Electronic devices are a very complex system. The defects and failures of the packaging process are also very complicated. Therefore, studying package defects and failures requires a systematic understanding of the packaging process so that the causes of defects can be analyzed from multiple perspectives. Package defect and failure research methodology The failure mechanism of the package can be divided into two categories: overstress and wear. Over-stress failures are often transient and catastrophic; wear failures are long-term cumulative damages that are often first expressed as performance degradation followed by device failure. Failed load types can be divided into mechanical, thermal, electrical, radiation, and chemical loads. The factors that affect package defects and failures are varied, and the material composition and properties, package design, environmental conditions, and process parameters all have an impact. Determine the basic factors for influencing factors and preventing package defects and failures. The influencing factors can be determined by experiments or simulations. The physical model method and numerical parameter method are generally used. For more complex defects and failure mechanisms, trial and error methods are often used to determine the key influencing factors, but this method requires long test times and equipment modifications, which are inefficient and costly. During the analysis of the failure mechanism, the use of fishbone maps (cause-effect diagrams) to display the influencing factors is a common method in the industry. Fishbone diagrams can explain the complex causes and the relationship between influencing factors and package defects, and can also distinguish multiple causes and group them. In production applications, there is a class of fishbone diagrams called 6Ms: the influence factors are analyzed from the six dimensions of machine, method, material, measurement, human and natural forces. This figure shows a fishbone diagram that shows the reasons for the lamination of the plastic chips, and analyzes them in terms of design, process, environment, and materials. Through the fishbone map, all the influencing factors are clearly displayed, which lays a good foundation for failure analysis. Failed load type As described in the previous section, the encapsulated load types can be divided into mechanical, thermal, electrical, radiation, and chemical loads. Classification of failure mechanisms Mechanical load: includes physical impact, vibration, stress (such as shrinkage stress) applied on the silicon chip by the filler particles and inertial force (such as the huge acceleration of the spacecraft). The response of the material to these loads may manifest itself as elastic deformation, plastic deformation, warpage, brittleness or soft fracture, interface delamination, fatigue crack generation and propagation, creep, and creep cracking. Thermal load: including the high temperature when the die-bonding agent is cured, pre-heating before wire bonding, molding process, post-curing, reprocessing of adjacent components, dip soldering, gas phase soldering, reflow soldering, and the like. External thermal loads can cause the material to undergo dimensional changes due to thermal expansion and also change the physical properties such as creep rate. In the event of a thermal expansion coefficient mismatch (CTE mismatch), local stress is induced and eventually the package structure fails. Excessive thermal loading may even cause combustion of flammable materials in the device. Electric load: Including sudden electric shock, voltage instability, sudden fluctuations in current transmission (such as poor grounding), current fluctuations, electrostatic discharge, and over-voltage stress. These external electrical loads may cause dielectric breakdown, voltage surface breakdown, thermal loss of electrical energy, or electromigration. It may also increase electrolytic corrosion, dendrite growth, leakage current, thermal degradation, etc. Chemical Loading: Including corrosion, oxidation and ionic dendritic growth caused by the chemical environment. Because moisture can penetrate through the molding compound, moisture is a major problem affecting the molded device in a humid environment. The moisture absorbed by the molding compound can extract the catalyst residues in the molding compound to form by-products into the metal base, the semiconductor material and various interfaces of the chip bonding, and induce degradation or even failure of the device performance. For example, the flux remaining on the device after assembly migrates to the chip surface through the molding compound. In the high-frequency circuit, slight changes in the properties of the medium (such as changes in dielectric constant, dissipation factor, etc. after absorbing moisture) are all critical. In devices such as high voltage converters, the change in the breakdown voltage of the package is critical. In addition, some epoxy polyamides and polyurethanes can cause degradation (sometimes also referred to as "reversal") if they are exposed to high temperatures and high humidity for long periods of time. Accelerated tests are often used to identify