Introduction to the mechanism and characteristics of epoxy resin for LED packaging

This article will introduce the mechanism, characteristics and application materials of epoxy resin encapsulation plastic powder, hoping to help IC packaging engineers in selecting materials and explaining the packaging mechanism.

Semiconductor (LED) packaging industry occupies the main position of domestic integrated circuit industry, and the question of how to choose electronic packaging materials has become more urgent. According to data, more than 90% of junction transistors and 70%~80% of integrated circuits have used molecular compound plastic packaging materials, and epoxy resin packaging plastic powder is the most common molecular compound plastic packaging material. This article will introduce the identity, characteristics and application materials of epoxy resin packaging plastic powder, hoping to help IC packaging engineers in selecting materials and analyzing packaging mechanisms.

1. The purpose of LED packaging

Semiconductor packaging is used to protect diodes, transistors, ICs, etc. from airtightness and humidity and temperature in the surrounding environment, and to prevent electronic components from mechanical vibration, impact, and damage that may cause changes in component characteristics. Therefore, the purposes of packaging are as follows:

(1) Prevent eczema and other rash from invading from the outside;

(2) Support the wire mechanically;

(3) Effectively discharge the heat generated inside;

(4) Provide a shape that can be held or held.

Semiconductor components encapsulated with ceramics and metal materials have better airtightness, but higher costs, and are suitable for applications with higher reliability requirements. Semiconductor components encapsulated with molecular compound plastics have poor airtightness, but low costs, so they have become the mainstream of consumer products such as televisions, telephones, computers, and radios.

2. Encapsulation of the molecular compound plastic material used

Most semiconductor products are packaged with epoxy resin. Its general properties include: formability, heat resistance, excellent mechanical strength and electrical insulation. At the same time, in order to prevent the degradation of the properties of the packaged product, the thermal expansion coefficient of the resin should be small, the water vapor permeability should be small, it should not contain impurities that affect the components, and the lead pins (LEAD) should have excellent adhesion. It is very difficult for a pure resin to fully meet the above-mentioned properties, so most of the resins are added with fillers, adhesives, hardeners, etc. to become composite materials for application. Generally speaking, epoxy resin has better electrical properties, adhesion and excellent low-pressure molding fluidity than other resins, and it is cheap, so it has become the most commonly used semiconductor plastic packaging material.

3. Composition of epoxy glue powder

In addition to epoxy glue, the commonly used encapsulation powder also contains hardeners, promoters, flame retardants, adhesives, release agents, fillers, pigments, lubricants and other ingredients, which are introduced as follows:

3.1 Epoxy Resin

The types of epoxy resins used in packaging plastic powder include bisphenol A series (BISPHENOL-A), NOVOLACEPOXY, cycloaliphatic epoxy resin (CYCLICALIPHATICEPOXY), epoxidized butadiene, etc. The epoxy resin used in packaging plastic powder must contain a low ion content to reduce the corrosion of the aluminum strip on the surface of the semiconductor chip. At the same time, it must have a high heat deformation temperature, excellent heat resistance and chemical resistance, and good reactivity to hardeners. You can choose a pure resin, or you can mix two or more kinds of resins for use.

This article will introduce the mechanism, characteristics and application materials of epoxy resin encapsulation plastic powder, hoping to help IC packaging engineers in selecting materials and explaining the packaging mechanism.

Semiconductor (LED) packaging industry occupies the main position of domestic integrated circuit industry, and the question of how to choose electronic packaging materials has become more urgent. According to data, more than 90% of junction transistors and 70%~80% of integrated circuits have used molecular compound plastic packaging materials, and epoxy resin packaging plastic powder is the most common molecular compound plastic packaging material. This article will introduce the identity, characteristics and application materials of epoxy resin packaging plastic powder, hoping to help IC packaging engineers in selecting materials and analyzing packaging mechanisms.

1. The purpose of LED packaging

Semiconductor packaging is used to protect diodes, transistors, ICs, etc. from airtightness and humidity and temperature in the surrounding environment, and to prevent electronic components from mechanical vibration, impact, and damage that may cause changes in component characteristics. Therefore, the purposes of packaging are as follows:

(1) Prevent eczema and other rash from invading from the outside;

(2) Support the wire mechanically;

(3) Effectively discharge the heat generated inside;

(4) Provide a shape that can be held or held.

