Double sided flexible printed board manufacturing process

FPC opening – double sided FPC manufacturing process

Except for some materials, the materials used in flexible printed boards are basically roll-shaped.
Since not all processes must be processed by the tape process,
some processes must be cut into sheets for processing.

For example, drilling holes in metalized holes of double-sided flexible printed boards can only be drilled in sheet form. Holes, so the first process of double-sided flexible printed boards is to open the material.

Flexible copper-clad laminates are extremely weak in external forces and can easily be injured.
If the material is damaged during the cutting process, it will have a serious impact on the qualification rate of subsequent processes.
Therefore, even if it seems to be a very simple material,
in order to ensure the quality of the material must also be given enough attention.
If the amount is relatively small, a manual cutter or a hob cutter may be used.
In large batches, an automatic cutter may be used.

Whether it is single-sided, double-sided copper foil laminate or cover film,
the accuracy of blanking size can reach ±O.33. The reliability of the material is high,
and the opened materials are stacked neatly and automatically,
and no personnel are required to receive the materials at the exit.
The damage to the material can be minimized,
and the use of a change in the size of the feed roller results in almost no wrinkles or scratches in the material.
And the latest device can also automatically cut the flexible printed circuit board after the tape process is etched,
the use of optical sensors can detect the corrosion positioning pattern,
automatic positioning of the material, open material accuracy of O. 3mm,
but this opened frame cannot be used as the orientation of the subsequent process.

FPC drilled through hole – double-sided FPC manufacturing process

The through hole of the flexible printed board can also be drilled with CNC as well as the rigid printed board,
but it is not applicable to the hole processing of the double-sided metallized hole circuit of the tape.
With the increasing density of circuit patterns and the small aperture of metallized holes,
the hole diameter of numerically controlled drilling has a certain limit.
Many new drilling techniques have now been applied to practical applications.
These new drilling techniques include plasma etching, laser drilling, micro-aperture punching,
chemical etching, etc.
These drilling techniques more easily meet the hole forming requirements of the tape process than CNC drilling.

The through hole of the flexible printed board can also be drilled with CNC as well as the rigid printed board,
but it is not applicable to the hole processing of the double-sided metallized hole circuit of the tape.
With the increasing density of circuit patterns and the small aperture of metallized holes, the hole diameter of numerically controlled drilling has a certain limit.
Many new drilling techniques have now been applied to practical applications.
These new drilling techniques include plasma etching, laser drilling,
micro-aperture punching, chemical etching, etc.
These drilling techniques more easily meet the hole forming requirements of the tape process than CNC drilling.

  1. CNC drilling

Most of the drilled holes in double-sided flexible printed boards are still drilled with numerical control drills.
The numerically controlled drill presses used in the numerically controlled drill presses and rigid printed boards are basically the same, but the drilling conditions are different.

Due to the thinness of the flexible printed board, it is possible to drill a plurality of holes,
and if the drilling conditions are good, 10-15 pieces can be drilled together.
Pads and covers can be made of paper-based phenolic laminates or glass cloth epoxy laminates.

Aluminum plates 0.2 to 0.4 mm thick can also be used.
The drills used in flexible printed boards are available on the market.
Drills for drilling holes in rigid printed boards and milling cutters for milling can
also be used for flexible printed boards.

The drilling conditions, milling cover film, and reinforcing plate shape are basically the same.
However, due to the soft adhesive used in the flexible printed board material,
it is very easy to attach to the drill bit, and the status of the drill bit needs to be checked frequently.

But also to properly increase the bit speed. Special care must be taken in the drilling of multi-layer flexible printed boards or multi-layer rigid-flex printed boards.

  1. punching

Impulse micro-apertures are not new technologies and have been used as mass production.
Since the tape rolling process is continuous production,
there are many examples of using punching to process the through-holes of the tape.

However, the batch punching technology is limited to the punch diameter O. The 6~0.8mm hole has a longer processing cycle than the drill hole of the CNC drilling machine and requires manual operation.
Because the size of the initial process is large, the punching die is also correspondingly large,
and the die price is very expensive. Although mass production is beneficial to reducing costs,
equipment depreciation is a burden, and small-batch production and flexibility
cannot compete with numerical control drilling. Therefore, it has not yet been popularized.

However, in the last few years, great progress has been made in the precision of punching technology and numerical control drilling,
and the practical application of punching on flexible printed boards has been very feasible.
The latest mold manufacturing technology enables the manufacture of punched holes with a diameter of 75 μm on a non-adhesive copper clad laminate with a substrate thickness of 25 μm.

The reliability of punching is also quite high, and even a diameter of 50 μm can be punched
if the punching conditions are appropriate. Hole.
The punching device has also been numerically controlled, and the mold can also be miniaturized,
so it can be well applied to the punching of the flexible printed board.
Numerically controlled drilling and punching cannot be used for blind hole machining.

  1. Laser drilling

The finest through-hole can be drilled with a laser.
The laser drills used to drill through-holes in a flexible printed board include an excimer laser drill,
an impact carbon dioxide laser drill, a YAG (yttrium aluminum garnet) laser drill, and argon. Laser drills and so on.

The impact carbon dioxide laser drill can only drill the insulating layer of the substrate,
while the YAG laser drill can drill the insulating layer and the copper foil of the substrate,
and the speed of drilling the insulating layer is obviously faster than the speed of drilling the copper foil.

professional R&D, designing, developing and manufacturing PCB, FPC FPCB,
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only use the same laser drilling machine for all drilling processing production efficiency can not be high.

