Abstract: This paper mainly analyzes and studies the problems (defects), unfilled, stomatal, pitting, crushing, cracking, flashover and sticky molds in the process of EMC encapsulation. Methods and countermeasures.
Plastic packaging with its unique advantages and become the mainstream of the current microelectronics package, accounting for more than 95% of the packaging market. The widespread use of plastic products, but also for the plastic packaging has brought unprecedented development, but almost all of the plastic products forming defects are always common, and whether it is the use of advanced transmission mold package, or the use of traditional single injection mold Packaging, are not completely avoided. In contrast, the traditional plastic mold forming defects are more likely, more types, 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 gel time, viscosity, mobility, mold release, adhesion, moisture resistance, heat resistance, , Stress, strength, modulus, etc .; B, mold, including the runner, gate, cavity, exhaust design and lead frame design matching degree; C, package form, different packages are often different D, process parameters, including clamping pressure, injection pressure, injection speed, preheating temperature, mold temperature, curing time, etc., to optimize the design of the package will greatly reduce the occurrence of defects;
In the following, the causes of the defects in the plastic forming are analyzed and studied, and the corresponding feasible and feasible solutions and countermeasures are put forward.
1. Package is not filled and its countermeasures
There are two cases of encapsulation forming without filling: one is the trend of the non-filling, mainly due to the packaging process and EMC performance parameters do not match; one is random not filled, mainly due to mold cleaning Improper, EMC insoluble impurities are too large, the mold feed port is too small and other reasons, causing the mold gate caused by plugging. From the package form point of view, in DIP and QFP are more prone to filling phenomenon, and from the appearance point of view, DIP is not filled mainly for the total and no part of the filling is not filled, QFP main corner is not filled. The main reason and countermeasures of not filling:
(1) due to the mold temperature is too high, or the packaging process and EMC performance parameters do not match the trend of the non-filling. After the preheating of the EMC in the high temperature reaction speed, resulting in EMC gel time is relatively short, poor mobility, the cavity is not fully filled, EMC's viscosity will rise sharply, the flow resistance also becomes larger, So that a good filling can not be obtained, resulting in a tendency to unfilled. This is more likely to occur in VLSI packages because these large-scale circuits often have a relatively large amount of EMC per mold, so that the temperature is often higher in order to achieve uniform heating in a short period of time, This is not filling the phenomenon. ) For this trend of the lack of filling is mainly due to EMC liquidity is not sufficient, can be used to improve the preheating temperature of EMC to make it even heat; increase the injection pressure and speed, so that EMC speed; Temperature, in order to slow down the reaction rate, the relative extension of the gel time of EMC, so as to achieve the full effect of filling.
(2) due to mold gate clogging, resulting in EMC can not be effectively injected, and due to improper cleaning of the mold caused by the exhaust hole blockage, will cause no filling, and this is not filled in the mold position is also irregular. Especially in small packages, due to the gate, the exhaust port is relatively small, so the most likely to cause clogging and produce no filling phenomenon. For this unfilled, you can use tools to remove clogs and apply a small amount of release agent, and after each mold package, use the ** and brush to the cartridge and mold on the EMC curing material removed.
(3) Although the packaging process and EMC performance parameters match well, but due to improper storage or expired, resulting in decreased mobility of EMC, viscosity is too large or too long gel time, will cause poor filling. The solution is to select the appropriate viscosity and gel time of the EMC, and in accordance with EMC's storage and use requirements for safekeeping.
(4) due to the lack of EMC caused by the lack of filling, this situation generally occurs in the replacement of EMC, packaging type or replacement of the mold, the solution is relatively simple, as long as the choice and package type and mold to match the amount of EMC, Can be resolved, but the amount should not be too much or too little.
2, encapsulation forming porosity and its countermeasures
In the process of packaging forming, the pores are the most common defects. According to the pores in the plastic body on the site can be divided into internal pores and external pores, and the external pores can be divided into the top pores and gate pores. Stomata not only seriously affect the appearance of plastic body, but also directly affect the reliability of plastic devices, especially the internal pores should pay more attention. Common stomata are mainly external pores, the internal stomata can not be seen directly, must be observed through the X-ray instrument, and the smaller internal pores Bp so that through the x-ray can not see clearly, which also to overcome the large pores difficult. So, to solve the problem of porosity defects, we must carefully study the formation of various types of stomatal processes. But strictly speaking, the stomata can not be completely eliminated, can only take measures to improve, the stomatal defects in the range of good control.