Semiconductor components encapsulated with ceramics and metal materials have better airtightness, but higher costs, and are suitable for applications with higher reliability requirements. Semiconductor components encapsulated with molecular compound plastics have poor airtightness, but low costs, so they have become the mainstream of consumer products such as televisions, telephones, computers, and radios.

2. Encapsulation of the molecular compound plastic material used

Most semiconductor products are packaged with epoxy resin. Its general properties include: formability, heat resistance, excellent mechanical strength and electrical insulation. At the same time, in order to prevent the degradation of the properties of the packaged product, the thermal expansion coefficient of the resin should be small, the water vapor permeability should be small, it should not contain impurities that affect the components, and the lead pins (LEAD) should have excellent adhesion. It is very difficult for a pure resin to fully meet the above-mentioned properties, so most of the resins are added with fillers, adhesives, hardeners, etc. to become composite materials for application. Generally speaking, epoxy resin has better electrical properties, adhesion and excellent low-pressure molding fluidity than other resins, and it is cheap, so it has become the most commonly used semiconductor plastic packaging material.

3. Composition of epoxy glue powder

In addition to epoxy glue, the commonly used encapsulation powder also contains hardeners, promoters, flame retardants, adhesives, release agents, fillers, pigments, lubricants and other ingredients, which are introduced as follows:

3.1 Epoxy Resin

The types of epoxy resins used in packaging plastic powder include bisphenol A series (BISPHENOL-A), NOVOLACEPOXY, cycloaliphatic epoxy resin (CYCLICALIPHATICEPOXY), epoxidized butadiene, etc. The epoxy resin used in packaging plastic powder must contain a low ion content to reduce the corrosion of the aluminum strip on the surface of the semiconductor chip. At the same time, it must have a high heat deformation temperature, excellent heat resistance and chemical resistance, and good reactivity to hardeners. You can choose a pure resin, or you can mix two or more kinds of resins for use.

3.2 Hardener

The hardeners used to crosslink epoxy resin on the top of the encapsulation plastic can be roughly divided into two categories:

(1) Carbonic anhydrides (ANHYDRIDES);

(2) Phenolic gum (PHENOLICNOVOLAC).

The following characteristics are compared between epoxy resin systems cured with phenolic resin and those cured with carbonic anhydride: ● The system cured with phenolic resin has less glue overflow, easier demoulding, better moisture resistance and better thermal stability than that cured with carbonic anhydride; ● The system cured with carbonic anhydride requires a longer curing time and a higher temperature post-curing (POSTCURE); ● The system cured with carbonic anhydride has better compatibility with components that are sensitive to surface leakage current; ● The system cured with phenolic resin has better thermal stability between 150-175°C, but the system cured with carbonic anhydride is better when the temperature is higher than 175°C.

In addition to electrical properties, the selection of hardeners also needs to consider factors such as workability, moisture resistance, survivability, price, and safety to the human body.

3.3 Accelerator (ACCELERATIVE OR CATALYST)

The curing cycle of epoxy encapsulated plastic powder is about 90-180 seconds. It must be able to cure in an instant, so it is necessary to add promoters to the plastic powder to shorten the curing time.

The epoxy resin plastic powder used in large quantities now contains hardeners and promoters, and has become partially cross-linked B-STAGE rubber after compounding. Before the packaging application is completed, the plastic powder itself will continue to undergo cross-linking and hardening reactions, so the plastic powder must be stored in a refrigerator below 5°C to inhibit the hardening speed of the plastic powder, and the plastic powder also has a shelf life. If you want to produce plastic powder that does not need to be stored at low temperatures and has a long shelf life (LNOGSHELFLIFE), you must use latent promoters (LATENTCATALYST). This type of promoter will not accelerate the hardening reaction at room temperature, and will only produce the effect of accelerating the hardening reaction at high temperatures. At present, Japan has produced epoxy resin powder that does not need to be stored at low temperatures. The key lies in the selection of latent promoters.

