What is flip chip

Flip chip is a pin less structure that typically contains circuit units. Designed for electrical and mechanical connection to circuits through an appropriate number of tin balls (covered by conductive adhesive) located on their surfaces.

Flip Chip

Flip chip, also known as flip chip, is a technology that deposits tin lead balls on I/O pads, then flips and heats the chip by combining the melted tin lead balls with a ceramic substrate. This technology replaces conventional wire bonding and gradually becomes the mainstream of future packaging. Currently, it is mainly used in high pulse CPU (central processing unit), GPU (graphic processor unit), Chipset (chipset) and other products. Compared with COB (On Board Chip Packaging), the chip structure and I/O (solder ball) direction of this packaging form are facing downwards. Due to the distribution of I/O leads on the entire chip surface, Flip chip has reached its peak in packaging density and processing speed. Especially, it can be processed using methods similar to SMT (Surface Mount) technology, making it the ultimate direction of chip packaging technology and high-density installation. There are three main types of flip chip connections: C4 (collapsed chip control connection), DCA (direct chip connection), and FCAA (flip chip adhesive).

C4 is a type of solder ball array connected to silicon wafers in a form similar to ultra-fine spacing BGA, with a typical spacing of 0.23 and 0.254mm. The welding ball diameter is 0.102 and 0.127mm. The solder ball component is 97Pb/3Sn. These solder balls can be completely or partially distributed on the silicon wafer. Due to the ability of ceramics to withstand high reflux temperatures, they are used as substrates for C4 connections. Usually, Au or Sn plated connection plates are pre distributed on the surface of ceramics, followed by C4 form flip chip connections.

The advantages of C4 connection are: 1) excellent electrical and thermal properties; 2) In the case of moderate solder ball spacing, the I/O number can be very high; 3) Not limited by pad size; 4) Can be suitable for mass production; 5) Can greatly reduce size and weight.

DCA is similar to C4 in that it is a type of ultra-fine spacing connection. The silicon wafer structure in DCA and C4 connection is the same, and the only difference between the two lies in the choice of substrate. The substrate used in DCA is a typical printing material. The composition of DCA solder balls is 97Pb/Sn, and the solder used to connect the solder pads is eutectic solder (37Pb/63Sn). For DCA, it is quite difficult for eutectic solder with a spacing of only 0.203 and 0.254mm to leak onto the connecting pads. Therefore, instead of using solder paste to leak, the top of the connecting pads is coated with lead tin solder before assembly. The volume of solder on the pads is very strict and usually requires more solder than other ultra-fine spacing components. The 0.051 and 0.102mm thick solder on the connecting pad is generally slightly domed due to its pre plating, and must be leveled before SMT, otherwise it will affect the reliable alignment of the solder ball and pad.

FCAA connections come in various forms and are currently in the early stages of development. The connection between silicon wafers and substrates is not made with solder, but with adhesive instead. The bottom of the silicon wafer in this connection can have solder balls or adopt structures such as solder bumps. The adhesives used in FCAA include various types such as isotropy and anisotropy, which mainly depend on the connection conditions in practical applications. In addition, the selection of substrates usually includes ceramics, printed circuit board materials, and flexible circuit boards. Inverted chip technology is one of the most advanced microelectronic packaging technologies today. It has raised the density of circuit assembly to a new height, and with the further reduction of electronic product volume in the 21st century, the application of flip chip will become increasingly widespread.

The characteristics of Flip Chip

Flip Chip packaging technology has many obvious advantages compared to traditional wire bonding processes, including superior electrical and thermal performance, high number of I/O pins, and reduced packaging size.

The thermal performance of Flip Chip packaging technology is significantly superior to conventional wire bonding processes. Nowadays, many electronic devices; Packaging such as ASICs (Application Specific Integrated Circuits), microprocessors, and SOC (System Level Chips) can consume 10-25W or even more power. The dissipation power of BGA devices with enhanced heat dissipation type lead bonding is only 5-10W. According to working conditions, heat dissipation requirements (maximum junction temperature), ambient temperature and air flow rate, packaging parameters (such as using external heat sinks, packaging and size, number of substrate layers, number of ball pins), etc., compared to Flip Chip packaging, it can usually generate 25W of dissipated power.

The outstanding thermal performance of Flip Chip packaging is determined by the low thermal resistance heat dissipation plate and structure. The heat generated by the chip is dissipated through the heat dissipation pins, internal and external heat sinks. The close contact between the heat sink and the chip surface results in a low junction temperature（ θ Jc). To reduce the thermal resistance between the heat sink and the chip, high thermal conductivity colloids are used between the two. Make it easier to dissipate heat inside the package. To further improve the heat dissipation performance, the external heat sink can be directly installed on the heat sink to obtain a low junction temperature for packaging（ θ Jc).

Another important advantage of Flip Chip packaging is its electrical performance. The wire bonding process has become a bottleneck in high-frequency and certain applications, and the use of Flip Chip packaging technology has improved electrical performance. Nowadays, many electronic devices operate at high frequencies, so signal integrity is an important factor. In the past, 2-3GHz was the upper frequency limit for IC packaging, while Flip Chip packaging could reach up to 10-40 GHz depending on the substrate technology used.

Testing of Inverted Chips

The testing of Flip Chip requires a dedicated testing machine. The PT-198A flip chip testing machine developed by Tektronics is a domestically leading fully automatic flip chip testing machine.

Tec Optoelectronic Inverted Chip Testing Machine

The Tec Optoelectronic Inverted Chip Testing Machine has the following main characteristics:

• Prober&Tester integrated all-in-one machine design, with industry-leading testing speed and accuracy;
  
• The Japanese made high-precision screw guide rail has strong mechanical rigidity and long service life. At the same time, it is combined with a precision grating ruler to achieve full closed-loop control of motion, with ultra-high positioning accuracy and long-term operational stability;
  
• The whole machine adopts an instrument marble platform and a sunken design, which has excellent running stability;
  
• Optimizing and reducing the size of the organization to improve the utilization rate of the factory building;
  
• The self-developed real-time weighing active electrode edge finder can effectively control the contact force between the probe and the chip electrode during each test, ensuring the consistency of needle marks;
  
• Autonomous inverted light receiving structure with higher optical sensitivity and accuracy, suitable for testing up to 6 "[* Note 1] Mini LED chips, conventional forward and reverse chips, and CSP chips;
  
• Optional integration balls of 1 ", 1.5", and 2 ", equipped with 7 levels of filters to meet the testing requirements of chips with different brightness levels, capable of measuring micro current illumination;
  
• Multi needle design can achieve the requirement of simultaneous measurement of multiple particles, improving the testing capacity of the machine;
  
• Equipped with automatic needle cleaning function;
  
• The machine is equipped with a zoom needle mark camera and automatic needle mark confirmation function during production, ensuring stable production.