What factors affect the reliability of electronic packaging? Detailed description of defects and failures

Date：2021-12-29 14:06:43 Views：2874

Electronic packaging is the process of converting chips into devices that can work reliably in the electronic manufacturing industry chain. Because bare chips can not withstand the load of the working environment for a long time and lack the necessary electrical signal connection, they can not be directly used in electronic equipment. Therefore, although different types of products are different, the main functions of electronic packaging are relatively similar, mainly including four functions: mechanical support, fixing the chip and other internal components in the specified position; Environmental protection to protect the chip from external loads such as water vapor, corrosion, dust and impact; Interconnection of electrical signals to provide electrical path and power supply for internal components; Heat dissipation to export the heat generated during chip operation in time [1]. According to different process stages, electronic packaging can generally be divided into zero level packaging (chip level interconnection), one level packaging (chip level packaging), two-level packaging (module level packaging) and three-level assembly.

Due to the large number of different types of materials involved in chips and packaging, the characteristics of some materials are very different. In the packaging process, if the internal defects, residual stress, deformation and other problems are not properly controlled, it is very easy to cause reliability problems in the packaging process or product service. With the increasing packaging density and diversification of functions, such as 3D packaging and heterogeneous integration technology, the reliability problem of multi field and multi-scale coupling in electronic packaging is more obvious.

Research methodology of packaging defect and failure

The failure mechanism of packaging can be divided into two categories: over stress and wear. Overstress failure is often instantaneous and catastrophic; Wear failure is a long-term cumulative damage, which is often first expressed as performance degradation, followed by device failure. The failure load types can be divided into mechanical, thermal, electrical, radiation and chemical loads.

There are many factors affecting packaging defects and failures, including material composition and properties, packaging design, environmental conditions and process parameters. Determining the influencing factors and the basic premise of preventing package defects and failures. The influencing factors can be determined by test or simulation, generally using physical model method and numerical parameter method. For more complex defects and failure mechanisms, trial and error method is often used to determine the key influencing factors, but this method requires long test time and equipment correction, low efficiency and high cost.

In the process of analyzing the failure mechanism, it is a common method in the industry to use fishbone diagram (cause and effect diagram) to show the influencing factors. Fishbone diagram can explain the relationship between complex causes and influencing factors and packaging defects, and can also distinguish and classify various causes. In production application, there is a kind of fishbone diagram called 6ms: analyze the influencing factors from six dimensions: machine, method, material, measurement, human and natural forces.

This figure shows a fishbone diagram showing the causes of lamination of plastic encapsulated chips, which is analyzed from four aspects: design, process, environment and materials. The fishbone diagram clearly shows all the influencing factors, which lays a good foundation for failure analysis.

Load type causing failure

As described in the previous section, the load types of packages can be divided into mechanical, thermal, electrical, radiation and chemical loads.

Classification of failure mechanism

Mechanical load:It includes physical shock, vibration, stress (such as shrinkage stress) and inertial force (such as huge acceleration of spacecraft) exerted by filled particles on silicon chip. The response of materials to these loads may be manifested as elastic deformation, plastic deformation, warpage, brittle or flexible fracture, interface delamination, generation and propagation of fatigue cracks, creep and creep cracking, etc.

Thermal load:Including the high temperature of chip adhesive curing, pre heating before wire bonding, forming process, post curing, reprocessing of adjacent components, immersion welding, gas phase welding and reflow welding, etc. The external thermal load will not only change the size of the material due to thermal expansion, but also change the physical properties such as creep rate. If the coefficient of thermal expansion mismatch (CTE mismatch) occurs, it will lead to local stress and eventually lead to the failure of packaging structure. Excessive thermal load may even cause combustion of flammable materials in the device.

Electrical load:Including current fluctuation, electrostatic discharge, over-current stress, etc. caused by sudden electric shock, unstable voltage or sudden oscillation during current transmission (such as poor grounding). These external electrical loads may lead to dielectric breakdown, voltage surface breakdown, heat loss or electromigration of electric energy. It may also increase electrolytic corrosion, dendritic crystal growth, cause leakage current, thermally induced degradation, etc.

