Common defects of EMC package forming and its countermeasures

This paper mainly analyzes and studies the problems (defects), such as unfilled, pores, pitting, punching, cracking, flashing, and sticking, which are often encountered in the process of forming EMC packages, and proposes effective solutions. Countermeasures.

Plastic packaging has become the mainstream of current microelectronic packaging with its unique advantages, accounting for more than 95% of the packaging market. The wide application of plastic packaging products has brought unprecedented development to plastic packaging, but almost all plastic molding products have defects in forming defects, whether using advanced transfer molding or traditional single injection molding. Packaging is not completely avoided. In comparison, the traditional plastic sealing mold has a large probability of forming defects, and the types are also large. The larger the size, the greater the probability of occurrence. The quality of plastic products is mainly determined by four factors: A, EMC performance, including gelation time, viscosity, fluidity, mold release, adhesion, moisture resistance, heat resistance, flashing properties , stress, strength, modulus, etc.; B, mold, mainly including the degree of matching between the runner, gate, cavity, exhaust port design and lead frame design; C, package form, different package forms often appear different Defects, so optimizing the design of the package form will greatly reduce the occurrence of defective defects; D, process parameters, mainly including mold clamping pressure, injection pressure, injection speed, preheating temperature, mold temperature, curing time and so on.

The following mainly analyzes and studies the causes of defects commonly encountered in plastic molding, and proposes corresponding effective and feasible solutions and countermeasures.

1. Package forming is not filled and its countermeasures

There are two main cases of package forming and unfilling: one is the tendency to be unfilled, mainly due to the mismatch between the packaging process and EMC performance parameters; one is random unfilled, mainly due to mold cleaning. Improper, the insoluble impurities in the EMC are too large, the mold feed port is too small, etc., causing blockage of the mold gate. From the perspective of package form, unfilled phenomenon is more likely to occur in DIP and QFP. From the aspect of appearance, DIP is not filled mainly in the form of completely unfilled and partially unfilled, and QFP mainly has corners not filled. The main reasons for unfilling and their countermeasures:

(1) There is a tendency to be unfilled due to the mold temperature being too high, or the packaging process does not match the EMC performance parameters. After the preheating, the reaction speed of the EMC is increased at a high temperature, so that the gelation time of the EMC is relatively short, and the fluidity is deteriorated. When the cavity is not completely filled, the viscosity of the EMC is sharply increased, and the flow resistance is also increased. As a result, a good filling is not obtained, thereby forming a tendency to be unfilled. This phenomenon is more likely to occur in VLSI packages, because the amount of EMC per mode of these large-scale circuits tends to be relatively large. In order to achieve uniform heating effect in a short time, the set temperature is often high, so it is easy to produce. This is not filled. For this kind of trending unfilling, which is mainly caused by insufficient EMC fluidity, it can be used to increase the preheating temperature of EMC to make it evenly heated; increase the injection pressure and speed, and accelerate the flow rate of EMC; The temperature is used to slow down the reaction rate and relatively extend the gelation time of the EMC to achieve a full filling effect.

(2) Due to the blockage of the mold gate, the EMC cannot be effectively injected, and the vent hole is blocked due to improper cleaning of the mold, which may cause unfilling, and the position of the unfilled in the mold is also irregular. In particular, in a small package, since the gate and the exhaust port are relatively small, it is most likely to cause clogging and unfilling. For this unfilled, the tool can be used to remove the blockage and apply a small amount of mold release agent, and after each mold package, use the ** and brush to remove the EMC curing material from the barrel and mold.

(3) Although the packaging process and the performance parameters of the EMC are well matched, the fluidity of the EMC is lowered due to improper storage or expiration, and the viscosity is too large or the gelation time is too short, which may cause poor filling. The solution is to choose EMC with suitable viscosity and gel time, and keep it in accordance with EMC's storage and use requirements.

(4) Unfilled due to insufficient EMC usage. This situation usually occurs when replacing EMC, package type or mold replacement. The solution is also simple. Just select the EMC quantity that matches the package type and mold. Can be solved, but the amount should not be too much or too little.

2, package forming pores and countermeasures

Porosity is the most common defect during package forming. According to the position where the pores are formed on the plastic body, the inner pores and the outer pores can be divided, and the outer pores can be further divided into the top pores and the gate pores. The air hole not only seriously affects the appearance of the plastic body, but also directly affects the reliability of the plastic sealing device, especially the internal air hole. The common pores are mainly external pores. The internal pores cannot be directly seen. They must be observed by X-ray apparatus, and the smaller internal pores Bp can not be seen clearly by X-rays. This also brings great resistance to pore defects. difficult. Then, in order to solve the problem of porosity defects, it is necessary to carefully study the formation process of various types of pores. However, strictly speaking, the pores cannot be completely eliminated, and measures can only be taken to improve them, and the pore defects are controlled within the scope of good products.

