Blanking

Technology suitable for both serial and small quantity production

Basic investigations on the effects of miniaturization on the blanking process have been first performed by Kals. It has been shown that the normalized forming force is constant while scaling down sheet thickness due to a lack of free surface. When sheet thickness is below a certain value, the forming force and the ultimate shear strength is increasing. Other investigations have shown that there is a dependency between tool and process parameters and the accuracy of the produced leadframes. The deflection of the leads in the plane of the sheet increases with decreasing width of the lead. The deflection is also influenced by different clearances between tool and die in a progressive tool. They also showed that increasing the strip holder pressure has a positive effect on the accuracy in most cases. Also the dynamic behaviour of the tool is affecting the accuracy, e.g. increasing blanking speed is resulting in a decreasing accuracy. Performed experiments have additionally shown that increasing the strip holder force is clearly improving the product quality. Other investigations on the deflection of a punch during the blanking process have shown an increase of deflection when the punch is eccentric relative to the die. A particular blanking process, so-called dam-bar cutting has been investigated by [21, 22]. This is a mechanical trimming process removing the dam-bar between the leads after the IC package is encapsulated. Due to the special shape of the specimen rectangular around the IC, investigations have to be performed considering the anisotropic behaviour of the material in the shearing line. Further investigations on different materials have shown that an increase of the angle α reduces the maximum cutting forces but is leading to an increasing burr height. An important aspect for industrial production is tool life. Therefore, investigations in [22] have been performed to show the dependency of the punch forces and thus the tool stresses from the clearance between punch and die and the used sheet material hardness. While tools made of tungsten carbide (WC) have the longest life time, tools made of bare HSS show the highest wear. An improvement can be reached by using coatings like PVD-TiN or plasma-nitriding. In this case, tools made of steel and WC have been compared according to their tool life in an industrial production process of blades for shavers. While the first only reaches a quantity of 50.000 parts, the latter exceeds 1.15 billion parts. Thus, it is obvious that the usage of the more expensive WC as tool material (factor 1.8 compared to steel) is more economical than the usage of steel. In general it can be said that blanking even at microscale using thin foils is an industrially well established technology.

[21] Lee, W.B.; Cheung, C.F.; Chan, L.K.; Chiu, W.M.: An investigation of process parameters in the dambar cutting of integrated circuit packages. J. of Materials Processing Technology 66 (1997), 63-72

[22] Cheung, C.F.; Lee, W.B.; Chiu, W.M.: An investigation of tool wear in the dam-bar cutting of integrated circuit packages. Wear 237 (2000), 274- 282

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