Chemical load:Including corrosion, oxidation and ion surface dendrite growth caused by chemical service environment. Because moisture can penetrate through plastic packaging materials, moisture is the main problem affecting plastic packaging devices in humid environment. The moisture absorbed by the plastic packaging material can extract the catalyst residue in the plastic packaging material, form by-products into the metal base, semiconductor materials and various interfaces of chip bonding, and induce the degradation or even failure of device performance. For example, the flux remaining on the device after assembly will migrate to the chip surface through plastic sealant. In high frequency circuits, the subtle changes of dielectric properties (such as the changes of dielectric constant and dissipation factor after moisture absorption) are very important. In high voltage converters and other devices, the change of package breakdown voltage is very important. In addition, some epoxy polyamides and polyurethanes can also cause degradation (sometimes referred to as "reversal") if they are exposed to high temperature and humidity for a long time. Accelerated test is usually used to identify whether plastic sealing materials are prone to such failure.

It should be noted that when different types of loads are applied, various failure mechanisms may interact on plastic encapsulated devices at the same time. For example, thermal load will cause thermal expansion coefficient mismatch between adjacent materials in the package structure, resulting in mechanical failure. Other interactions include stress assisted corrosion, stress corrosion cracking, field induced metal migration, cracks in passivation layer and electrolyte layer, package cracking caused by damp heat, acceleration of chemical reaction caused by temperature, etc. In these cases, the combined effect of failure mechanism is not necessarily equal to the sum of individual effects.

Classification of packaging defects

Packaging defects mainly include lead deformation, base offset, warpage, chip fracture, delamination, cavity, uneven packaging, burr, foreign particles and incomplete curing.

Lead deformation

Lead deformation usually refers to the lead displacement or deformation caused by the flow of plastic sealing material. It is usually expressed by the ratio X / L between the maximum transverse displacement X of lead and the length L of lead. Lead bending may cause electrical short circuit (especially in high-density I / O device packaging). Sometimes, the stress caused by bending will lead to the cracking of bonding point or the decrease of bonding strength.

The factors affecting wire bonding include packaging design, wire layout, wire material and size, molding plastic properties, wire bonding process and packaging process. The lead parameters affecting lead bending include lead diameter, lead length, lead breaking load, lead density and so on.

Base offset

Base offset refers to the deformation and offset of the carrier (chip base) supporting the chip.

As shown in the figure, the base offset is caused by the plastic sealing material. At this time, the uneven flow of plastic sealing material in the upper and lower molding cavities will lead to the base offset.

The factors affecting the base offset include the fluidity of plastic sealing material, the assembly design of lead frame and the material properties of plastic sealing material and lead frame. Packaging devices such as thin small size package (TSOP) and thin square flat package (TQFP) are prone to base offset and pin deformation due to the thin lead frame.

warping

Warpage refers to the bending and deformation of packaging devices out of plane. Warpage caused by plastic packaging process will lead to a series of reliability problems such as delamination and chip cracking. Warping will also lead to a series of manufacturing problems. For example, in plastic encapsulated ball grid array (PBGA) devices, warping will lead to poor coplanarity of solder balls, resulting in mounting problems in the reflow soldering process of the device assembled to the printed circuit board.

Warping modes include concave, convex and combined modes

The main causes of warpage include CTE mismatch and curing / compression shrinkage. The latter did not receive much attention at the beginning. In-depth research found that the chemical shrinkage of molding compounds also played an important role in the warpage of IC devices, especially in the packaging devices with different thicknesses on the upper and lower sides of the chip. In the process of curing and post curing, the plastic sealing material will undergo chemical shrinkage at high curing temperature, which is called "thermochemical shrinkage". By increasing the glass transition temperature and reducing the change of thermal expansion coefficient near TG, the chemical shrinkage during curing can be reduced.

Factors leading to warpage also include such factors as plastic packaging material composition, mold moisture, packaging geometry, etc. Packaging warpage can be minimized by controlling plastic packaging materials and components, process parameters, packaging structure and pre packaging environment. In some cases, warpage can be compensated by encapsulating the back of the electronic component. For example, the external connections of large ceramic circuit boards or multilayer boards are located on the same side, and their back packaging can reduce warpage.

Chip rupture

The stress generated in the packaging process can lead to chip rupture. The packaging process usually aggravates the micro cracks formed in the previous assembly process. Wafer or chip thinning, back grinding and chip bonding are all steps that may lead to chip crack initiation.