whether a molding compound is prone to such failure. It should be noted that, when different types of loads are applied, various failure mechanisms may simultaneously interact on the molded device. For example, thermal loading can cause thermal expansion mismatches between adjacent materials within the package structure, causing mechanical failure. Other interactions include stress-assisted corrosion, stress corrosion cracking, field-induced metal migration, cracking of passivation layers and electrolyte layers, cracking of packages due to moisture and heat, and acceleration of temperature-induced chemical reactions. In these cases, the combined effect of the failure mechanism does not necessarily equal the sum of the individual effects. Package defect classification Package defects mainly include lead deformation, pedestal deflection, warpage, chip cracking, delamination, voids, non-uniform encapsulation, burrs, foreign particles, and incomplete curing. Lead deformation Lead deformation usually refers to the displacement or deformation of the leads caused during the flow of the molding compound. It is usually expressed by the ratio x/L between the maximum lateral displacement x of the lead and the length L of the lead. Bending of the leads may cause electrical shorts (especially in high-density I/O device packages). Sometimes the stress caused by bending can cause cracking of bonding points or a decrease in bonding strength. Factors that affect wire bonding include package design, wire layout, wire materials and dimensions, molding compound properties, wire bonding processes, and packaging processes. Lead parameters that affect lead bending include lead diameter, lead length, lead break load, and lead density. Base offset Base offset refers to the deformation and offset of the carrier (chip base) supporting the chip. As shown in the figure, the offset of the base caused by the molding compound is caused. At this time, the uneven mold material flow in the upper and lower molding cavities may cause the base to shift. The factors affecting the offset of the base include the fluidity of the molding compound, the assembly design of the lead frame, and the material properties of the molding compound and the lead frame. Packaged devices such as Thin Small Package (TSOP) and Thin Quad Flat Package (TQFP) are susceptible to base shift and pin deformation due to thin lead frames. Warping Warpage refers to the out-of-plane bending and deformation of the packaged device. Warpage caused by the plastic sealing process can cause a series of reliability problems such as delamination and chip cracking. Warpage can also lead to a series of manufacturing problems, such as in plastic ball grid array (PBGA) devices, where warping can result in poor coplanarity of the solder balls and placement problems during reflow soldering of the device to a printed circuit board. . Warpage modes include three kinds of concave, convex and combination modes The causes of warpage mainly include CTE mismatch and cure/compression shrinkage. The latter did not receive much attention from the beginning. In-depth study found that chemical shrinkage of molding compounds also plays an important role in the warping of IC devices, especially on packaged devices with different thicknesses on the upper and lower sides of the chip. During the curing and post-curing process, the molding compound will undergo chemical shrinkage at high curing temperatures, known as "thermochemical shrinkage." By increasing the glass transition temperature and reducing the change in the coefficient of thermal expansion near the Tg, the chemical shrinkage that occurs during the curing can be reduced. Factors that cause warpage also include components such as molding compound, molding compound moisture, package geometry, and the like. By controlling the plastic material and composition, process parameters, package structure, and pre-encapsulation environment, package warpage can be minimized. In some cases, warping compensation may be performed by encapsulating the backside of the electronic component. For example, large ceramic circuit boards or multi-layer boards have external connections on the same side, and back-packing them can reduce warpage. Cracked chip Stress generated in the packaging process can cause the chip to crack. The packaging process usually increases the micro-cracks formed in the front assembly process. Wafer or chip thinning, back grinding, and die bonding are all steps that can lead to chip crack initiation. Cracked, mechanically-failed chips do not necessarily have electrical failures. Whether the chip rupture will cause the momentary electrical failure of the device also depends on the growth path of the crack. For example, if cracks appear on the backside of the chip, it may not affect any sensitive structures. Because silicon wafers are thin and brittle, wafer-level packaging is more prone to chip breakage. Therefore, it is necessary to strictly control the process parameters such as clamping pressure and forming pressure in the transfer molding process to prevent the chip from cracking. In the 3D stack package, chip cracking is apt to occur due to a lamination process. The