After more than 10 years of development, the SMT process of FPC has gradually unveiled the mystery. Its manufacturing process is being mastered by more and more manufacturers, but the level of FPC process technology varies among factories. With the continuous development of FPC and the improvement of the technical content of FPC products, higher requirements have been put forward for various manufacturers. The emergence of emerging technologies such as COF, NCP, and MCM on FPC greatly increases the complexity of FPC. The market corresponding to FPC is already a high-tech industry. Only by mastering the technology can it not be eliminated by the market.

1. Introduction of FPC

FPC is the abbreviation of Flexible Printed Circuit, also known as flexible circuit board or soft board. It is a highly reliable flexible printed circuit made of polyimide or polyester film as the base material.

According to the definition of JIS C5017, single-sided and double-sided printed circuit boards are single-sided flexible printed circuit boards that are laminated on PET or PI substrates with copper foil to form single-sided circuits, or PI as substrates on both sides The double-sided flexible printed circuit board that forms the circuit.

FPC is mainly used in: MP3, MP4 players, portable CD players, home VCD, DVD, digital cameras, mobile phones and mobile phone batteries, digital walkie-talkies, medical, automotive, aerospace and military fields.

 

 

 

The characteristics of FPC are:

advantage

shortcoming

Light weight, miniaturization and thinning

Long process, high requirements for process and environment, high cost

Flexible, can move and expand at will in three-dimensional space

High scrap rate

Large design area, three-dimensional wiring possible

Low mechanical strength

Good heat dissipation

Inspection method is more difficult than PCB

Can increase wiring density

Need to rely on fixture production, and put forward high requirements on the fixture design ability of engineers

The FPC industry can be divided into:

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Second, the SMT production method of FPC

2.1 RTR continuous tape and reel

roll-to-roll or reel-to-reel, abbreviated as RTR, full Chinese name: continuous reel method.

In the 1980s, a few large FPC manufacturers in the world began to build RTR production lines. As the process technology used at that time was not yet mature, the qualified rate of FPC products produced on the RTR production line was very low. In the late 1990s, due to the expansion of the TAB and COF markets and the initial maturity of RTR technology, the advantages of the continuous production of FPC equipment and processes began to be gradually exerted in the production of TAB and COF FPC. By the beginning of the 21st century, the technological development of RTR production of FPC has been relatively mature.

Schematic diagram of REEL TO REEL production line:

2.1.1 Advantages of Reel to Reel

Ø There is no need for the operator to attach the FPC to the fixture and other related work procedures.

Ø Avoid FPC creases and scratches.

Ø If FOG, NCP and other products with high requirements for a dust-free environment are to be used in the subsequent process, it can avoid personnel work and environmental pollution.

Ø Simplify packaging, transportation and operating conditions.

Ø It can meet the process requirements of ultra-thin, high-end FPC COF and other processes.

2.1.2 Disadvantages of Reel to Reel

Ø The production line equipment is highly specialized and the applicable product range is narrow.

Ø The investment cost of RTR production equipment is expensive.

Ø Suitable for ultra-large batch FPC production. Not suitable for multi-variety and small-batch production mode.

2.2 Fixture attached type

The focus of this type of production method is the attachment/and fixing of the FPC. The purpose of FPC attachment is to use a fixture to turn the FPC into a "PCB" so that the FPC can perform SMT like a "PCB". The simplified technological process is:

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Compared with REEL TO REEL, the fixture attachment method is low cost and easy to use, so using fixtures for FPC SMT is currently the most commonly used method in the industry. This article will explain the production method based on fixture attachment.

 

 

Third, the process points of the fixture attachment method

 

3.1 Pre-baking

 FPC materials are susceptible to moisture. When the damp FPC is welded at high temperature, blistering and delamination will occur, which will lead to scrap. Therefore, the FPC supplier is usually required to vacuum pack when the material is received. However, vacuum packaging is not foolproof. It is best to pre-bake the FPC before placement.

The pre-baking conditions need to be considered comprehensively based on the FPC material, FPC thickness, oven, baking tray, etc. After engineering experiments, the pre-baking conditions: temperature, baking time, and stacking quantity should be determined.

After baking, the FPC must be cooled to room temperature before it can be put into production, otherwise the hot FPC will cause the solder paste to collapse. There are two more places that need to be monitored: cooling time and overdue rebake time, which can also be determined after engineering experiments.

