Before discussing the assembly and assembly process of flip chips for PCB boards, it is necessary to understand what devices are called flip chips? In general, such devices have the following characteristics:
- The base material is silicon;
- Electrical surface and welding convex on the lower surface of the device;
- The ball spacing is generally 4-14mil, the ball diameter is 2.5-8mil, and the overall size is 1-27mm;
- After assembling on the substrate, you need to fill the bottom.
In fact, the reason why the flip chip is called “flip” is relative to the traditional metal Wire Bonding connection method (Wire Bonding) and the process after planting the ball. The electrical side of the traditional chip connected to the substrate through metal wire bonding is facing up, and the electrical side of the flip chip is facing down, which is equivalent to turning the former over, so it is called “flip chip”. After the chip is planted on the Wafer, it needs to be turned over and fed into the placement machine for easy mounting, and because of this flipping process, it is called “flip chip”.
First, PCB board with flip chip assembly process
In the semiconductor back-end assembly plant, there are now two methods of module assembly. In the two reflow processes, SMT components are first assembled on a separate SMT line consisting of a screen press, a SMT mounter and the first reflow. The partially assembled modules are then processed through a second production line, which consists of PCB flip chip mounters and reflow furnaces. The bottom fill process is completed in a dedicated bottom fill line, or combined with a PCB flip chip line.
Second, PCB board with flip chip assembly process introduction
Compared with other IC devices, such as BGA, CSP, PCB flip chip assembly process has its special, the process introduces the flux process and the bottom filling process. Because of the flux residue (impact on reliability) and the danger of bridging, the PCB board is not an acceptable assembly method to mount on the solder paste with flip chips. In the industry, no cleaning flux was introduced, and the chip dipping flux process became a widely used welding aid technology. At present, the main alternative method is to use no-wash flux, dip the device in the flux film, dip the device ball into a certain amount of flux, and then mount the device on the substrate, and then reflow welding; Alternatively, the flux is pre-applied to the substrate, and then the device is mounted and reflow welded. The flux plays the role of fixing the device before reflow, and wetting the welding surface enhances the weldability during reflow.
After the PCB board is welded with a flip chip, a glue (generally epoxy resin material) needs to be filled between the bottom of the device and the substrate. Bottom fill is divided into liquid and non-liquid (No-follow) bottom fill based on the “capillary flow principle”.
Flip chip assembly process flow (non-fluid bottom fill)
The PCB flip-chip assembly process is for C4 devices (the device welding salient material is SnPb, SnAg, SnCu or SnAgCu). Another process is to use anisotropic conductive adhesive (ACF) to assemble PCB flip chips. The conductive adhesive is applied to the substrate in advance, and the device is mounted on the substrate with a higher pressure at the sticker head. At the same time, the device is heated to make the conductive adhesive solidify. This process requires a very high accuracy of the mounter, and a high pressure and heating function of the mounter head. For the assembly of non-C4 devices (whose convex material is Au or other), the trend is to use this process. Here, we mainly discuss the C4 process, and the following table lists several ways of Bumping PCB boards with flip chips and connecting on the substrate.
A brief introduction to the assembly and assembly process of PCB flip chip
The flip chip geometry of PCB board can be described by a “small” word: small ball diameter (as small as 0.05mm), small ball spacing (as small as 0.1mm), and small overall size (1mm2). In order to obtain satisfactory assembly yield, the mounting equipment and its process has brought challenges, with the reduction of the diameter of the welding ball, the mounting accuracy requirements are getting higher and higher, and the accuracy of 12μm or even 10μm is becoming more and more common. The graphics processing ability of the camera is also very critical, and the small ball diameter and small ball spacing require a higher pixel camera to process.
Over time, the size of high-performance chips continues to increase, the number of Solder bumps increases, the substrate becomes thinner and thinner, and the bottom filling becomes necessary to improve product reliability.
If you have interest, please feel free to contact me, Cynthia at [email protected].