design factors that affect chip breakage in 3D packaging include chip stack structure, substrate thickness, mold volume, and mold sleeve thickness. Layered The lack of delamination or bonding refers to the separation between the molding compound and its adjacent material interface. The layered location may occur in any area of ​​the encapsulated microelectronic device; it may also occur in the packaging process, the post-package fabrication phase, or the device use phase. The poor bonding interface caused by the packaging process is the main factor that causes delamination. Poor adhesion can result from voids in the interface, surface contamination during packaging, and incomplete curing. Other influencing factors include shrinkage stress and warpage during curing and cooling. During the cooling process, mismatched CTEs between the molding compound and adjacent materials can also cause thermo-mechanical stress, resulting in delamination. Classification can be based on interface type Hollow In the packaging process, bubbles are embedded in the epoxy material to form voids that can occur at any stage of the packaging process, including transfer molding, filling, potting, and molding of the sealant to print in an air environment. By minimizing the amount of air, such as emptying or vacuuming, voiding can be reduced. Vacuum pressures in the range of 1 to 300 Torr (760 Torr at one atmosphere) have been reported. Filling simulation analysis believes that the bottom of the melt front and the chip contact, resulting in obstruction of mobility. A portion of the melt front flows upwards and fills the top of the mold half through the large opening area on the periphery of the chip. The newly formed melt front and the adsorbed melt front enter the top area of ​​the mold half, thereby forming blisters. Non-uniform packaging Non-uniform plastic body thickness can cause warping and delamination. Traditional packaging technologies, such as transfer molding, pressure molding, and potting packaging technologies, are not prone to inconsistent package defects. Wafer-level packaging is particularly prone to uneven plastic thickness because of its process characteristics. In order to ensure a uniform thickness of the mold layer, the wafer carrier should be fixed so that its inclination is at a minimum to facilitate blade installation. In addition, blade position control is required to ensure that the blade pressure is stable, resulting in a uniform thickness of the plastic seal layer. Before hardening, when the filler particles aggregate in a localized area in the molding compound and form a non-uniform distribution, different qualitative or non-uniform material compositions result. Insufficient mixing of the molding compound will cause different quality phenomena in the encapsulation process. Flash Burr refers to the molding compound that passes through the parting line and is deposited on the device pins during the molding process. Insufficient clamping pressure is the main reason for producing burrs. If the mold residue on the pin is not removed in time, it will cause various problems in the assembly stage. For example, bonding or adhesion is insufficient in the next packaging stage. Resin leakage is a relatively sparse form of flash. Foreign particles In the packaging process, if the package material is exposed to a contaminated environment, device or material, foreign particles will diffuse in the package and accumulate in the metal parts of the package (such as IC chips and wire bond points), resulting in corrosion and Other follow-up reliability issues. Incomplete cure Insufficient curing time or low curing temperature can lead to incomplete curing. In addition, a slight shift in the mix ratio will result in incomplete cure in the infusion of both packaging materials. In order to maximize the properties of the package material, it must be ensured that the package material is completely cured. In many packaging methods, post-curing methods are allowed to ensure complete curing of the packaging material. But also pay attention to ensure the exact proportion of packaging material ratio. Packaging failure classification Packaging failure occurs at either the package assembly stage or the device use stage. This is especially true when packaged microelectronic devices are assembled on a printed circuit board. At this stage, the device needs to withstand high reflow temperatures, which can result in delamination or cracking of the molding compound interface. Layered As described in the previous section, delamination refers to the separation of the plastic material from the adjacent material at the bonding interface. External loads and stresses that can cause stratification include moisture, moisture, temperature, and their combined effects. A type of stratification that often occurs during the assembly stage is called vapor induced (or steam induced) stratification, and the failure mechanism is mainly the water vapor pressure at relatively high temperature. When the packaged device is assembled on a printed circuit board, the solder melting temperature needs to reach 220° C. or higher, which is much higher than the glass transition temperature (about 