From the surface of the pores, the reasons for the formation seems very simple, but the cavity is not effectively discharged from the residual gas formed. In fact, the causes of stomatal defects are many factors, mainly in the following areas: A, packaging materials, including EMC's gel time, viscosity, mobility, volatile content, moisture content, air content, cake density , The diameter of the cake and the material does not match the diameter; B, mold, and the shape of the barrel, cavity shape and arrangement, gate and exhaust port shape and location; C, packaging process, Mainly with the preheating temperature, mold temperature, injection speed, injection pressure, injection time and so on.
In the process of encapsulation forming, the main causes and countermeasures of the top stomata, the gates and the internal stomata are:
(1), the formation of the top stomata is mainly two cases, one is due to various factors to make EMC viscosity sharp - rise, resulting in injection pressure can not be effectively transmitted to the top, so that the top of the residual gas can not be discharged and cause pore defects; One is the flow of EMC is too slow, so that the cavity is not fully filled began to occur curing cross-linking reaction, which will form a hole defect. The most effective way to solve this defect is to increase the injection speed, the appropriate adjustment of the preheating temperature will be some improvement.
(2), the main reason for the production of stomata is the flow of EMC in the mold too fast, when the cavity is full, there are some residual gas failed to discharge in time, and this time the exhaust has been blocked by the overflow, The final residual gas under the action of injection pressure, tend to be compressed and stay near the gate. An effective way to solve this stomatal defect is to slow down the injection speed and reduce the preheating temperature appropriately so that the flow rate of the EMC in the mold is slowed down. At the same time, the mold temperature can be increased in order to promote the escape of volatile substances.
(3), the formation of internal stomata is mainly due to the mold surface temperature is too high, so that the cavity surface of the EMC too fast or premature curing reaction, coupled with the rapid injection rate makes the exhaust port full, so that The internal part of the gas can not overcome the surface of the curing layer and remain in the internal formation of pores. This stomatal defect generally occurs in a large volume circuit package, and more in the gate and the middle position. To effectively reduce the incidence of this stomatal, first of all to properly reduce the mold temperature, followed by appropriate to consider the appropriate increase in injection pressure, but excessive pressure will lead to red wire, flash and other defects, the more appropriate pressure range is 8 ~ 10Mpa The
3, packaging forming pock and its countermeasures
In the package after forming, the surface of the package will sometimes appear a large number of fine holes, and the location is more concentrated, see Bu to be a pitting. These defects are often accompanied by other defects at the same time, such as not filling, cracking and so on. The main reason for this defect is that the cake is heated unevenly in the process of preheating, and the temperature difference between the parts is large, and the curing reaction is caused when the mold cavity is injected, so that the defect is formed. Causing the material is not uneven heating factors are more, but mainly in the following three cases:
(1), material cake broken angle. For the general broken angle of the cake, the defect length is less than 1/3 of the height of the cake, and in the preheater roller on the smooth rotation can be used, and in order to prevent the preheating dumping, you can break the cake Caught in the middle. When putting into the cartridge, it is advisable to place the damaged cake on the bottom or top, which can improve the temperature difference between the cakes. For serious damage to the cake, can only give up without.
(2), the material is placed improperly when placed. In the end of the preheating out of the cake, often found that both ends of the cake is relatively soft, while the middle of the relatively hard, large temperature difference. The general preheating temperature is set at 84-88 ℃, the temperature difference in the 8 ~ 10 ℃ or so, so the most likely to occur when the package forming pitting defects. To solve the large temperature caused by the pitting defects, you can warm up the gap between the material to leave a certain space to put, so that the cake can be fully uniform heat. Experience shows that in the feeding when the first vote in the middle of the cake after casting two ends of the cake, will also improve the temperature difference caused by the larger.
(3), preheating machine heating plate height unreasonable, can also cause uneven heating, which led to the emergence of pitting. This situation occurs in the same preheating machine on the use of different sizes of the cake, and did not adjust the height of the heating plate, making the heating plate and the cake away from the moment, so that the material is uneven heating. Experience has shown that the reasonable distance between them is 3-5mm, too close or too far are inappropriate.
