date of inclusion in the Authorisation List, for manufacturers or importers to apply for authorisation to continue manufacture of import of the material. If this is granted, it will be authorised for use for a further period of time. At the same time, a “Sunset Date” will be notified, which is 18 months after the “Latest Application Date. After the “Sunset Date,” the substance can no longer be used or placed on the market without an authorisation.

Wall’s presentation raised many questions from the audience, to which he responded reassuringly, stressing that there is no immediate need to panic, and that Electra and its counterparts in the industry have been working hard to develop liquid photoimageable solder mask formulations of equivalent performance which do not contain the “907” or “369” ingredients.

Bill Wilkie brought the formal proceedings to a close, with warm thanks to all who braved the weather forecast to participate, and wished everyone a safe journey home through the snow.

Note: Much thanks to Alun Morgan for the beautiful images.

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