The Influence of Post Mold Cure Process and Its Applications

Introduction

Post mold cure (PMC) is a crucial process within the electrical industry. It involves subjecting a molded part to elevated temperatures to accelerate the curing process and enhance the physical properties of the material. This process expedites cross-linking and aligns polymer molecules, resulting in stronger parts with improved high-temperature characteristics. Similar to tempering steel, post-mold curing of thermosetting materials can significantly enhance properties like tensile strength, flexural strength, and heat distortion temperature. Moreover, PMC is a common solution for addressing warpage issues and mitigating outgassing in IC packages.

Given the benefits of PMC, it finds widespread use in the electrical device industry, with many companies incorporating PMC into their production processes. Figure 1 showcases various chips packaged using PMC methods.

PMC Process in IC Encapsulation

The IC encapsulation process consists of two primary manufacturing steps. In the first step, the IC chip is encapsulated in a thermosetting epoxy molding compound (EMC), the most common material for IC packages. Pelletized EMC is injected into a hot mold die, forming the package around the IC die. After injection, the mold undergoes curing, aiming for good fill and partial cure.

The second step is the post mold cure (PMC) process, designed to enhance cure and reduce warpage in IC chips. PMC optimizes properties by ensuring the completion of chemical cross-linking. During PMC, the material undergoes further molecular rearrangement, leading to a greater degree of cross-linking. The heating can also break apart residual peroxides, initiating additional chains. The PMC heating process typically comprises three stages: heating from room temperature to PMC temperature, maintaining a constant temperature, and cooling to room temperature.

Applications of PMC

Numerous electronics companies, particularly semiconductor designers and manufacturers, have adopted PMC to enhance product performance. Table 1 provides an overview of products and associated companies that have benefited from PMC, underscoring its widespread use in the electronic market.

The Influence of PMC on Material Properties

The properties achieved through PMC are largely determined by the epoxy molding compound (EMC) used. Various epoxy resin structures impact EMC's flow characteristics, thermal performance, and electrical properties. The PMC process increases the molecular weight of the polymer, influencing mechanical, chemical, and thermal properties. Figure 2 demonstrates the physical property changes resulting from PMC, highlighting the substantial improvements achieved.

PMC serves to prevent issues such as warpage and outgassing during IC encapsulation. Warpage is a common concern in IC encapsulation, and PMC effectively mitigates this problem. It is also essential for addressing outgassing, which can lead to poor material properties and chemical decomposition. To ensure an effective PMC process, precise temperature control is required.

Furnace Selection for PMC

Selecting an appropriate furnace involves considering process requirements such as temperature, time, and atmosphere. Furnaces used for PMC can be categorized as batch or continuous furnaces. Batch furnaces are suitable for various part sizes but are limited in terms of production volume. They can only process one batch at a time. Continuous furnaces, on the other hand, are ideal for high-volume production and offer advantages like superior temperature uniformity, increased throughput, and reduced changeover times.

Converting from batch to continuous furnaces is recommended for steady part flow, and these furnaces offer superior flexibility. A critical consideration is the use of hot air convection furnaces that provide temperature uniformity, high energy efficiency, and environmental friendliness.

Furnace Control for PMC

Temperature and time control during PMC are critical for obtaining high-quality results. Proper temperature control is vital, as excessive temperatures can damage components, while insufficient heating leads to poor PMC quality. Uniform furnace temperatures are essential. A good furnace automatically shuts off when the temperature exceeds the set point to prevent distortion of parts.

In summary, post mold cure (PMC) is a vital process in the electrical industry, offering enhanced material properties and solutions to common issues like warpage and outgassing. Proper furnace selection, such as continuous furnaces with hot air convection heating, and precise temperature control are key factors in ensuring the success of the PMC process. This technology continues to play a pivotal role in enhancing the performance of electrical devices.

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