Abstract: This paper mainly analyzes and researches the problems (defects), unfilled, stoma, pitting, punching, cracking, flashing, sticking mold and so on that often appear in the process of EMC encapsulation forming and puts forward the effective solution 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. Plastic packaging products are widely used, but also for plastic packaging has brought unprecedented development, but almost all of the molding defects in plastic products is always prevalent, whether it is the use of advanced transfer molding package, or the use of the traditional single injection mold Packaging, are not completely avoided. In comparison, the traditional plastic mold forming a greater probability of defects, 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 the gel time, viscosity, fluidity, mold release, adhesion, moisture resistance, heat resistance, , Stress, strength, modulus, etc .; B, the mold, including the runner, gate, cavity, exhaust port design and lead frame design match degree; C, package, different packages often appear different So the optimization of the package design will greatly reduce the occurrence of bad defects; D, process parameters, including clamping pressure, injection pressure, injection speed, preheat temperature, mold temperature, curing time and so on.
The following mainly on the plastic forming the common defects in the causes of the analysis and research, and put forward the appropriate and feasible solutions and countermeasures.
1. Packaging is not filled and its countermeasures
There are mainly two kinds of phenomena that the encapsulation is unfilled: one is the tendency of unfilled, mainly due to the incompatibility between the encapsulation process and EMC performance parameters; one is the random unfilled, mainly due to mold cleaning Improper, EMC insoluble impurities is too large, the mold inlet is too small and other reasons, causing the mold gate caused by congestion. From the packaging point of view, DIP and QFP more prone to unfilled phenomenon, and from the appearance point of view, DIP is mainly filled with unfilled and partially unfilled, QFP exists mainly corner unfilled. The main reason of unfilled and countermeasures:
(1) Trend unfilled due to over mold temperature, or mismatch between the package process and EMC performance parameters. Preheated EMC at high temperatures faster reaction, resulting in the EMC of the gelation time is shorter, poor fluidity, the cavity is not yet fully filled, the EMC viscosity will rise sharply, the flow resistance also becomes larger, So that a good filling can not be obtained, so that there is a tendency of unfilled. This phenomenon is more likely to occur in VLSI packages because these large-scale circuits tend to consume large amounts of EMC per mode and are prone to produce temperatures that are set relatively high to achieve uniform heating in a short period of time This unfilled phenomenon. ) For this trendy underfill due primarily to inadequate EMC fluidity, the preheat temperature of the EMC can be increased to allow it to be uniformly heated; the injection pressure and speed are increased, the flow rate of the EMC is increased; the die is lowered Temperature, in order to slow down the reaction rate, the relative extension of EMC gel time, so as to achieve the full effect of filling.
(2) EMC can not be effectively injected due to blockage of the mold gate, and the exhaust hole is clogged due to improper mold cleaning, unfilled as well, and this unfilled position in the mold is also irregular. Especially in the small package, due to the gate, the exhaust port is relatively small, so the most likely to cause clogging and unfilled phenomenon. For this underfill, remove the blockage with a tool and apply a small amount of mold release agent, and clean the cylinder and mold EMC hardener with a brush and a brush after each mold package.
(3) Although the packaging process and EMC performance parameters match well, but due to improper storage or expired, resulting in reduced fluidity of EMC, viscosity is too large or gelling time is too short, will cause poor filling. The solution is to choose EMC with proper viscosity and gelling time and store it according to EMC's storage and use requirements.
(4) Unfilled due to insufficient amount of EMC, the solution usually appears when EMC, package type or mold change is replaced. If the EMC amount that matches the package type and mold is selected, Can be solved, but the amount should not be too much or too little.
2, packaging forming pores and its countermeasures
Stomata are the most common defects during the encapsulation process. According to stomatal parts in the plastic body can be divided into internal stoma and external stomata, and external stomata can be divided into the top stomata and gate stomata. Stomata not only seriously affect the appearance of plastic seals, but also directly affect the reliability of plastic seals, especially the internal stomata should pay more attention. Common stomata are mainly external stomata, internal stomata can not be directly seen, must be observed by X-ray machine, and smaller internal stoma Bp can not see through x-ray too, which also brings great impact to overcome the stoma defect difficult. Well, to solve the problem of stomatal defects, we must carefully study the formation of various types of stomatal processes. But strictly speaking, the stoma can not be completely eliminated, only to take measures to improve the many aspects, the stomatal defects in the range of good products.
