Types of defects in the EMC packaging forming process

灞波儿奔

Types of defects in the EMC packaging forming process [Copy link]

This article mainly analyzes and studies the common problems (defects) in the process of EMC packaging forming, such as unfilled, pores, pits, punching, cracking, overflow, and sticking mold, and proposes effective solutions and countermeasures.
Plastic packaging has become the mainstream of microelectronic packaging with its unique advantages, accounting for more than 95% of the packaging market. The wide application of plastic packaging products has also brought unprecedented development to plastic packaging, but almost all plastic packaging products have forming defects. Whether it is advanced transfer molding or traditional single injection molding, it is impossible to completely avoid it. In comparison, the probability of traditional plastic molding defects is higher, and there are more types. The larger the size, the greater the probability of occurrence.
The quality of plastic encapsulation products is mainly determined by four factors:
A. EMC performance, mainly including gel time, viscosity, fluidity, demoulding, adhesion, moisture resistance, heat resistance, flashing, stress, strength, modulus, etc.;
B. Mold, mainly including the matching degree of runner, gate, cavity, exhaust port design and lead frame design, etc.;
C. Packaging form, different packaging forms often have different defects, so optimizing the design of packaging form will greatly reduce the occurrence of bad defects;
D. Process parameters, mainly including mold clamping pressure, injection pressure, injection speed, preheating temperature, mold temperature, curing time, etc.
The following mainly analyzes the causes of common defects in plastic encapsulation, and proposes corresponding effective and feasible solutions and countermeasures.
1. Underfilling during package forming and its countermeasures
There are two main types of underfilling during package forming: one is a tendency of underfilling, which is mainly caused by the mismatch between the packaging process and the performance parameters of EMC; the other is a random underfilling, which is mainly caused by improper mold cleaning, too much insoluble impurities in EMC, too small mold feed port, etc., causing mold gate blockage. From the perspective of packaging form, underfilling is more likely to occur in DIP and QFP, while from the perspective of appearance, DIP underfilling is mainly manifested as complete underfilling and partial underfilling, and QFP mainly has corner underfilling. The
main reasons for underfilling and their countermeasures:
(1) Tendency of underfilling caused by excessive mold temperature, or the mismatch between the performance parameters of the packaging process and EMC. After preheating, the EMC reacts faster at high temperature, which shortens the gel time of EMC and makes its fluidity worse. When the cavity is not completely filled, the viscosity of EMC rises sharply, and the flow resistance increases, so that it cannot be filled well, thus forming a tendency of non-filling. This phenomenon is more likely to occur in VLSI packaging, because the amount of EMC used in each mold of these large-scale circuits is often large. In order to achieve the effect of uniform heating in a short time, the set temperature is often relatively high, so it is easy to produce this phenomenon of non-filling.) This tendency of non-filling is mainly caused by insufficient fluidity of EMC. You can increase the preheating temperature of EMC to make it evenly heated; increase the injection pressure and speed to speed up the flow rate of EMC; reduce the mold temperature to slow down the reaction speed and relatively prolong the gel time of EMC, so as to achieve the effect of full filling.
(2) Due to the blockage of the mold gate, the EMC cannot be effectively injected, and due to improper mold cleaning, the vent hole is blocked, which will also cause underfilling, and the position of this underfilling in the mold is also irregular. Especially in small packages, since the gate and vent are relatively small, it is most likely to cause blockage and underfilling. For this kind of underfilling, you can use tools to remove the blockage and apply a small amount of mold release agent. After each mold packaging, use a brush to clean the EMC curing material on the barrel and mold.
(3) Although the packaging process matches the performance parameters of EMC well, due to improper storage or expiration, the fluidity of EMC decreases, the viscosity is too high or the gel time is too short, which will cause poor filling. The solution is mainly to select EMC with suitable viscosity and gel time, and properly store it according to the storage and use requirements of EMC.
(4) Underfilling caused by insufficient EMC dosage. This situation usually occurs when changing EMC, packaging type or mold. The solution is relatively simple. Just choose the EMC dosage that matches the packaging type and mold to solve the problem. However, the dosage should not be too much or too little.
2. Package forming pores and countermeasures
In the process of package forming, pores are the most common defects. According to the location of the pores on the plastic package, they can be divided into internal pores and external pores, and external pores can be divided into top pores and gate pores. Pores not only seriously affect the appearance of the plastic package, but also directly affect the reliability of the plastic package device, especially internal pores should be paid more attention. Common pores are mainly external pores. Internal pores cannot be seen directly and must be observed through an X-ray device. Moreover, the smaller internal pores Bp cannot be seen clearly through x-rays, which also brings great difficulties to overcome pore defects. Therefore, to solve the problem of pore defects, it is necessary to carefully study the process of formation of various pores. However, strictly speaking, pores cannot be completely eliminated. We can only take measures in many aspects to improve them and control pore defects within the range of good products.