3.4 Flame retardant

Flame retardants in epoxy resin powder can be divided into organic and inorganic types. The organic type is brominated epoxy resin or tetrabromobisphenol A (TETRABROMOBISPHENOLA). The inorganic type is antimony trioxide (Sb203) powder. The two can be applied separately or together, and the flame retardant applied together is more effective.

3.5 Filler (LILLER)

In encapsulated plastic powder, fillers account for the largest proportion, about 70%, so fillers play a very important role in encapsulated plastic powder.

3.5.1 Adding appropriate amount of filler of appropriate quality to plastic powder has the following purposes:

(1) Reduce the shrinkage of plastic powder after hardening;

(2) Reduce the thermal expansion coefficient of epoxy resin;

(3) Improve heat conduction;

(4) Absorption reaction heat;

(5) Improve the mechanical and electrical properties of hardened rubber;

(6) Reduce the cost of plastic powder.

3.5.2 Types of fillers

The fillers used in epoxy plastic powder must not only improve electrical insulation and dielectric properties, but also have chemical stability and low hygroscopicity. Commonly used fillers include the following:

(1) Quartz;

(2) High-purity silicon dioxide (most widely used);

(3) Aluminum hydroxide

(4) Alumina;

(5) Mica powder;

(6) Silicon carbide.

3.5.3 Silicon dioxide (SiO2, Silica)

The thermal expansion coefficient of epoxy resin is about 65×10-6m/cm/℃ on average, which is much larger than the thermal expansion coefficient of metal embedded parts in the encapsulation resin. The frame (LEADFRAME) used in semiconductors is very different from epoxy resin. If pure resin is used to encapsulate semiconductor components, due to the difference in thermal expansion coefficients between each other and the heat generated when the components are working, internal stress and thermal stress will be generated, causing cracks in the encapsulation material. Therefore, the filler added to the plastic powder must not only be able to reduce the thermal expansion coefficient between the resin and the metal embedded parts, but also have good heat transfer performance.

Silica powder can be divided into crystalline silica and fused silica. Crystalline silica bricks have better thermal conductivity but a larger thermal expansion coefficient and poor resistance to thermal shock. Fused silica has poor thermal conductivity but a smaller thermal expansion coefficient and better resistance to thermal shock. Table 2 is a comparison of the properties of epoxy glue powder filled with molten and crystalline silica. It can be seen that in addition to poor thermal conductivity, molten silica has lower flexural strength and moisture resistance than crystalline silica.

In addition, the amount of filler used and the size, shape, and particle size distribution of the particles will affect the fluidity of the plastic powder during transfer molding and the electrical properties of the product after packaging. These factors must be considered when selecting fillers.

3.6 Coincidence Agent

Adding a small amount of binder to epoxy resin can produce the following effects:

●Increase the compatibility and affinity between fillers and gum;

●Increase the contact force between the rubber powder and the buried components;

●Reduce water absorption;

● Improve flexural strength;

●Reduce the viscosity of plastic powder during forming and improve fluidity;

●Improve the thermal dissipation factor (THERMALDUSSIPATIONFACTOR), loss factor (LOSSFAC-TOR) and leakage current (LEAKAGECURRENT) of the rubber powder.

3.7 Release agent (Japanese: ELEASEAGENT)

Epoxy glue has excellent adhesion and will also produce contact force on the production model, which will affect the demolding after the processing and packaging is completed. Therefore, a release agent is added to improve the demolding ability between the glue powder and the production model. Commonly used release agents include: wax, stearic acid, zinc stearate, calcium stearate, etc. The type and amount of release agent depends on the plastic powder formula (glue, hardener, filler). The amount of release agent should be appropriate. If the amount is too little, it will make demolding difficult; on the contrary, if the amount is too much, it will not only easily contaminate the production model, but also reduce the adhesion between the glue powder and the embedded frame and leads, directly affecting the moisture resistance and durability of the components. The following figure shows the relationship between the amount of release agent added and the contact force. The more release agent is added, the more the contact force between the glue powder and the embedded parts decreases.