Broken, mechanically failed chips do not necessarily have electrical failure. Whether the chip rupture will lead to the instantaneous electrical failure of the device also depends on the growth path of the crack. For example, if cracks appear on the back of the chip, they may not affect any sensitive structures.

Because silicon wafers are thin and brittle, wafer level packaging is more prone to chip breakage. Therefore, the process parameters such as clamping pressure and forming conversion pressure in the transfer molding process must be strictly controlled to prevent chip breakage. Chip breakage is easy to occur in 3D stacked packaging due to lamination process. The design factors affecting chip fracture in 3D Packaging include chip stack structure, substrate thickness, molding volume and die sleeve thickness.

layered

Delamination or weak bonding refers to the separation between the plastic sealing material and its adjacent material interface. The delamination position may occur in any area of the plastic encapsulated microelectronic device; It may also occur in the packaging process, post packaging manufacturing stage or device use stage.

Poor bonding interface caused by packaging process is the main factor causing delamination. Interface cavity, surface contamination during packaging and incomplete curing will lead to poor bonding. Other influencing factors include shrinkage stress and warpage during curing and cooling. During cooling, CTE mismatch between plastic sealing material and adjacent materials will also lead to thermal mechanical stress, resulting in delamination.

Layers can be classified according to the interface type

empty

In the packaging process, bubbles are embedded into the epoxy material to form a cavity. The cavity can occur at any stage of the packaging process, including transfer molding, filling, potting and plastic sealing materials, as well as printing in the air environment. Voids can be reduced by minimizing the amount of air, such as emptying or vacuuming. It is reported that the vacuum pressure range used is 1 ~ 300torr (one atmospheric pressure is 760torr).

The filling simulation analysis shows that the bottom melt front is in contact with the chip, which hinders the fluidity. Part of the melt front flows upward and fills the top of the half die through the large open area around the chip. The newly formed melt front and the adsorbed melt front enter the top area of the half die to form foaming.

Uneven packaging

The non-uniform thickness of the plastic package will lead to warpage and delamination. Traditional packaging technologies, such as transfer molding, pressure molding and pouring packaging, are not easy to produce packaging defects with uneven thickness. Wafer level packaging is particularly easy to lead to uneven plastic packaging thickness due to its process characteristics.

In order to ensure a uniform thickness of the plastic seal layer, the wafer carrier should be fixed with a minimum inclination to facilitate the installation of the scraper. In addition, it is necessary to control the scraper position to ensure that the scraper pressure is stable, so as to obtain a uniform plastic sealing layer thickness.

Before hardening, when the filler particles gather and form uneven distribution in the local area of the plastic sealing material, it will lead to different or uneven material composition. The insufficient mixing of plastic packaging materials will lead to the occurrence of heterogeneity in the packaging and potting process.

Burr

Burr refers to the molding compound that passes through the parting line and is deposited on the device pins in the plastic packaging process.

Insufficient clamping pressure is the main cause of burr. If the die residue on the pin is not removed in time, it will lead to various problems in the assembly stage. For example, insufficient bonding or adhesion in the next packaging stage. Resin leakage is a sparse form of burrs.

Foreign particles

In the packaging process, if the packaging material is exposed to the polluted environment, equipment or materials, foreign particles will diffuse in the packaging and gather on the metal parts in the packaging (such as IC chip and lead bonding point), resulting in corrosion and other subsequent reliability problems.

Incomplete curing

Insufficient curing time or low curing temperature will lead to incomplete curing. In addition, in the pouring of the two packaging materials, a slight deviation of the mixing proportion will lead to incomplete curing. In order to maximize the characteristics of the packaging material, it is necessary to ensure that the packaging material is fully cured. In many packaging methods, post curing is allowed to ensure the complete curing of packaging materials. And pay attention to ensure the accurate proportion of packaging materials.

Classification of package failure

In the packaging assembly stage or device use stage, packaging failure will occur. Especially when the packaged microelectronic devices are assembled on the printed circuit board, it is more likely to occur. At this stage, the devices need to withstand high reflux temperature, which will lead to delamination or fracture of the plastic packaging interface.

layered

As mentioned in the previous section, delamination refers to the separation of the plastic sealing material from the adjacent material at the bonding interface. External loads and stresses that may cause delamination include water vapor, moisture, temperature and their combined action.

