Is Encapsulation Ancient for Package Protection?

Encapsulation innovation: IDEALmold^TM 3Ge, Image source: ASMPT

Encapsulation

In simple terms, encapsulation refers to the process of enclosing plastic material around an object, so that a protection volume forms around the object like a capsule. This method has already been in use since the invention of integrated microelectronics (‘Integrated Circuits’ (ICs)) in the back-end semiconductor packaging and assembly process around 40 years ago.

There are many forms of encapsulation such as injection, glob topping, compression and transfer. The purpose of transfer encapsulation is to protect IC assemblies and their interconnections against mechanical interference and hostile environments when in use. This transfer moulding technology might seem ‘ancient’ in comparison to others, but it has made great progress thanks to advances in Plastic Encapsulated Microelectronics (PEM) packages, encapsulation techniques, and even the plastic material used to protect ICs.

Understanding Transfer Molding

Transfer molding is an encapsulation method where a thermoset plastic material known as ‘epoxy’ is loaded in pellet form and preheated inside a metal pot, then forced to flow into the mould cavities where the PEM in question to be protected is. This process uses a heated plunger, via a plumbing system utilizing a series of gate and runners (see Figure 1).

Figure 1: Complete Transfer Molding Process, Image source: ASMPT

The advantages of transfer moulding are that it facilitates standardization, with less variation in package thickness. This is because it holds tighter tolerances for parts that are more intricate to form. This in turn generates a higher Unit-Per-Hour (UPH) for high cavity count applications. This means shortening the production cycle, a faster setup time, and finally lower operating costs. In addition, transfer molding tooling is cost efficient when moulding a large number of PEMs within a typical 30-second machine cycle time.

Figure 2: Advances in Transfer Moulding - PGS Technique. Image source: ASMPT

One key technical advantage of transfer moulding is its Pinnacle Gating System (PGS). The PGS uses pin gates (see Fig. 2), which are ideal when a packaging designer needs to maximize a substrate’s size that produces a high quantity PEM. For example, when mould flow induced stresses must be properly controlled, specifically by minimizing flow jetting in order to reduce stress to the wire bond to minimize ‘wire sweeping’ (Fig 3.)

Figure 3: Wire sweeping. Image source: ASMPT

The PGS technique is also cost-efficient as it optimizes the available space in a lead frame or substrate, resulting in a high UPH output due to minimal wastage of usable space. It also reduces epoxy wastage (which is eco-friendly) and provides an operational gain via lowered cost-per-unit.

Encapsulating Power-Automotive Packages

Figure 4: Examples of Automotive and Power Packages by ASMPT

Transfer molding is a useful protection technique for power packages, deemed for as applications that carry a high current load of more than 100A, for example, sensing and protection of Electronic Control Units (ECUs) in automobiles. Transfer moulding provides both mechanical and environmental protection to power packages - both Single Side Cooling (SSC) and Dual Sided Cooling (DSC).

One of the most formidable challenges with such packages is to encapsulate them with a package thickness that consumes no more than 40 grams of epoxy and a good moulding outcome.

Generally, such packages are thicker laterally, requiring dual epoxy pellets with higher tonnage and higher force to encapsulate them. Such transfer molding machines require dual pellet and dual row pot-plungers in order to mold these thicker packages (> 4mm) while ensuring a high UPH. In addition, to get a good moulding outcome (free of voids or incomplete fills), high pressure is used to ensure sufficient packaging material is applied to coalesce the epoxy once cured. This means such transfer moulding machines have a force tonnage in excess of 170 tons versus the usual force of 80 tons typically.

SSC and DSC power packages may have exposed metal surfaces. In such cases, it is critical that there is minimal-to-zero flash formation on the exposed surface, because such exposed metal surfaces use a heat sink to dissipate heat generated from the IC when supplied with electric current.

This requires two things: special anti-flash formation designs incorporated into the mould tooling, and the use of film-assisted moulding technology.

The anti-flash designs ensure that exposed metal surfaces are restrained enough to prevent resin or epoxy seeping into the streak lines of the metal lead frame while the film supplements the design by blocking metal surface imperfection.

What’s more, besides the transfer mould tool design, special handling mechanisms are needed to both load and unload the packages, especially for automotive sensing package that usually possess intricate body profiles. Thus, the handling mechanism that loads and unloads in the mould tool must be robust enough to ensure no mechanical damage to the package before and after encapsulation. Such pick-and-place mechanisms are usually more sophisticated for automotive packages, as they require force and pressure sensing capabilities to handle intricate bodies and weight, especially for large ECUs.

Last but not least, the demoulding process. Since automotive power packages use high quality eco-friendly epoxy moulding compounds (free of halogen elements), they are generally very reliable, but have an increased tendency to stick to the moulds and tools during the demoulding process. Thus, a good anti-stick metal coat is applied to the mould tool to minimize this during ejection after the moulding cycle is completed.

Summary

In conclusion, encapsulation technology is far from ‘ancient’. The advances in tool and machinery designs for both handling and minimizing damage of intricate power automotive packages are just of the ways that the technology is keeping pace with the times to better protect semiconductor packages as demands and requirements increase, so as to deliver reliable, problem-free moulding outcomes.