introduction
In recent years, plastic encapsulated devices have been more and more used in highly reliable technical fields because of their advantages in size, weight, cost and performance. However, due to the inherent structure and material characteristics of plastic packaging devices, moisture intrusion, corrosion, cracking and internal delamination have become unique potential defects of plastic packaging devices, which directly affect the long-term reliability of plastic packaging devices. Plastic encapsulated devices are almost always sold as commercial off the shelf devices. Although manufacturers continue to improve device function design and packaging technology and master the technology of how to control production process for high yield, they leave the long-term quality and reliability verification of products for final use. In particular, users with small purchase volume, including domestic military users, generally do not purchase directly from manufacturers, and it is difficult to require agents or traders to carry out reliability tests or screening tests. Therefore, it is difficult to guarantee the purchase of model batch plastic encapsulated devices with guaranteed quality grade; In addition, due to various reasons, it is difficult for domestic military users to purchase high-grade products. The functions of high-grade products that can be purchased often can not meet the requirements of designers, making users generally use low-grade plastic packaging devices. Because there are certain risks in using low-grade products in high reliability systems, how to use effective assessment and evaluation techniques to evaluate the reliability of plastic encapsulated devices has attracted extensive attention. Through some analysis and Research on the common failure mechanism of plastic packaging devices, this paper recommends a set of optimized and practical test process, which has been successfully applied to the secondary screening test and evaluation test of low-grade plastic packaging devices selected for high reliability application equipment.
1. Common failure mechanisms of plastic encapsulated devices
Due to the inherent structure and material characteristics of plastic packaging devices, plastic packaging devices are vulnerable to a variety of defects that can lead to premature failure. Moisture intrusion, corrosion, cracking and internal delamination initially made the plastic encapsulated devices considered to be more prone to failure, and these factors just limited the use of plastic encapsulated devices in high stress and high reliability environments.
1.1 erosion
Plastic packaging devices are non airtight devices sensitive to humidity. Although great progress has been made in packaging materials, chip passivation and processing technology, plastic packages will absorb moisture from the atmosphere over time. When the packaging material contains any ionic impurities, it may cause corrosion to the chip metallization. When this happens, the corrosion of metallization generally begins in the bonding area in the lead. High temperature and applied voltage are the factors that accelerate this electrochemical corrosion mechanism. In the case of moisture and impurity contamination, these wire bonds themselves are more susceptible to erosion. The ionic impurities are hydrolyzed and can react with the aluminum in the gold aluminum intermetallic compound at the welding site to produce chemical or electrochemical corrosion.
Failure modes caused by wire bonding and chip metallization erosion The formula includes electrical parameter drift, leakage, short circuit and open circuit. The erosion of a solder joint may not directly lead to failure, but it can increase the contact resistance and eventually make the device lose its function. Lead frames are also susceptible to erosion or stress erosion fracture mechanisms. In the presence of moisture and foreign contamination, any sand holes, cracks or voids in the lead frame coating will erode the substrate. Fracture may also occur during the final process of lead molding. The most sensitive place of lead frame erosion is at the junction between plastic sealant and lead frame.
1.2 "popcorn" effect
When the moisture absorbed plastic packaging device is exposed to the high temperature of reflow soldering or if the package is integrated into the molten solder or wave solder, the internal moisture will turn into steam and expand rapidly. It is easy to have various effects, resulting in internal delamination, bonding damage, metallization corrosion or thermal expansion, and even the phenomenon of popcorn, that is, the so-called "popcorn" effect. Compared with through-hole devices, plastic SMD is more sensitive to this. All plastic encapsulated devices have wet steam sensitivity levels. SMD devices are divided into 8 levels in the American ipc/jedec j- std--020d standard. Level 1 is the lowest level of moisture sensitivity, and level 8 is the highest level. That is, the higher the level, the more sensitive the device is to moisture, and the more prone it is to thermal damage in the process. According to aesi/ipc- sm-786, the internal moisture content is greater than 0.11% of the weight, the welding temperature is greater than 220 ℃ or the welding temperature conversion rate is greater than 10 C, which will significantly increase the risk of burst of a plastic package. In particular, the temperature and time of lead-free reflow soldering are higher and the risk of "popcorn" effect is higher.
When repairing printed circuit boards and replacing defective components, it may aggravate the problem that adjacent components suffer from additional heat and thermal stress, and may lead to internal crack propagation or delamination in a larger area, or even open circuit, increased contact resistance and metal wire disconnection.
1.3 failure caused by temperature deformation
The plastic packaging device adopts the overall molded packaging structure, including metal frame, chip, bonding material between chip and substrate, inner lead and plastic molding compound. The thermal expansion coefficients of plastics, frames and silicon chips are different. When subjected to continuous temperature changes, it will lead to temperature deformation between the package and the chip, and eventually lead to fatigue or cracks in the cross sections of different materials. Then moisture and impurities are easy to enter. Any moisture related failure mechanism mentioned above may occur.