From the surface of the pores, the cause of the formation seems to be very simple, just that there is residual gas in the cavity that is not effectively discharged. In fact, there are many factors that cause pore defects, which are mainly manifested in the following aspects:
A. In terms of packaging materials, it mainly includes the gel time, viscosity, fluidity, volatile content, moisture content, air content, cake density, and the mismatch between the cake diameter and the cylinder diameter of EMC;
B. In terms of molds, it is related to the shape of the barrel, the shape and arrangement of the cavity, the shape and position of the gate and the vent, etc.;
C. In terms of packaging process, it is mainly related to the preheating temperature, mold temperature, injection speed, injection pressure, injection time, etc.
The main reasons for the formation of top pores, gate pores and internal pores and their countermeasures:
(1) There are two main situations for the formation of top pores. One is that due to various factors, the viscosity of EMC rises sharply, resulting in the inability to effectively transmit the injection pressure to the top, so that the residual gas at the top cannot be discharged, resulting in pore defects; the other is that the flow speed of EMC is too slow, so that the curing cross-linking reaction begins before the cavity is completely filled, which will also form pore defects. The most effective way to solve this defect is to increase the injection speed. Appropriate adjustment of the preheating temperature will also improve the situation.
(2) The main reason for the gate pores is that the EMC flows too fast in the mold. When the cavity is full, some residual gas cannot be discharged in time. At this time, the exhaust port has been blocked by the overflow material. Finally, the residual gas is often compressed and remains near the gate under the action of the injection pressure. The effective way to solve this pore defect is to slow down the injection speed and appropriately reduce the preheating temperature to slow down the flow of EMC in the mold. At the same time, in order to promote the escape of volatile substances, the mold temperature can be appropriately increased.
(3) The main reason for the formation of internal pores is that the temperature of the mold surface is too high, causing the EMC on the cavity surface to cure too quickly or too early. In addition, the faster injection speed makes the exhaust port area full, so that some of the internal gas cannot overcome the surface curing layer and remains inside to form pores. This type of pore defect generally occurs in large-volume circuit packaging, and often appears at the gate end and the middle position. To effectively reduce the incidence of this type of pores, first of all, the mold temperature should be appropriately lowered, and then the injection pressure can be appropriately increased. However, excessive pressure increase will cause other defects such as punching and overflow. The more appropriate pressure range is 8-10Mpa
.
After the package is formed, a large number of tiny holes sometimes appear on the surface of the package, and the positions are relatively concentrated, which looks like a patch of pitting. These defects often appear simultaneously with other defects, such as underfilling, cracking, etc. The main reason for this defect is that the material cake is heated unevenly during the preheating process, and the temperature difference between different parts is large. After being injected into the mold cavity, the curing reaction is inconsistent, resulting in pitting defects. There
are many factors that cause uneven heating of the material cake, but there are mainly three situations:
(1) The material cake is broken and chipped. For general broken and chipped material cakes, the length of the defect is less than 1/3 of the height of the material cake, and it can be used only when it rotates smoothly on the preheating machine roller. In order to prevent tipping during preheating, the broken material cake can be sandwiched in the middle. When putting it into the barrel, it is best to place the broken material cake at the bottom or top, so as to improve the temperature difference between the material cakes. For severely damaged material cakes, they can only be abandoned.
(2) Improper placement of the material cake during preheating. When the material cake is taken out after preheating, it is often found that the two ends of the material cake are relatively soft, while the middle is relatively hard, and the temperature difference is large. Generally, when the preheating temperature is set at 84-88℃, the temperature difference is about 8-10℃, which is the most likely to cause pitting defects during packaging. To solve the pitting defects caused by the large temperature difference, a certain gap can be left between the material cakes during preheating so that each material cake can be fully and evenly heated. Experience shows that when feeding, first feeding the middle material cake and then the two end material cakes can also improve this defect caused by the large temperature difference.
(3) The unreasonable height of the preheating machine heating plate can also cause uneven heating, resulting in pitting. This situation often occurs when using different sizes of material cakes on the same preheating machine without adjusting the height of the heating plate, so that the distance between the heating plate and the material cake is far or near, so that the material cake is heated unevenly. Experience has shown that the more reasonable distance between them is 3-5mm, and it is not appropriate to be too close or too far.