A kind of delamination that often occurs in the assembly stage is called water vapor induced (or steam induced) delamination, and its failure mechanism is mainly the water vapor pressure at relatively high temperature. When the packaging device is assembled on the printed circuit board, the melting temperature of solder needs to reach 220 ℃ or even higher, which is much higher than the glass transition temperature of molding compound (about 110 ~ 200 ℃). Under the reflux high temperature, the water vapor existing between the plastic sealing material and the metal interface evaporates to form water vapor. The generated steam pressure acts together with the thermal mismatch between the materials and the stress caused by moisture absorption and expansion, which eventually leads to the weak bonding or delamination of the interface, and even the rupture of the package. Compared with traditional lead-based solder, lead-free solder has higher reflow temperature and is more prone to delamination.

Hygroscopic expansion coefficient (CHE), also known as moisture expansion coefficient (CME)

The failure mechanism of moisture diffusion to the packaging interface is an important factor of delamination caused by water vapor and moisture. Moisture can diffuse through the package or along the interface between the lead frame and the molding compound. It is found that when there is good bonding between the molding compound and the lead frame interface, the moisture mainly enters the package through the plastic package. However, when the bonding interface is degraded due to poor packaging process (such as oxidation caused by bonding temperature, lead frame warpage caused by insufficient stress release, excessive trimming and formal stress, etc.), delamination and microcracks will be formed on the packaging contour, and moisture or water vapor will be easy to diffuse along this path. What's worse, moisture will lead to the hydration of polar epoxy adhesive, which weakens and reduces the chemical bonding at the interface.

Surface cleaning is the key requirement to achieve good bonding. Surface oxidation often leads to delamination (the example mentioned in the previous article), such as copper alloy lead frame exposed to high temperature. The presence of nitrogen or other synthetic gases is conducive to avoiding oxidation.

Lubricants and adhesion promoters in molding compounds promote delamination. Lubricants can help to separate the molding compound from the mold cavity, but will increase the risk of interface delamination. On the other hand, the adhesion promoter can ensure good adhesion between the molding compound and the chip interface, but it is difficult to remove it from the mold cavity.

Delamination not only provides a path for water vapor diffusion, but also the source of resin cracks. The delamination interface is the location of crack initiation. When subjected to excessive external load, the crack will expand through resin. The results show that the delamination between the chip base ground and the resin is the most likely to cause resin cracks, and the interfacial delamination in other locations has little effect on resin cracks.

Gas induced cracking (popcorn phenomenon)

The further development of water vapor induced stratification will lead to gas-phase induced fractures. When the water vapor in the package escapes through the crack, it will produce a popping sound, which is very similar to the sound of popcorn, so it is also called popcorn phenomenon. Cracks often extend from the chip base to the bottom of the plastic seal. In the welded circuit board, it is difficult to find these cracks by visual inspection. Large and thin plastic packaging forms such as QFP and TQFP are most likely to produce popcorn; In addition, it is also easy to occur in devices with large ratio of chip base area to device area and large ratio of chip base area to minimum plastic sealing material thickness. The popcorn phenomenon may be accompanied by other problems, including the fracture of the bonding ball from the bonding disk and the silicon pit under the bonding ball.

Cracks in plastic encapsulated devices usually originate from stress concentration areas (such as edges and burrs) on the lead frame and expand in the thinnest plastic encapsulated area. Burr is a small-scale deformation of the lead frame surface in the stamping process. Changing the stamping direction to make the burr on the top of the lead frame, or etching the lead frame (Molding) can reduce cracks.

Reducing moisture in plastic packaging devices is the key to reduce popcorn phenomenon. High temperature baking is often used to reduce moisture in plastic encapsulated devices. Previous studies have found that the allowable safe moisture content in the package is about 1100 × 10^-6（0.11 wt.%)。 Baking at 125 ℃ for 24 hours can fully remove the moisture absorbed in the package.

Brittle fracture

Brittle fracture often occurs in low yield strength and inelastic materials (such as silicon chips). When the material is over stressed, the sudden and catastrophic crack propagation will originate from small defects such as cavities, inclusions or discontinuities.