Plastic encapsulated devices will also encounter the formation of harmful gold aluminum Intermetallics at the bonding site, which will continue to thicken with time and high temperature. Excessive intermetallics will lead to Kirkendall holes. With the production of intermetallics and the appearance of voids, the contact resistance increases and the strength decreases. In the environment of temperature change, the internal stress between various mesophase alloys will lift the bonding and cause open circuit, while maintaining contact during cooling, which is easy to cause the phenomenon of time-off and time-on. The user must be aware of this potential problem, and it is required to protect the plastic packaging device from the possible harmful temperature and relative humidity environment during handling and storage.
2. Recommended test procedures for plastic encapsulated devices
The advantages of plastic packaging devices include low cost, light weight and better physical and electrical performance. However, if plastic packaging devices are subject to the same screening and batch identification tests that air tight devices usually bear, the cost advantage of plastic packaging devices will disappear. In the traditional quality assurance system, the most expensive and time-consuming procedures are electrical testing, sophistication and hast (high temperature service life) test. For complex devices such as microprocessors, memories, high-speed / high-resolution a/d and d/a converters and ASICs, it costs a lot to develop programs, purchase high-tech sockets, design and manufacture test boards, and finally carry out tests and tests. Therefore, many domestic military users hope to tailor the electrical test and stress test, and use the technology with high cost-effectiveness ratio to evaluate the reliability of plastic packaging devices without affecting the reliability requirements of the whole system. According to the characteristics and common failure mechanism of plastic encapsulated devices, the following test process fully considers the consistency of batch quality of plastic encapsulated devices, device life and moisture resistance at high temperature. At the same time, in order to reduce the cost of testing and samples, the test evaluation starts with the nondestructive method that never destroys the samples, and then goes to the destructive evaluation test method.
2.1 external visual inspection
Plastic packaging devices are non air tight devices. For external visual inspection of other packaging forms, the plastic packaging devices do not need to be checked for air tightness of glass seals, packages or caps, but must focus on the integrity of molding compounds. Among them, new requirements are put forward for the inspection of package deformation, foreign objects in the coating, holes and cracks, leads, etc., and the conformity of (finished products) batches is required to be evaluated. External visual inspection is generally conducted before and after screening and identification test.
In recent years, the proliferation of counterfeit devices has seriously destroyed the production quality of plate assembly users. During acceptance, we can identify the authenticity of mixed batch, refurbished and counterfeit products by checking whether the identification information such as the model, batch and place of origin of the samples are clear and consistent, and whether the sealing surface has wear marks and defects.
2.2 X-ray inspection
X-ray inspection is a method to detect whether the internal structure of the sample is defective by using the X-ray intensity after the X-ray decays through each part of the sample according to the different X-ray absorptivity and transmissivity of different parts of the sample. The X-ray fluoroscope has reached the spatial resolution of submicron level, which can realize multi angle rotation of the measured object to form images of different angles, and can also provide two-dimensional section or three-dimensional imaging through computer layered scanning technology. X-ray inspection can not only clearly show the internal structure of the tested sample, but also clearly identify the internal defects of plastic encapsulated devices, including design, structure, material and process defects such as plastic encapsulated foreign bodies and holes, inaccurate chip positioning and bonding holes, lead frame teeth, bonding internal lead offset and fracture. It is worth noting that X-ray inspection, as a non-destructive technology, is not absolute. It may cause damage to MOS and other sensitive devices by exposing them to abnormally high doses. Therefore, when performing X-ray inspection, it is necessary to estimate the radiation impact caused by sensitive devices.
In recent years, we have inspected many inferior imported plastic packaging devices. The bonding wire collapses, there is no bonding lead, and the internal structure of the same batch of samples is inconsistent (mixed batch). These inferior products directly affect the successful operation of the electronic machine and system, and there are great hidden dangers in reliability.
2.3 acoustic scanning microscope inspection
Acoustic scanning microscope is an instrument that uses ultrasonic pulses to detect defects such as voids in samples. With the development of acoustic scanning microscope inspection technology, the detection mode of acoustic signals has been expanded from single point detection A-scan, cross-section detection B-scan, cross-section detection C-scan to multi-layer cross-section detection X-Scan, projection detection T-scan, etc. No matter which mode, the location analysis of interface defects is based on whether the phase change of acoustic signal occurs after the interface reflection to determine whether there are defects at the interface. In general, we recommend to use C-scan mode, i.e. c--sam, to inspect the internal interfaces of plastic encapsulated devices.
Plastic encapsulated devices are non airtight devices sensitive to humidity. Compared with sealed devices, they have poor environmental resistance, especially the failure caused by moisture intrusion, corrosion and stress. Once the plastic packaging device is invaded by moisture, it is easy to cause delamination under the condition of isothermal welding. The acoustic scanning microscope technology shows high sensitivity and effectiveness for abnormal phenomena such as package pores and impurities, welding base displacement, welding wire deviation, chip cracks, interface cracks, peeling, poor chip bonding, etc. For plastic encapsulated devices in highly reliable applications, 100% c--sam screening is generally conducted to eliminate devices that may have delamination defects.