From the surface of the pores, the reason for the formation seems to be very simple, but the residual gas in the cavity is not effectively discharged. In fact, there are many factors causing porosity defects, mainly in the following aspects: A. Packaging materials, mainly including EMC gelation time, viscosity, fluidity, volatile matter content, moisture content, air content, and cake density. , the diameter of the cake does not match the diameter of the material; B, the mold, and the shape of the barrel, the shape and arrangement of the cavity, the shape and position of the gate and the exhaust port; C, packaging technology, Mainly related to preheating temperature, mold temperature, injection speed, injection pressure, injection time and so on.

The main causes of top vents, gate vents and internal vents during package forming and their countermeasures:

(1) There are two main situations in the formation of the top pores. One is that the EMC viscosity is sharply increased due to various factors, so that the injection pressure cannot be effectively transmitted to the top end, so that the residual gas at the top end cannot be discharged and the pore defects are caused; One is that the flow rate of EMC is so slow that the curing cross-linking reaction begins to occur when the cavity is not completely filled, which also creates pore defects. The most effective way to solve this kind of defect is to increase the injection speed, and the proper adjustment of the preheating temperature will be somewhat improved.

(2) The main reason for the generation of gate pores is that the flow rate of EMC in the mold is too fast. When the cavity is full, some residual gas is not discharged in time, and the exhaust port has been blocked by the overflow material. Finally, the residual gas is often compressed and left in the vicinity of the gate under the action of the injection pressure. An effective way to solve this porosity defect is to slow down the injection speed, appropriately reduce the preheating temperature, so that the flow rate of EMC in the mold is slowed down; and at the same time, in order to promote the escape of volatile substances, the mold temperature can be appropriately increased.

(3) The reason for the formation of internal pores is mainly because the temperature of the surface of the mold is too high, so that the EMC of the surface of the cavity is too fast or the curing reaction occurs prematurely, and the faster injection speed makes the exhaust port part full, so that the exhaust port is full. Part of the internal gas cannot overcome the solidified layer of the surface and remains inside to form pores. Such stomata defects generally occur in large-volume circuit packages and occur mostly at the gate end and intermediate positions. To effectively reduce the incidence of such pores, firstly, the mold temperature should be appropriately reduced. Secondly, the injection pressure can be appropriately increased. However, excessive pressure increase can cause other defects such as punching and flashing. The suitable pressure range is 8~10Mpa. .

3. Package forming pitting and its countermeasures

After the package is formed, a large number of fine pores sometimes appear on the surface of the package, and the positions are relatively concentrated, and it is a pitting. These defects are often accompanied by other defects, such as unfilled, cracked, and the like. The reason for this defect is mainly that the cake is heated unevenly during the preheating process, and the temperature difference of each part is large. After the injection into the cavity, the curing reaction is inconsistent, so that pitting defects are formed. There are also many factors that cause the cake to be unevenly heated, but there are three main situations:

(1), the cake is broken and not angled. For the cake with general broken corners, the length of the defect is less than 1/3 of the height of the cake, and it can be used smoothly on the roller of the preheater, and in order to prevent the pouring during preheating, the broken cake can be used. Sandwiched in the middle. When the cartridge is placed, it is preferred to place the broken cake on the bottom or top so that the temperature difference between the cakes can be improved. For the severely damaged cake, you can only give up.

(2) When the cake is preheated, it is improperly placed. When the cake is taken out at the end of preheating, it is often found that the two ends of the 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 ° C, the temperature difference is about 8 to 10 ° C, so that the pitting defects are most likely to occur when the package is formed. In order to solve the pitting defects caused by the large temperature difference, a certain gap can be left between the cakes during the preheating, so that the cakes can be uniformly and uniformly heated. Experience has shown that it is also possible to improve the defects caused by the large temperature difference after the middle cake is thrown at the time of feeding.

(3) The height of the heating plate of the preheater is unreasonable, which may also cause uneven heating, which leads to the occurrence of pitting. In this case, when different sizes of cakes are used on the same preheater, the height of the heating plate is not adjusted, so that the distance between the heating plate and the cake is too close, so that the cake is unevenly heated. Experience has shown that the reasonable distance between them is 3-5mm, too close or too far is not suitable.