110-200° C.) of the molding material. At high temperature, the water vapor between the molding compound and the metal interface evaporates to form water vapor. The resulting vapor pressure interacts with the thermal mismatch between the materials and the stress caused by the moisture expansion and eventually causes the interface to bond poorly or otherwise. Layers can even cause the package to break. Lead-free solders have higher reflow temperatures than conventional lead-based solders and are more prone to delamination problems. Moisture absorption coefficient (CHE), also known as moisture expansion coefficient (CME) The failure mechanism of moisture diffusion to the interface of the package is an important factor in the delamination caused by moisture and moisture. Moisture can diffuse through the package or spread along the interface between the lead frame and the molding compound. It was found that when there is a good bond between the molding compound and the lead frame interface, moisture enters the package mainly through the plastic package. However, when the bonding interface is degraded due to a poor packaging process (eg, due to the oxidation of the bonding temperature, insufficient lead frame warping caused by stress release, or excessive trimming and form stress, etc.), a layered layer is formed on the package outline. And micro-cracks, and moisture or water vapor will easily spread along this path. Worse, moisture can cause hydration of polar epoxy adhesives, weakening and reducing the chemical bonding of the interface. Surface cleaning is a key requirement for good bonding. Surface oxidation often leads to the occurrence of delamination (as mentioned in the example above), and exposure of copper alloy lead frames to high temperatures often leads to delamination. The presence of nitrogen or other synthesis gas helps to avoid oxidation. Lubricants and adhesion promoters in molding compounds promote delamination. Lubricants can help separate the molding compound from the mold cavity, but increase the risk of interface delamination. Adhesion promoters, on the other hand, ensure good adhesion between the molding compound and the chip interface but are difficult to remove from the mold cavity. Stratification not only provides a path for vapor diffusion, but also a source of resin cracks. The layered interface is where cracks start, and when subjected to external loads, cracks propagate through the resin. Studies have shown that delamination between the ground of the chip base and the resin is most likely to cause cracks in the resin, and interface delaminations at other locations have little effect on resin cracks. Gas phase induced cracks (popcorn phenomenon) The further development of water vapor induced stratification will lead to gas phase induced cracks. When the water vapor in the package escapes through the crack, it will produce popping sound, which is very similar to the popcorn sound. It is also called the popcorn phenomenon. Cracks often extend from the chip base to the bottom of the plastic package. In the soldered circuit board, it is difficult to find these cracks by appearance inspection. Large and thin plastic packaging forms such as QFP and TQFP are most likely to produce popcorn phenomenon; in addition, they are likely to occur in devices where the area of ​​the chip base is relatively large compared with the device area, and the area of ​​the chip base is relatively large compared with the minimum thickness of the molding compound. The popcorn phenomenon may be accompanied by other problems including the breakage of the bonding ball from the bonding disk and the silicon pits under the bonding ball. Crevices within the encapsulated device usually originate from stress concentrating areas (such as edges and burrs) on the lead frame and expand within the thinnest encapsulated area. Burrs are small dimensional distortions in the stamping process that occur on the surface of the lead frame. Changing the direction of punching so that the burrs are on top of the lead frame, or etching the lead frame (molding) can reduce the cracks. Reducing moisture in the molded device is the key to reducing the popcorn phenomenon. High-temperature baking is often used to reduce moisture in the molded device. Previous studies have found that the allowable safe moisture content in the package is about 1100×10^-6 (0.11 wt.%). Baking at 125°C for 24 hours can fully remove the moisture absorbed in the package. Brittle fracture Brittle fractures often occur in low yield strength and non-elastic materials (such as silicon chips). When the material is overstressed, sudden, catastrophic crack propagation can originate from small defects such as voids, inclusions, or discontinuities. Ductile fracture Plastic packaging materials are prone to brittle and tough fracture modes, depending on environmental and material factors, including temperature, viscoplastic properties of the polymer resin, and fill loading. Even in highly loaded molding materials containing brittle silicon fillers, ductile fracture may still occur due to the viscoplastic properties of the polymeric resin. Fatigue fracture When the molding material is subjected to periodic stress