From the stomatal surface, the formation of the reason seems to be very simple, but the residual gas in the cavity is not formed effectively discharged. In fact, the causes of stomatal defects are many, mainly in the following aspects: A, packaging materials, including EMC gel time, viscosity, fluidity, volatile content, moisture content, air content, cake density , Cake diameter and material diameter does not match the diameter, etc .; B, the mold, with the shape of the barrel, the shape and arrangement of the cavity, the shape and location of the gate and exhaust port and so on; C, packaging technology, Mainly with the preheating temperature, mold temperature, injection speed, injection pressure, injection time and so on.
In the package forming process, the top of the stomatal, sprue and internal porosity of the main reasons and countermeasures:
(1) There are mainly two kinds of formation of the top stomata, one is that the EMC viscosity is sharply increased due to various factors, the injection pressure can not be effectively transmitted to the top, so that the top residual gas can not be discharged and cause stomatal defects; One is that the flow of EMC is too slow, so that the cavity begins to undergo a cure cross-linking reaction without fully filling, which can also cause stomatal defects. The most effective way to solve this defect is to increase the speed of injection molding, and the proper adjustment of the preheating temperature will also be some improvement.
(2) The main reason of the sprue of the gate is that the flow speed of the EMC in the mold is too fast. When the cavity is full, some of the residual gas can not be discharged in time, and the exhaust port has been blocked by overflow material. Finally, the residual gas injection pressure in the role, often will be compressed and stay near the gate. The effective way to solve this kind of stomatal defects is to slow down the injection speed and lower the preheating temperature appropriately to slow the flow of EMC in the mold. Meanwhile, in order to promote the escape of volatile substances, the mold temperature can be properly raised.
(3) the formation of internal pores is mainly due to the mold surface temperature is too high, so that the cavity surface of the EMC too fast or too early curing reaction, coupled with faster injection speed makes the exhaust port is full, so that Part of the internal gas can not overcome the surface of the solidified layer leaving the formation of pores inside. This stomatal defects usually occur in large volume circuit packaging, and appear more in the gate end and the middle position. To effectively reduce the incidence of such porosity, we must first properly reduce the mold temperature, and secondly, consider the appropriate injection pressure to increase, but too much pressure will cause fouling, flashover and other defects, the more appropriate pressure range is 8 ~ 10Mpa .
3, Packaging pitting and its countermeasures
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. These defects often accompanied by other defects at the same time, such as unfilled, cracking and so on. The main reason for this defect is the material cake in the preheating process of uneven heating, the larger the temperature difference between the parts, causing the curing reaction after injection into the cavity inconsistent, that the formation of pitting defects. Cause uneven heating cake material factors are more, but mainly in the following three cases:
(1), Pie bread damage notch. For ordinary nicked cake, the length of the defect is less than 1/3 of the height of the cake, and it can be used smoothly on the preheater roller, and in order to prevent dumping when preheating, the damaged cake Sandwiched in the middle. When put into the barrel, it is best to place the damaged cake on the bottom or top, which can improve the temperature difference between the cake. For severely damaged cake, can only give up without.
(2), preheated cake placed properly. At the end of the preheat cake removed, often will find the 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 is about 8 ~ 10 ℃, so the most vulnerable to pitting defects during encapsulation. To solve the large temperature difference caused by pitting defects, you can preheat the material between the cake left a certain gap to place, so that all the cake can be fully and evenly heated. Experience has shown that the first vote in the vote when the first vote after the cake material at both ends of the vote will also improve the larger temperature difference caused by the defects.
