February 18, 2013, anysilicon
The name leadframe (or lead-frame) is actually very accurate.
Leadframe is an alloy frame that consists of the package leads and the paddle. The silicon die is attached on the paddle and the leads are connected to the die with wirebonds. That’s it.
In the following photo you can see the paddle (center) which is typically used for ground signal and many leads around the package stretching from the package edge to a point where the wirebonder can reach.
Many types of packages are based on leadframe, such as QFN, QFP, LQFP, TQFP, PLCC, SOIC and TSOP.
Like any other material, leadframe requires tooling. The tooling can either be open tools,which are available and can be used free of charge, or custom tools that are associate with an NRE cost.
Leadframes are produced in a long strip, which allows them to be quickly processed on assembly machines.
Packaging houses have many leadframes tooling on stock that can be available for your project. Therefore, it is wise to look around and ask several packaging houses whether they have open tools for your leadframe. This will save you the NRE required for creating the tooling.
If open tools are not available then you’ll have no choice but to pay for a custom leadframe.
But, there are also some technical issues that may require the use of a custom leadframe:
For most application the length of the bond wire is not critical and there is no need to have a custom leadframe.
But for some designs, particularly RF applications, the length of the wirebonds may be vital. With a custom leadframe you can determine the distance between the die and the leads to achieve the exact wirebond length required.
In cases where the chip is too small for the selected package, a custom leadframe is needed to ensure the length of the wirebonds does not exceed the maximum length allowed by the package design rules.