4, packaging and forming the wire and its countermeasures
In the package forming, EMC appears molten state, due to a certain degree of melt viscosity and flow rate, so naturally have a certain momentum, this momentum on the gold wire, it is easy to make gold wire deviation, serious will cause gold Wire thrust. This phenomenon is very common in the process of plastic, and can not be completely eliminated, but if you choose the appropriate viscosity and flow rate can still be controlled within the range of good products. The effect of melt viscosity and flow velocity on the impact of gold can be explained by establishing a mathematical model. It is assumed that the molten EMC is the ideal fluid, then the force 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. From the formula we can see that the larger the η, the greater the F, the bigger the F, the bigger the F, and the larger the F.
To improve the incidence of wire defects, the key is how to choose and control the melt viscosity and flow rate of EMC. In general, EMC's melt viscosity is from high to low to a high change process, and there is a low viscosity period, so choose a reasonable injection time, so that the mold cavity in the low-viscosity flow of EMC to reduce momentum. The choice of a suitable flow rate is also an effective way to reduce the impact of the impact of many factors that can affect the flow rate, mold temperature, mold flow, gate and other factors to consider. In addition, the long gold of the packaging products than the short gold wire packaging products more prone to red wire phenomenon, so the chip size and size of the island to match, to avoid the phenomenon of small island chip to reduce the degree of red silk. )
5, encapsulation forming cracking and its countermeasures
In the process of packaging and forming, sticky mold, EMC moisture absorption, the expansion coefficient of each material does not match, etc. will cause cracking defects.
For the sticky mold caused by the cracking phenomenon, mainly due to the curing time is too short, EMC stripping performance is poor or mold surface contamination and other factors. In the forming process, you can take to extend the curing time to make it fully cured; in terms of materials, can improve the performance of EMC stripping; in operation, the mold surface can be cleaned before each mold, the mold surface can also be coated with appropriate Of the release agent. For the phenomenon of cracking caused by EMC moisture absorption, in the process, to ensure that the custody and recovery of normal temperature in the process, to avoid the occurrence of moisture; in the material, you can choose a high Tg, low expansion, low water absorption, high cohesive force of the EMC The For the material expansion coefficient does not match the cracking caused by the phenomenon, you can choose with the chip, frame and other materials to match the expansion coefficient
6, packaging forming material and its countermeasures
In the process of packaging forming, the flash is a common form of defects, and this defect itself has no effect on the performance of packaged products, will only affect the subsequent solderability and appearance. The reason for the occurrence of the flash can be considered from two aspects. First, the material, the resin viscosity is too low, the particle size distribution of the material is unreasonable and so will cause the occurrence of flash, in the viscosity of the allowable range, you can choose a larger viscosity resin And adjust the particle size distribution of the filler to improve the filling volume, so that you can improve the anti-flash performance from EMC itself; Second, the packaging process, the injection pressure is too large, clamping pressure is too low, the same can cause the production of flash , This can be improved by appropriately lowering the injection pressure and increasing the clamping pressure. As the plastic mold after long-term use of the surface wear or the base is not smooth, resulting in a large gap after clamping, will also cause flash material, and production to see the phenomenon of serious flash material is often caused by this reason, Reduce wear, adjust the leveling of the base, to solve this flash material defects.
7, package forming sticky mold and its countermeasures
The reason and countermeasure of the formation of viscose die is: A, the curing time is too short, the adhesive mold caused by the incomplete curing of EMC can prolong the curing time and increase the clamping time to make it fully cured. B, EMC itself stripping performance Poor viscosity caused by the mold can only be improved from the material to the EMC's stripping performance, or packaging process, the appropriate external release agent; C, mold surface contamination can also cause sticky mold, you can clean the mold To solve; D, mold temperature is too low will also cause viscous phenomenon, can be appropriate to improve the mold temperature to be improved. 8. Conclusion
In short, there are many types of defects in plastic forming, in different forms of packaging have different forms of expression, the probability and location are also very different, the reasons are more complex, and implicated each other, mutual influence, it should be in the Research on the basis of comprehensive consideration, to develop a corresponding effective solution and countermeasures.