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3.2 FPC attaching and fixing

The attaching and fixing methods of FPC vary according to different fixture designs. After years of development, FPC fixtures have had many mature design methods. Excluding the need for special products, and taking into account the operability and ease of maintenance, we usually design the fixture into two parts: the base and the pallet.

3.2.1 Base

Its function is to fix the pallet and locate the FPC. Although the base design is simple, it is indispensable as an integral part of the fixture. One of the BASE designs is shown on the right:

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3.2.2 Pallet

The pallet serves to fix the FPC.

Ø According to the different fixing methods and materials, it can be divided into: silica gel attached pallet, high temperature adhesive tape attached pallet, magnetic pallet, etc.

Ø According to different materials, it can be divided into aluminum alloy pallets and synthetic stone pallets. Due to the low cost of aluminum alloys, it is most widely used.

3.2.2.1 Silica gel attachment pallet

It uses the viscosity of silica gel to fix the FPC. After the pallet is processed, it is necessary to print silica gel or paste a silica gel film on the pallet.

 The advantages of this kind of pallet are: cost-saving and repeated use; saving personnel and high efficiency; smooth surface, which is conducive to printing; convenient and quick removal of the plate after the furnace.

Disadvantages are: sink the position of the silicone paste, which increases the complexity of the design and production of the fixture; the position of the silicone paste corresponding to the FPC cannot have components; the life of the silicone is short and maintenance (replacement when the silicone fails, etc.) work large.

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3.2.2.2 High temperature adhesive tape attached to the pallet

It is to paste high-temperature adhesive paper on the FPC, and no post-process is required after the pallet processing is completed.

The advantage of this kind of pallet is that it is well fixed and does not fall off after passing the furnace; no special maintenance is required.

The disadvantages are: high-temperature adhesive tape costs high; personnel waste and low efficiency; it is easy to scratch the surface of the FPC when removing the adhesive paper from the board behind the furnace; it is easy to cause the FPC to attach adhesive.

The high cost of high temperature adhesive tape is one of the main drawbacks of this method. In order to reduce this cost, we have found a white Japanese high temperature resistant adhesive tape. This type of adhesive tape can be recycled 5 times, greatly reducing the cost of high temperature adhesive tape. cost.

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3.2.2.3 Magnetic pallet

It embeds a high-temperature magnet in the pallet when processing the pallet. After positioning the FPC on the pallet, cover the pressed sheet. The pressed sheet contains iron elements, which can generate magnetic attraction with the magnet in the pallet. Firmly clamp the FPC between the pallet and the pressing sheet.

The advantage of this kind of pallet is that it is well fixed and does not fall off after passing the furnace; no special maintenance is required. The cost is one-time investment, no follow-up costs are accumulated, and the total cost is low; it can significantly improve the problem of gold fingers attaching tin.

The disadvantages are: complex production and long cycle; high initial investment cost; easy deformation of the pressed sheet; not suitable for FPC with PSR or PSC type solder mask.

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Pallet with high temperature resistant magnet at the bottom

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Pressed sheet made of stainless iron material

The above three fixture solutions do not say which one is necessarily better than the other. When we select the fixture design plan, we need to comprehensively consider the characteristics of the product, the order quantity and other factors.

The design and manufacture of fixtures directly affect the FPC production through rate. For FPC, the craftsman needs to make a set of FPC fixture design specifications suitable for the company's products to guide and standardize the fixture production of the tooling fixture department or outsourcing suppliers.

3.3 FPC solder paste printing

Although the FPC becomes a "PCB" after being attached and fixed, the surface of the FPC will still be uneven. This unevenness comes from: the deformation of the FPC itself, the thickness of the attached material, the thickness of the reinforcing plate or the adhesive. When the FPC is not flat, it will cause the problems of continuous tin, less tin, and more tin in printing.

In order to eliminate the influence of FPC unevenness, the following measures can be taken:

3.3.1 Optimize the attachment position and quantity of high temperature adhesive tape

The deformation of FPC is especially obvious when high-temperature adhesive tape is used for attachment, which will lead to unfavorable release and less tin. In order to reduce the impact, it is necessary to add high-temperature adhesive tape near the printing area, or make the adhesive tape sticking position close to the printing area without adding adhesive tape, and at the same time, ensure that the adhesive tape is at least 8mm away from the pad to prevent the adhesive tape from getting stuck. The thickness interferes with the pad and causes the printing of multiple tin or continuous tin.