3.8 Pigment

Pigments are usually added depending on the color of the finished product. The general packaging glue powder uses coal black as the pigment, so the finished product has a black appearance.

3.9 Lubricant

In order to increase the fluidity of the rubber powder during processing, some lubricants can sometimes be added to reduce the viscosity. However, this often results in a decrease in the glass transition temperature (TgGLASSTRANSISTIONTEMPERATURE) of the rubber powder and deterioration of the electrical properties. Therefore, if lubricants are required, it is best to use reactive diluents so that the diluent molecules can react with the rubber to prevent the deterioration of T2 and electrical properties.

4. Basic characteristics of epoxy resin plastic powder

We have mentioned some of the properties that plastic powder should have before, and we will further discuss these properties below.

4.1 Heat resistance

4.1.1 Glass transition temperature, Tg

If heat degradation is the main consideration for heat resistance, Tg can be used as a reference value. The Tg value of plastic powder mainly depends on the cross-linking density of the plastic powder: Tgl=Tg0+k/ncTgi: Tg after cross-linkingTg0: Tg before cross-linkingK: Experimental constant nc: The average number of atoms before two cross-linking points. The higher the cross-linking density, the higher the Tg value; the better the heat resistance, the higher the heat deformation temperature. The Tg value of general packaging plastic powder is about 160℃. Too high Ts will make the product too hard and brittle, reducing the resistance to thermal shock.

4.1.2 Tg calibration

There are many methods to calibrate Tg. At present, our institute uses thermal expansion meter (DIALTOMETER), DSC (DIFFERENTIAL CANALINING CALORIMETRY), rheometer (RHEOMETRIC), TBA (Torsion ALBRAID ANALYZER) and other spectrometers to calibrate Tg value.

4.2 Corrosion resistance

Among the causes of failure proposed by failure analysts engaged in plastic packaging circuits, aluminum strip corrosion (CORROSION OF ALUMINUN METALLIZATION) accounts for the highest proportion, so corrosion resistance is actually the primary consideration for packaging plastic powder.

4.2.1 Causes of Corruption

As for epoxy resin plastic powder, the main cause of aluminum strip corrosion is the chloride ions and hydrolyzable chloride contained in the plastic powder. When the moisture in the atmosphere expands into the interior of the semiconductor through the resin itself and the interface between it and the lead, the invading water vapor will combine with the ionic impurities in the resin, especially Cl-, and increase the mobility of the impurities. When these impurities reach the surface of the chip, they will form a corrosion reaction with the aluminum strip, destroying the extremely thin aluminum layer and causing semiconductor failure.

4.2.2 Prevention of Corruption

(1) Reduce the content of impurities

For semiconductor packaging companies, it is necessary to choose packaging glue powder with low chloride ion content. The content of ionic impurities in general plastic powder is below 10ppm. Since epoxy resin uses EPICHLOROHYDRIN in the synthesis process, there is no way to prevent the presence of chloride ions. Therefore, the resin must be alcoholized to remove most of the chloride ions before it can be used to produce packaging plastic powder. Table 3 shows the ion content and conductivity of epoxy glue packaging glue powder from Japanese manufacturers.

(2) New Corrosion Inhibitor (CORROSION INHIBITOR)

Adding corrosion inhibitors to the glue powder can reduce the corrosion rate of the aluminum strip, interfere with the corrosion reaction of the anode or cathode, and thus reduce the speed of the overall corrosion reaction (OVERALLREACTION). The selected inhibitor must have the following properties: ① The inhibitor cannot contain ions that are harmful to the operation of the circuit; ② The increased ionic conductivity after adding the inhibitor cannot produce side reactions that are harmful to the circuit; ③ The inhibitor must be able to form a complex (COMPLEX); ④ For organic inhibitors, it cannot react with epoxy resin and has stability during the hardening process of the transfer surface; ⑤ For inorganic inhibitors, the ions produced by it cannot penetrate into the Si or SiO: insulating layer to avoid affecting the operation of the circuit.

Generally, inorganic corrosion inhibitors are the most effective, among which ammonium tungstate and calcium citrate are commonly used.