4. Package forming and punching and its countermeasures

When the package is formed, the EMC is in a molten state. Because of its certain melt viscosity and flow velocity, it naturally has a certain impulse. This impulse acts on the gold wire, which easily causes the gold wire to shift, which will seriously cause gold. The wire is broken. This kind of squeezing phenomenon is very common in the process of plastic sealing and cannot be completely eliminated, but if the proper viscosity and flow rate are selected, it can be controlled within the range of good products. The influence of EMC's melt viscosity and flow velocity on the thrust of gold wire can be explained by establishing a mathematical model. It can be assumed that the molten EMC is an ideal fluid, then the impulse F=KηυSinQ, K is a constant, η is the melt viscosity of EMC, υ is the flow velocity, and Q is the angle between the flow direction and the gold wire. It can be seen from the formula that the larger η, the larger the υ, the larger the F; the larger the Q, the larger the F; the larger the F, the more severe the rushing.

To improve the incidence of defects in the wire, the key is how to select and control the melt viscosity and flow rate of the EMC. In general, EMC's melt viscosity is a change from high to low and then high, and there is a low viscosity period, so choose a reasonable injection time, so that the EMC in the cavity flows in the low viscosity period to reduce momentum. Choosing a suitable flow velocity is also an effective way to reduce the impact force. There are many factors affecting the flow velocity, which can be considered from the factors such as injection speed, mold temperature, mold flow path and gate. In addition, the packaged product of long gold wire is more prone to tapping than the packaged product of short gold wire, so the size of the chip is matched with the size of the island, and the small island chip phenomenon is avoided to reduce the degree of punching. )

5. Package forming cracking and countermeasures

In the process of package forming, the sticking mold, EMC moisture absorption, and the expansion coefficient of each material do not match, etc., which will cause cracking defects.

The cracking phenomenon caused by the sticking mold is mainly caused by factors such as too short curing time, poor release property of EMC, or contamination of the mold surface. In the forming process, the curing time can be extended to make it fully cured; in terms of materials, the release property of EMC can be improved; in terms of operation, the surface of the mold can be cleaned before each mold, or the surface of the mold can be coated with an appropriate amount. Release agent. For the cracking caused by EMC moisture absorption, in the process, it is necessary to ensure the occurrence of moisture absorption during storage and recovery of normal temperature; in the material, EMC with high Tg, low expansion, low water absorption and high adhesion can be selected. For the cracking phenomenon caused by the mismatch of the expansion coefficients of the materials, the expansion coefficient of the material such as the chip and the frame can be selected to match.

6. Package forming flash and its countermeasures

In the process of package forming, flash is a common form of defect, and the defect itself has no effect on the performance of the packaged product, only affecting the subsequent solderability and appearance. The cause of flashing can be considered from two aspects. First, in terms of materials, the resin viscosity is too low, and the particle size distribution of the filler is unreasonable, which can cause the occurrence of flash. Within the allowable range of viscosity, a resin with a large viscosity can be selected. And adjust the particle size distribution of the filler to increase the filling amount, so that it can improve its anti-flash performance from EMC itself; second, the packaging process, the injection pressure is too large, the clamping pressure is too low, can also cause the generation of flash This defect can be improved by appropriately reducing the injection pressure and increasing the clamping pressure. Due to the wear of the surface of the plastic mold after long-term use or the unevenness of the pedestal, the gap after the mold clamping is large, which will also cause flashing. The serious flash phenomenon seen in the production is often caused by this reason. Reduce the wear and adjust the flatness of the pedestal to solve this flash defect.

7. Package forming sticking mold and its countermeasures

The reasons for the formation of the packaged adhesive mold and its countermeasures: A, the curing time is too short, the adhesive is not fully cured by the EMC, the curing time can be extended, the clamping time can be increased to make it fully cured; B, EMC itself release properties The poorly formed sticking mold can only improve the release property of EMC from the material aspect, or the appropriate mold release agent during the process of package forming; C, the surface of the mold can also cause sticking, which can be cleaned by the mold. To solve; D, the mold temperature is too low will also cause the sticking phenomenon, which can be improved by appropriately increasing the mold temperature.

8. Conclusion

In short, there are many types of defects in plastic molding, and there are different expressions in different packaging forms. The probability and location of occurrence are also very different. The causes are also complicated, and they are related to each other and affect each other. Therefore, they should be separated. On the basis of research, comprehensive considerations have been made to formulate corresponding effective solutions and countermeasures.

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