within the ultimate strength range, it will break due to cumulative fatigue fracture. The wet, thermal, mechanical or composite loads applied to the plastic material will generate cyclic stresses. Fatigue failure is a type of wear failure mechanism. Cracks generally initiate at discontinuities or defect locations. The fatigue fracture mechanism includes three stages: crack initiation (stage I); stable crack propagation (stage II); sudden, uncertain, catastrophic failure (stage III). Under cyclic stress, the fatigue crack propagation of phase II refers to the steady growth of fracture length. The crack growth rate of the plastic material is much higher than the typical fatigue crack propagation of the metal material (about 3 times). Accelerated failure factors Environmental and material loadings and stresses, such as moisture, temperature, and contaminants, accelerate the failure of the encapsulated device. The plastic encapsulation process plays a key role in the failure of the package. Characteristics such as moisture diffusion coefficient, saturated moisture content, ion diffusion rate, thermal expansion coefficient and hygroscopic expansion coefficient of the plastic material greatly affect the failure rate. The factors leading to acceleration acceleration are mainly moisture, temperature, pollutants and solvent environment, residual stress, natural environmental stress, manufacturing and assembly loads, and comprehensive load stress conditions. Moisture can accelerate the delamination, cracking, and corrosion failure of plastic microelectronic devices. In plastic devices, moisture is an important failure acceleration factor. Mechanisms associated with the acceleration of accelerated failure by moisture include the degradation of the bonding surface, the moisture absorption expansion stress, water vapor pressure, ion migration, and changes in the characteristics of the molding compound. Moisture can change the glass transition temperature Tg, elastic modulus and volume resistivity of the molding compound. Temperature is another key failure acceleration factor. The effect of temperature on package failure is usually evaluated using the temperature rating associated with the glass transition temperature of the molding compound, the thermal expansion wash of various materials, and the resulting thermo-mechanical stress. Another influencing factor of temperature on the failure of the package is the change in temperature-related properties of the package material, moisture diffusion coefficient, and diffusion between metals. Pollutants and solvent-borne environmental pollutants provide a venue for the initiation and expansion of failures. The sources of pollution are atmospheric pollutants, moisture, flux residues, unclean examples in plastic sealants, corrosive elements resulting from thermal degradation, and die bonding. The byproduct (usually epoxy) is discharged from the agent. Plastic packages are generally not subject to corrosion, but moisture and contaminants can diffuse in the molding compound and reach the metal sites, causing corrosion of the metal parts within the encapsulated device. Residual stress Chip bonding produces stress alone. The level of stress depends mainly on the characteristics of the die bonding layer. Because the shrinkage of the molding compound is greater than other packaging materials, the stress generated during molding is considerable. A stress test chip can be used to determine the assembly stress. Natural environmental stress In the natural environment, the molding compound may degrade. Degradation is characterized by the breaking of polymeric bonds, often the transformation of solid polymers into viscous liquids containing monomers, dimers and other low molecular weight species. Elevated temperatures and enclosed environments often accelerate degradation. The ultraviolet rays and atmospheric ozone layer in sunlight are powerful catalysts for degradation and can be degraded by cutting off the molecular chains of epoxy resins. Separating the encapsulated device from the environment that is susceptible to degradation and using polymers that are resistant to degradation are methods of preventing degradation. Products that need to work in a hot and humid environment require anti-degradation polymers. Manufacturing and Assembly Loads Both manufacturing and assembly conditions can cause package failures, including high temperatures, low temperatures, temperature changes, operational loads, and loads imposed on bond wires and die pads due to molding compound flow. The popcorn phenomenon that occurs when plastic components are assembled is a typical example. Comprehensive load stress conditions Failure acceleration factors such as temperature and moisture often coexist during manufacturing, assembly, or operation. Combined loads and stress conditions often further accelerate failures. This feature is often applied to accelerated test designs for the purpose of identifying defective components and identifying vulnerable component devices. Active Line Array,Powered Array Speakers,Powered Line Array Speakers,Active Line Array Speakers NINGBO LOUD&CLEAR ELECTRONICS CO.,LIMITED , https://www.loudclearaudio.com
