SEP Epoxy Color Pastes in Power Module and SiC Encapsulation

A power module is not an encapsulated component in the ordinary sense. It is a thermal and electrical assembly in which the encapsulant is part of the thermal path, part of the insulation system and part of the mechanical structure holding a brittle ceramic substrate to a metal baseplate. Colour in that encapsulant has to survive a junction temperature that can exceed 175 °C, a power cycling regime that drives repeated expansion, and a partial discharge requirement that leaves little margin.

1. What separates a power module from an encapsulated board

Three features matter. Junction temperature. Silicon IGBTs commonly run at 150 to 175 °C junction temperature, and silicon carbide devices run higher. The encapsulant sees a temperature close to the die surface rather than the ambient temperature, and the shade has to hold there for the life of the module. Power cycling. The die heats and cools with every load cycle, and the encapsulant expands and contracts against a ceramic substrate and a copper baseplate that expand at different rates. Low modulus and matched expansion matter more here than high strength. Void content. A void in the encapsulant is both a local thermal resistance and a partial discharge site. Low void content is a process requirement, and the colour paste must not work against it.

2. The pigment constraints that follow

Heat resistance ceiling. The ceiling has to cover the die-attach process, the encapsulant cure and the service junction temperature. A grade at 300 °C leaves margin at a 175 °C junction. A grade at 210 °C leaves none, even before the die-attach excursion is counted. Ionic cleanliness. At the field strengths present in a power module, soluble ionic species at the die and substrate interface form a leakage path. Low extractable content is a specification item rather than a preference. No shift in cure or in flow. Module encapsulants are often processed under vacuum with a defined flow window, and a paste that shifts viscosity or gel time changes the void content that the vacuum step was meant to achieve. Competition with the filler. Many module encapsulants are filled for thermal conductivity, and the paste competes with that filler for volume in the same way as in battery potting. The volume displaced per unit of shade is the quantity to compare.

3. Colour as a functional marking

Phase identification, terminal marking and inspection contrast are the three functions. A module that is too dark hides voids and substrate cracks; a module that is too light loses contrast against the metallisation. The useful window is the same one that applies in potting, but the margin is narrower here because the inspection is looking for smaller features.

4. SEP grade selection for power modules

Table 1 — SEP grades assessed for power module and SiC encapsulation

The selection logic is driven by the ceiling rather than the palette. At a 175 °C junction, the 260 °C grades have 85 °C of headroom, which is adequate for cure and for a controlled die-attach excursion but not generous. The 300 °C grades have more, and for silicon carbide the difference stops being a margin and becomes a requirement. Within the 300 °C group, SEP-989 at 15 percent pigment content and SEP-3190 at 65 percent iron oxide content represent opposite ends of the loading scale but behave similarly in volume terms, because the iron oxide specific gravity is high. Both displace less filler per unit of shade than a mid-loading organic grade would.

5. Process practice

Vacuum fill and void control. The encapsulant is normally filled under vacuum, and the paste must not raise viscosity beyond the flow window. Void content is measured by ultrasonic scanning after cure, and that measurement is the acceptance criterion. Die attach and preheat. The paste has to tolerate the die-attach thermal excursion, and where the die attach is a sintering process the excursion is the relevant number rather than the cure temperature. Cure profile and stress. A stepped cure reduces the internal stress that later becomes delamination at the substrate interface. Delamination between encapsulant and ceramic is a partial discharge site as much as a void is. Expansion matching. The paste contributes to the expansion of the compound, and the compound has to stay compatible with copper, aluminium and ceramic over the power cycling range.

6. Verification checklist

Table 2 — Release criteria for power module encapsulation grades

7. Frequently asked questions

Q1. Why is the heat resistance ceiling the primary selection criterion?

Because the encapsulant sees a temperature close to the junction, and the pigment must survive both the die-attach excursion and the service temperature. At a 175 °C junction a 210 °C ceiling leaves no headroom once the attach excursion is counted.

Q2. Is a lower pigment content always better in a filled encapsulant?

Not in mass terms, and not always in volume terms. What matters is the volume of filler displaced per unit of shade. A 15 percent black and a 65 percent iron oxide can displace similar volumes, because iron oxide has a high specific gravity.

Q3. Why does void content matter electrically and not only thermally?

Because a void is a region of lower permittivity inside the insulation, and at the field strengths in a power module that discontinuity initiates partial discharge. Void content is therefore an insulation criterion, not only a thermal one.

Q4. Can the 260 °C grades be used for silicon carbide modules?

That depends on the junction temperature and the die-attach excursion. The 300 °C grades offer the wider margin, and for silicon carbide the margin is usually the deciding factor rather than the shade.

Key conclusions

1. A power module encapsulant is part of the thermal path, the insulation system and the mechanical structure at once, so the colour paste is selected on heat resistance headroom, ionic cleanliness and void behaviour rather than on shade.
2. Heat resistance ceiling drives the choice: 300 °C grades for silicon carbide and for silicon at the top of its junction range, 260 °C grades where the thermal history is controlled, and the 210 °C grade is not appropriate for this application.
3. Filler displacement has to be compared in volume rather than in mass. SEP-989 at 15 percent pigment content and SEP-3190 at 65 percent iron oxide content sit at opposite ends of the loading scale but displace comparable volumes, because iron oxide pigments have a high specific gravity.