Coplanarity, simply put, is the measure of how well a set of points lies within a single plane. In the context of semiconductor packaging and assembly, it refers to the planarity of component leads, solder balls (in Ball Grid Arrays or BGAs), or other contact points intended to make electrical connections with a printed circuit board (PCB) or other substrates.
Why is coplanarity so important? The answer lies in the need for reliable and consistent electrical connections. Non-coplanar leads or solder balls can lead to several problems:
- Open Joints: If a lead or solder ball is significantly higher than its neighbors, it might not make proper contact with the PCB pad during the soldering process. This results in an open joint, meaning no electrical connection is established.
- Weak Solder Joints: Conversely, a lead or solder ball that is too low might make contact, but the resulting solder joint will be thin and weak. These joints are prone to failure due to mechanical stress, thermal cycling, or vibration.
- Short Circuits: In extreme cases, a severely non-coplanar lead could bend and touch an adjacent lead, creating a short circuit.
- Assembly Challenges: Components with poor coplanarity can be difficult to place accurately on the PCB during automated assembly, leading to misalignments and further connection issues.
- Reduced Reliability: Ultimately, all these issues contribute to reduced product reliability, increased failure rates, and potential field failures, which can be costly for manufacturers.
Factors Affecting Coplanarity:
Several factors can influence the coplanarity of semiconductor components:
- Manufacturing Processes: The lead forming, trimming, and singulation processes during package fabrication can introduce variations in lead height. Similarly, variations in solder ball placement and reflow during BGA manufacturing can affect coplanarity.
- Material Properties: The materials used in the package body and leads can expand or contract at different rates with temperature changes, leading to warpage and non-coplanarity.
- Handling and Storage: Improper handling and storage can also damage component leads, causing them to bend or deform, thereby affecting coplanarity.
- PCB Surface Quality: While coplanarity primarily refers to the component, the planarity of the PCB pads also plays a role. Uneven PCB surfaces can exacerbate the effects of component non-coplanarity.
Measuring and Controlling Coplanarity:
Coplanarity is typically measured using specialized equipment such as laser scanners, coordinate measuring machines (CMMs), or automated optical inspection (AOI) systems. These systems accurately measure the height of each lead or solder ball relative to a reference plane.
To control coplanarity, manufacturers employ various techniques:
- Optimized Manufacturing Processes: Precise control over lead forming, trimming, and solder ball placement processes is essential.
- Material Selection: Choosing materials with matched coefficients of thermal expansion (CTE) can minimize warpage.
- Package Design: Package designs that minimize stress and warpage are crucial.
- Handling Procedures: Implementing strict handling and storage procedures to prevent lead damage.
- Coplanarity Specifications: Setting and adhering to strict coplanarity specifications for components.
In conclusion, coplanarity is a vital parameter in semiconductor manufacturing that directly impacts the reliability and performance of electronic devices. By understanding the factors that affect coplanarity and implementing appropriate measurement and control techniques, manufacturers can ensure planar precision and deliver high-quality, reliable products.