4.3 Low coefficient of thermal expansion (CTE)

We have already mentioned that the difference in CTE between the rubber and the embedded parts causes internal stress and product deformation. Here we will specifically introduce the effect of CTE on rubber powder.

4.3.1 Relationship between GTE and internal mechanical stress

The internal stress can be expressed by DANNENBERG'S equation:

σ: internal stress (internal stress) O: thermal expansion coefficient (CTE) E: elastic modulus (elastic modulus) S: cross-section area (cross section area) R: resin (resin): embedded component, frame, chip interface, lead frame, c Hello p) From equation (⑷), we can clearly see that the greater the CTE difference between the resin and the embedded component, the greater the internal stress generated. The cracks (CRACK) caused by the internal stress will become a channel for external moisture and pollution to invade, further causing component failure, so the epoxy resin powder must have a low CTE value. At present, some people also reduce the elastic modulus to reduce the internal stress. 4.3.2 Factors affecting CTE The CTE value can be controlled by Tg or the degree of cross-linking density. In addition, the following factors will also affect CTE: 1) moisture pollution;

(2) Loss of plasticizer or lubricant;

(3) Stress elimination;

(4) Unreacted chemicals;

(5) Post-hardening time and temperature.

For epoxy resin powder, the key to low CTE value must start from the filler. A powder formula engineer must always keep Tg and CTE in mind as a reference and a tool for finding problems that need to be solved, because low CTE and high Tg are very important for thermal shock resistance.

4.4 Formability

The broad meaning of formability includes dimensional stability after semiconductor packaging, mold release, fluidity during processing, etc.

4.4.1 Fluidity and swirl flow test (SPIRALFOWTEST)

Since the glue powder itself is a partially cross-linked B-STAGE resin, improper storage or storage for too long will increase the degree of cross-linking and hardening of the glue powder, resulting in a decrease in fluidity. At this time, the glue powder with poor fluidity should be discarded. The quality of fluidity is generally judged by the size of the swirl flow value obtained in the swirl flow test. The current packaging specification is 25-35 inches. A swirl flow value that is too low indicates that the fluidity of the glue powder is poor, and there is no way to fill the model during molding; a swirl flow value that is too high indicates that the fluidity of the glue powder is too high, which is easy to break the metal wire of the embedded part and cause glue overflow.

4.4.2DSC and plastic powder fluidity

In addition to the vortex test, we can also use differential scanning calorimeter (DSC) to test whether the rubber powder still has good fluidity.

The first exothermic peak is the polymerization reaction heat released when the rubber powder hardens. The higher the exothermic peak, the more reaction heat the rubber powder has, which also means that the rubber powder has less hardening during storage, so it has good fluidity. The lower the exothermic peak, the more the rubber powder has hardened and can only release a small amount of reaction heat, which means that the rubber powder has lost its fluidity. Using the above principle, we can find the corresponding relationship between the exothermic peak height and the swirl value.

If the stored rubber powder is found to have a reduction of more than 10% in the height of the exothermic peak after DSC analysis, it means that the rubber powder has lost its excellent fluidity and should be discarded and not used again.

4.5 Electrical characteristics

Electrical properties are a crucial property for epoxy glue powder, and dielectric properties are the focus of consideration. For packaging materials, the smaller the dielectric constant, the better the electrical insulation. The dielectric constant is affected by frequency changes, temperature, and humidity. The change in dielectric constant is far more important than the initial value of dielectric constant. In addition, the tight sealing of the product is very important, which will directly affect the electrical properties. If the product package does not have a gap, in addition to providing a path for eczema contamination, corona (CORONA) will be generated under special conditions of receiving voltage, causing the electric field to concentrate at the front of the gap, resulting in internal discharge and insulation damage.