3.3.2 Avoidance of reinforcing plate and adhesive

Reinforcing plates and adhesives are another important factor that causes FPC unevenness. We can design some sink grooves on the pallet to eliminate their influence and make the surface of the FPC as flat as possible. However, the processing accuracy should be considered when designing the depth of the sink groove, as long as the pad area of ​​the FPC is not interfered. Otherwise, due to the low processing accuracy, the sink groove is too deep, which will cause the problem of excessive tin and continuous tin.

The distance between the reinforcement board and the backing glue should be considered when designing the pad. Otherwise, the slot avoidance distance is insufficient, which will cause excessive tin and continuous tin printing. As shown on the right.

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This distance needs to be set differently according to the thickness of the reinforcing plate or adhesive, the material and thickness of the FPC, and the type of pad. Usually a distance of 8mm can cope with most situations.

3.3.3 The thickness and opening size of the pressing piece in the magnetic clamp

How to reduce the impact of unevenness between the tablet and FPC on printing?

First of all, choose the thickness of the tablet. The material of the tablet is stainless iron. It is recommended to choose stainless iron with a thickness of 0.06mm as the tablet.

Secondly, the opening of the tablet can be enlarged to reduce the influence of the thickness of the tablet. The recommended distance between the edge of the tablet opening and the edge of the pad is: at least 8mm in the printing direction and at least 6mm in the non-printing direction.

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3.4 FPC patch

When the FPC is attached to the fixture, the FPC becomes a "PCB", and the unevenness of the FPC has been solved, so the patch is very simple, and it is not much different from the PCB patch. However, because FPC has few components and must be assembled on the board for placement, how to efficiently use the placement machine is the main problem of FPC placement.

3.4.1 When the FPC is the incoming material of the puzzle

The placement of incoming FPC panels is relatively simple. At this time, we only need to pay attention to the impact of the bad board rate of each panel on the placement efficiency. It must be ensured that when the board failure rate is the highest, the CYCLE TIME of the placement machine is still the highest.

3.4.2 When the FPC is a single board

Single-board FPC needs to be made into jigs and jigs before it can be put into production. The number of puzzles directly affects the placement efficiency. Because the FPC is mainly based on CHIP, the number of ICs and connectors is not much, so we recommend the number of panels = a multiple of the number of positioners on each cantilever of the placement machine. For example, Siemens D4 placement machine, the number of positioners for each cantilever is 12, then the number of FPC panels can be 12/piece or a multiple of 12/piece. The advantage of such a puzzle is that each CYCLE can take full components without waste, and it is convenient for program optimization (manual fine-tuning after automatic optimization).

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FPC single board is assembled into 12 pieces

Affected by the positioning accuracy of the fixture and the accuracy of the FPC positioning hole, the position of each FPC will be more or less deviated. Therefore, it is necessary to identify the MAKR points for each FPC during placement. For example, if 12 puzzles need to identify 24 MARK points, the efficiency loss is great. Recommend a solder paste MARK point technology, as shown on the right. Using this technology, not only can the MARK points be reduced to two, but also the quality problems of erection and false welding can be significantly improved. However, before using the solder paste MARK point, it is necessary to confirm whether the camera of the placement machine has a blue light and a 45-degree light source, otherwise the camera needs to be modified and a 45-degree blue light source is sufficient.

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3.5 FPC sub-board

The incoming FPC of the splicing board needs to be divided into different boards. According to the different connection methods of the FPC splicing board, different sub-boarding methods need to be selected.

3.5.1 Sub-board mode of micro-connection

Using micro-connected FPC, the connection part is slightly connected to the FPC panel, and the FPC can be separated by gently tearing it by hand. However, the connection strength of the micro-connection is too weak, and the FPC will fall off from the jigsaw if accidentally, causing the printing to be offset. Therefore, for FPC with high component density and small pad spacing, it is not suitable for micro-connection method to join the board, and the connecting rib method should be used for connection.

 

3.5.2 Splitting method of connecting ribs

As shown on the right, it is an FPC connected by connecting ribs.

There are 3 ways to divide the board:

Ø Manual splitting with a carving knife or blade, with low accuracy and efficiency, and low cost.

Ø Die cutting board. The accuracy is low, the life is short, but the cost is low, and the efficiency is high.

Ø Steel mold split plate. High precision, long life, high efficiency, but the cost is very high.

Need to choose different ways of splitting according to product characteristics and order volume.