A typical case of device failure caused by internal delamination: a four channel codec is reflowed After the circuit was connected, it was found that the circuit was invalid, and c--sam found that the plastic packaging and chip were layered, as shown in Figure 1; However, it can be observed by X-ray that the sample chip is peeled off from the plastic packaging and the root of some bonding wires is broken, as shown in Figure 2. It is consistent with the open circuit of some pins in the electrical measurement, which indicates that the open circuit failure of the plastic encapsulated device is caused by the pull off solder joint of the plastic encapsulated layer caused by the delamination failure.
2.4 destructive physical analysis (DPA)
Dpa: destructive physicalanalysis aims to use physical and chemical methods and means to determine whether there are undiscovered design, material and processing defects in the product batch. When DPA finds obvious major defects, it usually indicates that the manufacturer is out of control during the production process, and the batch needs to be replaced. Therefore, it is recommended to conduct DPA before screening and identification of batches. DPA focuses on package integrity, assembly quality and chip defects, and evaluates the consistency of batch product packages and chips. According to gjb4027a--2006 (3), DPA of plastic packaging devices includes 7 inspections: external visual inspection, X-ray inspection, acoustic scanning microscope inspection, internal visual inspection, bonding strength, scanning electron microscope (SEM) inspection and glass passivation layer integrity inspection.
The non-destructive external visual inspection, X-ray inspection and acoustic microscope inspection have been described above, and will not be repeated here. Internal visual inspection requires inspection of bonding, internal lead, chip installation, scribing and chip, metallization, glass passivation layer and diffusion zone, and judgment of invasion of foreign objects of plastic packaging materials and cavity, corrosion, peeling and bulge of metal layer. In addition, it is also necessary to pay attention to the particle shape and size of filled quartz sand particles to investigate whether they will damage the glass passivation layer. The bonding strength test results shall not be regarded as whether the DPA inspection is qualified or not But it can be used for reference. SEM inspection requires to evaluate the wire bonding, the integrity of glass passivation layer and the quality of metallization layer of chip interconnect. The integrity inspection of glass passivation layer requires the evaluation of cracks, cavities and pinholes in the glass passivation layer.
Military users usually need to purchase foreign plastic encapsulated integrated circuits, while the imported plastic encapsulated devices purchased are of mixed quality, uncontrollable quality and generally low quality grade. Most of them are industrial grade, and a few are commercial grade. There is no military grade. In 2008, a laboratory conducted DPA on 674 batches of plastic encapsulated devices, and 87 batches were found to be unqualified, accounting for 13%, of which 80 batches of C-sam were unqualified, accounting for 12%; In addition, the problems of renovation, counterfeiting, mixed batch, corrosion and stratification are serious, and the product quality is worrying. Therefore, DPA is an important means to prevent and eliminate unqualified batches in advance.
2.5 life test
Life test is carried out under bias voltage to identify potential failure mechanism and quantify device reliability. It can detect defects related to design, metallization integrity, silicon contamination, manufacturing and assembly. For example, the organic packaging materials are accelerated to aging under high temperature, the insulation performance and protection performance deteriorate, or release substances harmful to the device, such as water vapor, sodium ions and heavy metal impurities, resulting in internal pollution and corrosion of the device. High temperature service life (HTOL) test is life test. The test shall be carried out according to gjb548b-2005 method 1016, condition D, and 1000 h under the maximum operating junction temperature.
2.6 humidity resistance test
Moisture resistance test is to test the device in a high temperature and high humidity environment, so that moisture invades the package to produce defects such as delamination and cracking. This test can identify the sensitivity of the device to the stress caused by moisture, so that the device can be properly packaged, stored and handled to avoid mechanical damage. Pressure cooker, 85 9c/85%rh and hast tests are moisture resistance tests. The high accelerated stress and temperature (HAST) test is rapidly replacing the 85/85 test. The hast test is carried out according to jesd 22-a110. The conditions are usually 130 C, 85%rh, 250 H.
PEMs after high-temperature hot water or hast test shall be removed after welding process, and then acoustic scanning inspection shall be conducted to analyze the layering of lead frame and packaging material, chip and packaging material, and judge whether this batch of devices pass the hot water resistance test. A large number of practical data confirmed that it passed The test evaluation of the above process can well eliminate the plastic components that may enter the equipment and have potential reliability problems, and play a good role in ensuring the reliability of the equipment.
2. Concluding remarks
Due to the inherent structural characteristics of plastic packaging devices, erosion, delamination and temperature deformation are common failure phenomena. In order to effectively reduce the risk of COTS plastic packaging devices used in military projects, this paper recommends a set of test process including non-destructive screening, DPA, life test and moisture resistance test, which can effectively evaluate the batch quality consistency of plastic packaging devices High temperature service life and moisture resistance and other important reliability index levels.