C4 or C2 Bumps in PCB Microelectronics?

C4 and C2 bumps for flipchip assemblies are among the top techniques that require close attention during PCB microelectronics assembly. Those two are flipchip (FC) bump connection techniques and are crucial for efficient and cost-effective microelectronics assembly.

In short, C4 and C2 are interposers that connect a small die in a FCBGA, using flipchip on one side and BGA on the other side. This connection technique therefore significantly reduces a board or substrate real estate by making the same amount of connections in a traditional BGA using these FCBGA connections.

In the C4 technique, Fig. 1, solder bumps are deposited on chips pads situated on the top side of a silicon wafer during the final wafer processing step. A chip or die is then mounted to external circuitry though a substrate, which can be another organic material circuit board. It is then flipped over to match bottom pads and then reflowed to complete the process.

Fig. 1 (Courtesy of Ansys/DfR Solutions)

For more details, check out our latest SMT007 column. In the meantime, here are some tips that’ll give you a basic understanding.

• C4 is different than wire bonding where a chip is sitting face up. In this case, die pads are connected to the pads of an interconnect circuit, like a substrate, BGA, or glass or ceramic, thereby connecting to outside circuitry.
• C4 is an abbreviation for controlled collapse of chip connection. It has long been associated with the ball-grid array (BGA) packaging process. The “collapse” part of C4 is when the BGA balls undergo IR reflow and are reflowed.
• It’s important to emphasize that C4 is well-proven. It’s a commodity known technology for decades, used in BGA assemblies.
• C2 differs from C4 in that C4 has a controlled collapse of balls or chip connections. However, C2 doesn’t have that. There’s no ball collapse like in C4 because Cu pillars cannot be melted like the solder on a BGA ball.
• C2 is typically used for finer pitch devices when the BGA pad has a pitch of 180 µm or less, which is about seven mils or less pitch pad of a BGA.
• C2 and C4 bumps can be used either on organic or inorganic substrates. Organic substrates are based on FR4, Rogers, or Polyimide Inorganic substrates are alumina (AI203), aluminum nitride (AIN), and gallium arsenide (GaAs).

Considering the various C4 and C2 aspects and differences, it’s always best for OEMs to first collaborate with their PCB microelectronics assembly houses to get a solid foundation of the pros and cons before launching into an advanced product.