4.6 Moisture resistance Moisture enters semiconductor components and interacts with ionic impurities, reducing insulation, increasing leakage current and corroding aluminum circuits, which is the main reason for reduced reliability. There are two ways for moisture to enter packaged products: ● Enter through the surface contour of the resin group (BULKOFPLASTIC); ● Enter through the interface between the resin and the IC bracket by capillary phenomenon. Take a 14-pin DIP (DUAl+INLINEPACKAGE), open a hole on the top, the bottom of the hole can reach the surface of the chip, and then connect a container with a gas inlet and outlet to the hole of the DIP and seal it. Then immerse the assembly in 100% RH water vapor or water, and pass dry nitrogen (0% RH) into the container. Water vapor will enter the container according to the two ways mentioned above. We use the detector to measure the water vapor contained in the outflowing nitrogen and obtain the total water vapor penetration rate Pt (the sum of the two water vapor penetration rates). Pt is the sum of the water vapor penetration rate Pb through the gum group and the water vapor penetration rate P1 through the interface capillary penetration, and Pt = Pb + P1. We can use the same material of gum to seal the bottom of the container, measure Pb in the same way, and then subtract Pt from Pb to find the value of Pl.

The above method is used to evaluate plastic powder. If the component is to have a 10-year operating life guarantee, the PI value should be below 70. We can use this method to evaluate the moisture resistance of epoxy glue powder.

4.7 Heat release during curing Plastic powder releases polymerization reaction heat during curing. If the formula is not properly prepared, the heat release will be too large, which may cause cracks and give component stress. Therefore, chemical engineers should consider that the heat release during curing should not be too large when studying the plastic powder formula.

In fact, the cross-linking of plastic powder can be divided into two stages. First, gelation, then hardening. The gelation speed of low molecular weight gum is faster than that of high molecular weight. If the concentration of the promoter is small, the gelation time is determined by heat or power; if the concentration of the promoter is large, the gelation time is determined by the expansion of the molecules to the correct reaction position:

● For faster gelation, increase the heat, the resulting material has a low cross-linking density, high CTE, and large thermal shrinkage. ● For slower gelation, reduce the heat, the resulting material has a higher cross-linking density, low CTE, and less thermal shrinkage.

4.8 Flame resistance

In the UL specification, epoxy resin plastic powder with 94V-O as the standard can meet this specification.

4.9 Adhesion and demoulding properties

As mentioned above, the bonding strength of the rubber will decrease if the amount of release agent increases. If the amount of release agent is reduced, the bonding strength between the rubber and the lead wire can be increased, but the bonding strength between the production model and the molded product will also increase, causing difficulty in demolding. Therefore, the amount of release agent should be selected to take both adhesion and demolding properties into consideration.

4.10 Low α-Particle Effect (LOW α-PARTICLEEFFEC)

Epoxy resin powder uses silicon dioxide as filler, and silicon dioxide is a natural mineral that contains trace amounts of radioactive elements such as uranium and thorium. These radioactive elements will emit alpha particles during the decay process. Dynamic RAM's and CCD's and other conductive components will be affected by alpha particles and cause soft errors (SOFFERROR). Static RAM's, ROM's, PROM's and EPROM's and other components are not affected by alpha particles.

When alpha particles pass through the active device regions, they collect electrons in the N-region and holes in the P-region before the electrons and holes recombine. If enough charge is collected in a particular region, it will disturb the stored data or logic states. If the number of electrons collected and generated exceeds the critical charge, it will cause a so-called soft error.

In addition to fillers, substrates and metallurgical strips also emit alpha particles, but fillers are the main source of alpha particles. In order to prevent the alpha particle effect, in addition to using polyimide as a protective coating, silicon dioxide with low radioactive element content can be used as a filler. Japan has produced silicon dioxide with a radioactive element content of less than 1ppb. These silicon dioxides are refined through alcoholization and are more expensive. For high-quality conductor components, it is necessary to find a way to prevent the alpha particle effect.

4.11 Persistent Survivability

At present, the gelling time of most rubber powders is about 30 seconds. After hardening, it usually needs to be post-hardened and refrigerated. If we want to develop a rubber powder that can harden quickly and survive at room temperature (max 40-45℃) for more than 6 months without losing its fluidity, we must study and improve the potential enhancer.

This article only briefly introduces the composition, selected materials and basic properties of epoxy resin encapsulation powder. It is hoped that the semiconductor industry can have a comprehensive understanding of the composition of plastic powder, and it is also hoped that it can provide guidance for colleagues in the selection of epoxy resin plastic powder and the research of packaging mechanism.