(3), preheater heating plate height is unreasonable, will also cause uneven heating, resulting in pitting. This situation occurred in the same preheater using different sizes of cake, without adjusting the height of the heating plate, making the heating plate and the cake suddenly far and near, so that the cake heating uneven. Experience has shown that the more reasonable distance between them is 3-5mm, too close or too far are not suitable.
4, packaging forming wire and its countermeasures
In the package forming, EMC showed a molten state, due to a certain degree of melt viscosity and flow rate, so naturally have a certain momentum, the impact on the gold wire, it is easy to shift the gold wire, a serious gold Broken wire. This phenomenon is often in the process of plastic molding 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 impact of EMC melt viscosity and flow velocity on the gold wire can be explained by establishing a mathematical model. It can be assumed that the molten EMC is the ideal fluid, then the momentum F = KηυSinQ, K is 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: 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.
The key is to choose and control the melt viscosity and flow rate of EMC in order to improve the incidence of the cut-off defect. In general, the melt viscosity of EMC is a process from high to low to high, and there is a low viscosity period. Therefore, choose a reasonable injection time to reduce EMC in the mold cavity during the low viscosity period momentum. Choose a suitable flow rate is also an effective way to reduce the momentum, the impact of the flow rate of many factors, from the injection speed, mold temperature, mold runner, gate and other factors to consider. In addition, the long gold wire package products are more likely to happen than the short-blown package of silk products, so the chip size and size of the island to match, to avoid the phenomenon of the island chip, in order to reduce the degree of shredding. )
5, package forming cracking and countermeasures
In the package forming process, sticky mold, EMC moisture absorption, the expansion coefficient of the material mismatch, etc. will cause cracking defects.
Crack caused by the sticky mold phenomenon, mainly due to curing time is too short, poor EMC mold release or mold surface contamination caused by other factors. In the forming process, you can take to extend the curing time to make it fully cured; in terms of materials, EMC can be improved mold release performance; in operation, the mold surface can be cleaned before each mold, the mold can also be coated with the right amount Release agent. For EMC moisture cracking caused by the phenomenon, in the process, to ensure that the storage and return to normal temperature to avoid the occurrence of moisture absorption; in the material, you can choose with high Tg, low expansion, low water absorption, high adhesion EMC . For the materials caused by the expansion coefficient does not match the cracking phenomenon, you can choose with the chip, frame and other materials to match the expansion coefficient
6, package forming material and its countermeasures
Flash over is another common form of defect during the encapsulation process, and the defect itself has no effect on the performance of the packaged product and only affects subsequent solderability and appearance. The causes of flash can be considered from two aspects. First, the material, the resin viscosity is too low, the filler particle size distribution is unreasonable and so will cause flash occurred, within the allowable viscosity range, you can choose a larger viscosity resin , And adjust the particle size distribution of the filler to increase the amount of filling, so that it can improve its anti-spill performance from EMC itself; Second, the packaging process, the injection pressure is too large, the clamping pressure is too low, the same can cause spillage , This defect can be ameliorated by appropriately reducing the injection pressure and increasing the clamping pressure. As the long-term use of plastic mold surface wear or pedestal is not flat, resulting in a larger gap after the mold, will result in flash material, and production to see the serious flash phenomenon are often caused by this reason, you can try Reduce wear, adjust the flatness of the base, to solve this flash defects.
7, package molding sticky mold and its countermeasures
Encapsulation forming sticky sticking causes and countermeasures: A, the curing time is too short, EMC is not completely cured caused by the sticky mold, you can extend the curing time and increase the mold clamping time to fully cure; B, EMC itself mold release performance Poor adhesion caused by the adhesive can only be improved from the material aspects of EMC mold release performance, or encapsulation process, the appropriate external mold release agent; C, mold surface contamination can also cause sticky mold, you can clean the mold To solve; D, the mold temperature is too low will cause sticky mold phenomenon, you can properly increase the mold temperature to be improved. 8. Conclusion
In short, many types of plastic molding defects in different forms of packaging have different manifestations, the probability and location are also very different, the causes are more complex, and implicate each other, so each should be in the separate Based on the research, considering and formulating the corresponding effective solutions and countermeasures.