4. Punching wires during package forming and its countermeasures
During package forming, EMC is in a molten state. Due to its certain melt viscosity and flow rate, it naturally has a certain impact force. This impact force acts on the gold wire, which can easily cause the gold wire to deviate, and in severe cases, it can cause the gold wire to break. This punching phenomenon is very common in the process of plastic packaging and cannot be completely eliminated, but if the appropriate viscosity and flow rate are selected, it can still be controlled within the range of good products. The influence of the melt viscosity and flow rate of EMC on the impact force of the gold wire can be explained by establishing a mathematical model. It can be assumed that the molten EMC is an ideal fluid, then the impact force F = KηυSinQ, K is a constant, η is the melt viscosity of EMC, υ is the flow rate, and Q is the angle between the flow direction and the gold wire. It can be seen from the formula: the larger the η, the larger the υ, the larger the F; the larger the Q, the larger the F; the larger the F, the more serious the punching.
To improve the incidence of punching defects, the key is how to select and control the melt viscosity and flow rate of EMC. Generally speaking, the melt viscosity of EMC changes from high to low and then to high, and there is a low viscosity period, so choose a reasonable injection time to make the EMC in the mold cavity flow during the low viscosity period to reduce the impact. Choosing a suitable flow rate is also an effective way to reduce the impact. There are many factors that affect the flow rate, which can be considered from the injection speed, mold temperature, mold runner, gate and other factors. In addition, the packaged products with long gold wires are more prone to wire punching than the packaged products with short gold wires, so the size of the chip should match the size of the island to avoid the phenomenon of large island and small chip to reduce the degree of wire punching. )
5. Package forming cracking and its countermeasures
In the process of package forming, mold sticking, EMC moisture absorption, and mismatch of expansion coefficients of various materials will cause cracking defects.
The cracking caused by mold sticking is mainly caused by factors such as too short curing time, poor demolding performance of EMC, or contamination of the mold surface. In the forming process, the curing time can be extended to fully cure; in terms of materials, the demoulding performance of EMC can be improved; in terms of operation, the mold surface can be cleaned before each mold, or an appropriate amount of demoulding agent can be applied to the mold surface. For the cracking phenomenon caused by EMC moisture absorption, in terms of technology, it is necessary to ensure that moisture absorption is avoided during storage and restoration to normal temperature; in terms of materials, EMC with high Tg, low expansion, low water absorption and high adhesion can be selected. For the cracking phenomenon caused by the mismatch of expansion coefficients of various materials, materials that match the expansion coefficients of chips, frames, etc. can be selected.
6. Packaging forming flash and its countermeasures
In the process of packaging forming, flash is another common defect form, and this defect itself has no effect on the performance of the packaged product, but only affects the subsequent solderability and appearance. The reasons for the flashing can be considered from two aspects. First, in terms of materials, low resin viscosity and unreasonable filler particle size distribution can cause flashing. Within the allowable range of viscosity, you can choose a resin with a higher viscosity, adjust the filler particle size distribution, and increase the filling amount. This can improve the anti-flash performance of the EMC itself. Second, in terms of packaging technology, excessive injection pressure and low mold clamping pressure can also cause flashing. This defect can be improved by appropriately reducing the injection pressure and increasing the mold clamping pressure. Due to the wear of the surface of the plastic encapsulation mold after long-term use or the uneven base, the gap after mold clamping is large, which can also cause flashing. The serious flashing phenomenon seen in production is often caused by this reason. You can try to reduce wear and adjust the flatness of the base to solve this flashing defect.
7. Sticking during encapsulation and its countermeasures
The causes and countermeasures of sticking during encapsulation: A. The curing time is too short, and the EMC is not completely cured, which causes sticking. The curing time can be appropriately extended, and the mold closing time can be increased to fully cure it; B. The sticking caused by the poor demoulding performance of the EMC itself can only be improved from the material aspect, or the appropriate release agent can be added during the encapsulation process; C. Contamination on the mold surface can also cause sticking, which can be solved by cleaning the mold; D. Too low mold temperature can also cause sticking, which can be improved by appropriately increasing the mold temperature.
8. Conclusion
In short, there are many types of defects in plastic encapsulation, which have different manifestations in different packaging forms, and the probability and location of occurrence are also very different. The causes are also relatively complex, and they are intertwined and affect each other. Therefore, it should be based on separate research and comprehensive consideration to formulate corresponding effective solutions and countermeasures.