Ductile fracture

Plastic packaging materials are prone to brittle and ductile fracture modes, which mainly depend on environmental and material factors, including temperature, viscoplastic properties of polymer resin and filling load. Even in high loading plastic packaging materials containing brittle silicon filler, ductile fracture may still occur due to the viscoplastic properties of polymer resin.

fatigue fracture

When the plastic sealing material is subjected to periodic stress within the limit strength range, it will break due to cumulative fatigue fracture. Cyclic stresses are generated by wet, thermal, mechanical or combined loads applied to plastic packaging materials. Fatigue failure is a kind of wear failure mechanism, and cracks usually sprout at discontinuities or defects.

The fatigue fracture mechanism includes three stages: crack initiation (stage I); Stable crack propagation (stage II); Sudden, uncertain and catastrophic failure (stage III). Under cyclic stress, fatigue crack growth in stage II refers to the stable growth of crack length. The crack growth rate of plastic packaging materials is much higher than the typical value of fatigue crack growth of metal materials (about 3 times).

Accelerated failure factors

Environmental and material loads and stresses, such as moisture, temperature and pollutants, will accelerate the failure of plastic encapsulated devices. Plastic packaging process plays a key role in packaging failure, such as moisture diffusion coefficient, saturated moisture content, ion diffusion rate, thermal expansion coefficient and moisture absorption expansion coefficient of plastic packaging materials will greatly affect the failure rate. The main factors leading to failure acceleration are moisture, temperature, pollutants and solvent environment, residual stress, natural environmental stress, manufacturing and assembly load and comprehensive load stress conditions.

Moisture can accelerate delamination, cracking and corrosion failure of plastic encapsulated microelectronic devices. Moisture is an important failure acceleration factor in plastic encapsulated devices. The mechanisms related to the failure acceleration caused by moisture include the degradation of bonding surface, hygroscopic expansion stress, water vapor pressure, ion migration and the change of plastic sealing material properties. Moisture can change the glass transition temperature Tg, elastic modulus and volume resistivity of plastic sealing materials.

Temperature is another key failure acceleration factor. The effect of temperature on package failure is usually evaluated by using the temperature level related to the glass transition temperature of molding compounds, the thermal expansion and washing of various materials, and the resulting thermal mechanical stress. Another influencing factor of temperature on packaging failure is that it will change the temperature related packaging material properties, moisture diffusion coefficient and intermetallic diffusion.

Pollutants and solvent environmental pollutants provide a place for the initiation and expansion of failure. The pollution sources mainly include air pollutants, moisture, flux residue, unclean examples in plastic sealing materials, corrosive elements produced by thermal degradation and by-products (usually epoxy) discharged from chip adhesives. Generally, the plastic package will not be corroded, but moisture and pollutants will diffuse in the plastic packaging material and reach the metal part, causing the corrosion of the metal part in the plastic packaging device.

Residual stress chip bonding will produce single stress. The stress level mainly depends on the characteristics of the chip bonding layer. Because the shrinkage of molding compound is greater than that of other packaging materials, the stress generated during molding is quite large. The stress test chip can be used to determine the assembly stress.

Natural environmental stress in the natural environment, the plastic sealing material may degrade. Degradation is characterized by the breaking of polymerization bonds, which is often the transformation of solid polymers into viscous liquids containing monomers, dimers and other low molecular weight species. Elevated temperatures and confined environments often accelerate degradation. Ultraviolet rays in sunlight and atmospheric ozone layer are powerful catalysts for degradation, which can lead to degradation by cutting off the molecular chain of epoxy resin. Isolation of plastic encapsulated devices from the environment prone to degradation and the use of polymers with anti degradation ability are all methods to prevent degradation. Products that need to work in hot and humid environments require anti degradable polymers.

Manufacturing and assembly loads manufacturing and assembly conditions can lead to package failure, including high temperature, low temperature, temperature change, operating load and load imposed on bonding lead and chip base due to plastic packaging material flow. The popcorn phenomenon during the assembly of plastic encapsulated devices is a typical example.

In the process of manufacturing, assembly or operation, failure acceleration factors such as temperature and moisture often exist at the same time. Combined load and stress conditions often further accelerate failure. This feature is often used in accelerated test design for the purpose of screening defective components and identifying failure prone packaging devices.