PCB Tech - Analyze the definition of high-density PCB soft board

Generally, the high-density PCB soft board is defined by the process capability of fine lines and micro-holes. The line pitch (Pitch) is less than 150μm, and the micro-hole diameter is less than 150μm (the definition of IPC). The density PCB soft board is further reduced. The application of high-density PCB soft board can distinguish several fields:

1. IC carrier board: such as CSP, BGA, etc.

2. Information products: such as hard disk (Hard Disk), ink jet printer (Ink Jet Printer)

3. Consumer products: such as cameras (Camera), mobile phones (Mobile Phone)

4. Office automation products: such as Facsimile Machine (Facsimile Machine)

5. Medical products: such as hearing aid (Hearingaid), electric shock device (Defibrillator)

6. LCD module

It can be known that the demand for high-density PCB soft boards in the past two years has grown much higher than that of traditional FPC. This article will introduce several high-density PCB soft boards that are in greater demand.

Two, application

1. TBGA (Tape Ball Grid Array)

IC packaging has been developing towards light weight. Many companies design to use flexible boards as IC substrates. In addition to high density, excellent heat transfer and electrical properties are also the main reasons for considering using them.

TBGA uses a flexible board as the carrier of the IC, which has a fine line and thin effect. Currently, one layer of copper is more commonly used, and the use of two layers of metal is gradually increasing. The size of TBGA mass production ranges from 11mm*11mm to 42.5mm*42.5mm, and the number of pins ranges from 100 to 768. Figure 6 shows SONY's TBGA with a flexible board as the loading board. A larger number of TBGA packages should be 256 and 352 feet, 35mm*35mm packages. Products using TBGA are mainly microprocessors, chipsets, memory, DSP, ASIC and PC network systems.

2. Chip Scale Package

CSP chip-level package CSP’s package emphasizes the package volume of chip size. There are currently four main types: Rigid Substrate, Lead Frame, and Flex Interposer. And wafer type (Wafer Level), in which the soft substrate uses a high-density PCB soft board as the IC carrier substrate. The bigger difference from TBGA is the size after the assembly is completed, because TBGA adopts the fan-out method The assembly entity is much larger than the IC, and the CSP adopts the Fan-in method. The actual assembly entity does not exceed 1.2 times of the IC, so it is called chip-level assembly. The assembly IC used has Flash memory., SILM, ASIC and digital signal processor (DSP), etc., are used in digital cameras, camcorders, mobile phones, memory cards and other products. Wire Bonding is generally used to connect the IC and the flexible carrier board, but recently, the Flip Chip method is gradually used. As for the connection with the PCB, the method of solder ball array (BGA) is mainly used. The overall size of the CSP package depends on the size of the IC. The general size ranges from 6m*6mm to 17m*17mm, and the package spacing ranges from 0.5mm to 1.0mm. TI’s μStar BGA three-dimensional cross-sectional view is a typical soft-board CSP structure. One of the representatives. Tessera's μBGA is the originator of CSP, and several domestic companies are authorized to produce it.

3. LCD driver IC assembly

In the past, LCD driver ICs were mostly assembled by TAB (Tape Automatic Bonding) of high-density PCB soft boards. The TAB external pins (internal pins and IC Bonding) were connected to the LCD through anisotropic conductive adhesive (ACF). The ITO electrode of the panel is connected to each other, and the minimum pitch can reach 50μm. The TAB Tape used is 48mm and 7mm wide, and the typical number of 1/0 is 380. The main application products are mobile phones, camcorders, notebook computers, etc. However, when the LCD driver IC is assembled in TAB mode, only the driver IC is assembled, and other passive components must be carried by another PCB. This will cause the entire LCD assembly size to be unable to be effectively reduced, so someone started Use the COF (Chip on Flex) method which is different from the TAB method but belongs to the high-density PCB soft board assembly to make the LCD driver IC assembly. In addition to the drive IC, the COF method assembly can place some passive components of resistance and capacitance. It can also be placed on it by surface adhesion, which solves the problem of complex and over-sized structures caused by reusing PCBs. This is currently a popular LCD assembly method, which has a very potential market, but it is thin, thin, and And the issue of material adhesion will be the challenge for soft board manufacturing (line width 150μm), equipment research and development (transmission with thickness below 50μm) and material supply (no glue and copper adhesion).

4. Inkjet head of inkjet printer

The inkjet head drive assembly of the inkjet printer also uses a high-density soft board. At present, it uses a non-adhesive type soft board with a resolution of about 150μm. However, there is a trend of gradual downward development. 24mm is the most common. At present, this high-density PCB soft board is almost monopolized by 3M.

5. Hard Disk Drive (HDD) read head

Due to the rapid development of information, the Internet, and digital imaging, data storage equipment has shown rapid growth in storage capacity and access speed. Not only the hard disk drives of PCs and notebooks, but also the high-capacity memory devices of digital cameras and digital video recorders, etc., all need to use the so-called R/WFPC-the one used by the read-write head. Soft board. Not only the design of high-density structure, but also for situations where the operating temperature may be as high as 80°C, and high-speed dynamic vibration is required without breaking the wire, the stringent requirements for reliability can be seen in general.

3. Development trends and technical requirements

1.Fine Lines&Microvia

For example, COF's PITCH will be reduced to 25μm~50μm, which will challenge the substrate (copper adhesion, thickness), wiring process (photosensitive resolution, etching control, equipment transmission) and so on. The aperture is as small as 50μm or even smaller; there are also requirements for blind and buried holes, which will inevitably drive non-machined drilling processes such as lasers.

2.Microvias

When the pore size is as small as 50μm, traditional machine drills can no longer handle it. Laser hole burning must be used to directly etch the PI film, which is most commonly used in IC substrates such as CSP and TAB.

3.Flying Leads

Some high-density PCB soft boards with special structures can be etched away by dry or wet PI film, leaving so-called flying leads, which can be directly hot-pressed or welded to the hard board.

4.Small Coverlay Openings

Since the analysis of Coverlay Opening will reach 50μm or less, and the number of openings will increase greatly, traditional punching methods can no longer be achieved, so PIC (Photoimageble Coverlayer) was developed to meet future needs.

5.Surface Finish

Lead-free processes such as nickel gold, electroplated soft and hard nickel gold, and pure tin used in hard boards will be the mainstream that can be expected.

6.Microbump Array

CSP, Flip Chip and other reliability and intensive requirements, MBA is bound to be a major challenge for soft board production.

6.7Dimensional Control

The result of miniaturization is that the precision also needs to be greatly improved. Material selection, layout design, equipment considerations, process control and compensation value design are all great challenges to be faced.

8.Inspection AOI and non-contact electrical test matching inspection

This is a reference direction to solve the pre-shipment inspection of high-density PCB soft boards in the future.

Concluding remarks

In order to maintain high growth rates and profits, flexible substrates will inevitably develop towards high-scale applications. At this time, improvements and breakthroughs in materials must be made at the same time. Among them, non-adhesive base materials and photosensitive protective film will act as traditional flexible substrates. Enter the important role of high-density PCB soft boards.