Leadframe: Precision Metal Stamping Mold Design and Processing Technology

Lead frame is a key metal component of semiconductor and information products. With the booming semiconductor and information industry, its market demand is huge and is growing rapidly. Lead frame stamping die represents the highest level of precision dies. Not only should it have advanced die design technology and high precision processing equipment (optical projection grinder and wire cutting discharge machine are indispensable tools). This article explains the design and processing technology of lead frame stamping die.

precision medical stamping parts

Leadframe related process description

A. Gold wire bonding (Wire bonding)
The gold wire is extremely small, its diameter is about 30 μm, and no one in China manufactures moulds for drawing gold wires.
B. Wire bonding material
Leadframe materials are mainly iron-nickel alloy (due to the nickel content of 42%, also known as 42 alloy) and copper alloy (oxygen-free copper, deoxidized copper). The former accounts for about 20% of the use rate, and the latter is about 80%.

Leadframe stamping products processing keypoints

1)The front end of the inner lead (Inner lead) of the leadframe requires high flatness, and the flat area is at least 0.1 mm (more than three times the diameter of the gold wire), so the embossing process must be used (coining). 

2) Each inner lead foot interval (Lead space) requires to maintain correct and uniform, the embossing process will reduce this interval, so it is necessary to control the embossing depth and suppress the lateral reverberation (Twist) of the lead foot. 

3) The accuracy of the position of the inner lead foot must be kept correct to facilitate the wire bonding of the last project. The corresponding policy is to punch and cut the inner lead foot first and then punch and cut the outer lead foot of the stamping process sequence to be designed properly, as well as the design of the stamping process adjustment stage (correction stage) to suppress the shift of the lead foot position of the wireframe during the stamping process. 

4) The flatness of the leadframe is required to be high to facilitate the stability and smoothness of the wire bonding when the project is conveyed later. The countermeasure is to suppress the amount of recoil when punching and cutting the guide foot to a minimum degree, and the direction of the recoil is consistent. The wireframe material should be applied to the stress relief project before punching and processing. 

5) The deformation of the guide foot within the leadframe should be twisted or offset to a minimum extent to facilitate the operation of subsequent works. The solution in the mould should pay attention to the strong pressure design of the pressure plate, set the most appropriate mould gap and the role of components (punch and mother die) of the edge to maintain the best state, mould guide device rigidity.

Key points of leadframe stamping mold design

1)Mold clearance 
  The clearance of the  leadframe stamping die is 3-5% of the plate thickness (3% for copper alloy, 4-5% for 42 alloys), and the clearance between the press plate and punch will be smaller, which is less than 50% of the die clearance.

2)Pressing plate 
  The pressing force of the press board is required to be high to suppress the distortion and deformation caused by the punching process and to improve the quality of the punching surface of the guide foot. It should concentrate pressing position near the punching load area (i.e. the punch guide area), and the pressing place of the press board is designed to be protruding to increase the local pressure so that the material is subjected to compressive stress and to prevent the occurrence of distortion or reverse curvature, as shown below.

3)Punching sequence 
  A proper punching sequence design is the most effective way to improve the punching deformation or distortion of the guide foot, which is difficult to achieve by subsequent annealing operations.
The following are the basic principles of punching sequence consideration.
A. Punch the inner guide foot first, then punch the outer guide foot.
B. Punching the short guide foot first and then the long guide foot or punching the long guide foot first and then punching the short guide foot can be done.

4)Form of mother die 
  The shape of the mother die designed as a straight section followed by pushing or full pushing; following the processing method, the shape of the mother die is designed as a straight section followed by pushing or full pushing. In the former case, the length of the straight section is 3 mm, the pushing angle is 1/2°, and the processing method is grinding. In contrast, the pushing angle is taken in the latter case, and the processing method is wire-cutting and electrical discharge processing. 

5)Adjustment station design 
  To increase the strength of the die or make the die have sufficient fixed space, the empty station is an important part of the design of a continuous punching die. In addition, to suppress the twist or deformation of the leadframe in the punching process, the design of the adjusting station is the main point that must consider.

6)Die rigidity and guide method 
A. The mould guide method uses a double guide, that is, the main guide post (Main guide post) and auxiliary guide post (Sub guide post), and use the way.

B. The number of external guide posts adopts double design, six external guide post designs when the mould size is less than 600 mm, and eight external guide post designs when the mould size is 800 mm.

C. Use a high rigidity roller guide to improve the accuracy and rigidity of the guide.

D. The inner guide adopts the full guide type (also known as the three-plate full guide method), i.e. the inner guide passes through the punch plate, press plate and mother plate.

Leadframe stamping mold demand trends

1)Die towards miniaturization 
  Due to the development of wire cutting and electrical discharge machining (WEDM) technology to improve its processing accuracy and surface quality, the use of the WEDM method for the mother module or pressure plate in the block of processing gradually has the possibility of replacing the grinding process, to reduce the number of stamping stations (can reduce empty station design ) and make the size of the mould significantly reduced. To match the specifications of the high-speed punching machine, the punching and cutting die size of the multi-foot wire frame (100 pins or more) reaches more than 1200 mm in length, so its punching production method must adopt the serial arrangement type of punching machine.

2)Die components towards miniaturization and high precision 
  Take the multi-pin number wire frame with the guide foot punch as an example, its shape and size trend towards the shape size reduction, edge length reduction, punch thickness thinner development, and its accuracy trend towards high precision, low processing surface roughness progress. In order to achieve such high precision (dimensional tolerance ±2 μm or less) and low surface roughness (0.3 μm Ra or less), it is necessary to use high-precision grinding equipment and low surface roughness wire-cutting and electrical discharge machining machines.

The key technology of leadframe stamping die processing

The key technology of wireframe stamping die processing

1)High precision/surface roughness grinding 

A.Optical projection grinding.

B.High speed reciprocating grinding.

C. Tooling grinding.

D.Lapping of the mirror surface.

2)Wire-cutting and electrical discharge machining (WEDM) 

A.Oil type WEDM.

B. Water type wire cutting EDM.

C.Low degradation layer wire cutting electric discharge processing.

3)Mold material and treatment technology 

A.Mold heat treatment technology.

B.PVD, CVD, TD, etc.

C.Diamond coating for super hard mould material.

D.Super hard die material with super power particles.

With the leadframe products increasingly miniaturised,  leadframe mould has to cooperate towards small size and high precision. At this time, the material strip layout to prevent warpage, gap control to reduce burr, appropriate heat treatment to extend, to ensure mould life and precision machining to meet the requirements are indispensable.