In general, the copper foil is first etched. The pattern of holes is first formed, and then the insulating layer is removed to form a through hole so that the laser can drill holes with extremely small pore diameters.
However, the accuracy of the position of the upper and lower holes may restrict the borehole diameter.
If it is a blind hole, as long as one side of the copper foil is etched away,

there is no vertical position accuracy problem. This process is identical to the plasma and chemical etching holes described below.

The holes that are currently excimer laser processed are the finest.
The excimer laser is an ultraviolet ray that directly destroys the structure of the basal layer resin,
so that the resin molecules are dispersed and the heat generated is extremely small, so the extent of damage around the hole can be limited to a minimum in the heat, and the wall of the hole is smooth and vertical.

If the laser beam can be further reduced, holes of 10 to 20 um in diameter can be processed. Of course, the greater the ratio of the plate thickness to the hole, the harder the wet copper plating will be.
The problem with the drilling of excimer laser technology is that the decomposition of the polymer causes the carbon black to adhere to the wall of the hole.

Therefore, some measures must be taken to clean the surface before plating to remove the carbon black. However, when laser processing blind holes, the uniformity of the laser also has certain problems, resulting in bamboo-like residues.

The biggest difficulty with excimer lasers is slow drilling and high processing costs. Therefore, it is limited to the processing of high-precision, high-reliability microholes.

Impact carbon dioxide lasers generally use carbon dioxide gas as the laser source and radiate infrared light.
Unlike the excimer laser, which burns and decomposes resin molecules due to thermal effects, it is thermally decomposed, and the processed hole shape is different from that of the excimer laser.

Much more, the diameter of the hole that can be processed is basically 70 to 100 um,
but the processing speed is significantly faster than that of the excimer laser and the drilling cost is much lower.
Even so, it is much more expensive to process than the plasma etch and chemical etch methods described below, especially when the number of holes per unit area is large.

The impact carbon dioxide laser should pay attention to the processing of blind holes,
the laser can only be emitted to the surface of the copper foil, the organic matter on the surface does not have to be completely removed, in order to stably clean the copper surface,
chemical etching or plasma etching should be used as a post-treatment.
From the perspective of technical possibilities, the laser drilling process is basically not difficult for the tape winding process, but considering the balance of processes and the proportion of equipment investment,

it is not dominant, but the tape chip automation welding The narrow width of the process (TAB, Tape Automated Bonding) and the use of the tape winding process can increase the drilling speed. Practical examples are already available in this area.

Hole Metallization – Double Sided FPC Manufacturing Process

The hole metallization of a flexible printed board is basically the same as that of a rigid printed board.

In recent years, there has been a substitution of electroless plating, using a direct electroplating process that forms a carbon conductive layer technology. Hole metallization of flexible printed boards also introduces this technology.

Because of its softness, the flexible printed board requires special fixtures.
The fixture can not only fix the flexible printed board but also must be stable in the plating solution.
Otherwise, the thickness of the copper plating is not uniform,
which is also caused by the disconnection during the etching process.

And the important reason for bridging. In order to obtain a uniform copper plating layer,
the flexible printed board must be tightened in the fixture, and the position and shape of the electrodes must be worked hard.

Hole metallization outsourcing processing,
to avoid as much as possible outsourcing to the factory without the experience of hole-free flexible printed board,
if there is no special plating line for flexible printed board, the quality of the hole can not be guaranteed.

Copper foil surface cleaning – FPC manufacturing process

In order to improve the adhesion of the resist mask, the surface of the copper foil is cleaned before the resist mask is coated, even though such a simple process requires special care for the flexible printed board.

Chemical cleaning and mechanical polishing processes are commonly used for cleaning.
For the production of precise patterns, most of the occasions involve the combination of two clean flow processes for surface treatment. Mechanical polishing uses a method of throwing a brush.
Excessive hardness of the brushing material can cause damage to the copper foil. If it is too soft,
it will be insufficiently ground. Nylon brushes are generally used,
and the length and hardness of the brush must be carefully studied. Use two roller brushes,
placed above the conveyor belt, the direction of rotation and the direction of the belt transfer,
but if the roller pressure is too large, the substrate will be subject to a lot of tension and be elongated,
This is one of the important causes of dimensional changes.

If the surface of the copper foil is not clean, adhesion to the resist mask is poor,
which reduces the yield of the etching process. Recently,
due to the improvement of the quality of the copper foil,
the surface cleaning process can also be omitted in the case of a single-sided circuit.
However, with a precision of less than 100μm, surface cleaning is an indispensable process.

Resist Coating – Double Sided FPC Manufacturing Process

Now, the coating method of the resist is classified into the following three methods depending on the precision and the yield of the circuit pattern: a screen printing method, a dry film/lighting method, and a liquid resist light sensing method.

Now, the coating method of the resist is classified into the following three methods depending on the precision and the yield of the circuit pattern: a screen printing method, a dry film/lighting method, and a liquid resist light sensing method.

Anti-corrosion inks are printed directly on the surface of the copper foil using the screen printing method.
This is the most commonly used technology and is suitable for mass production with low cost.
The accuracy of the formed line pattern can reach the line width/spacing 0.2~O. 3mm,

but not for more precise graphics. With the refinement of this method can not gradually adapt.
Compared with the dry film method described below, there is a need for a certain technical operator,
and the operator must be trained for many years, which is an unfavorable factor.

The dry film method can produce a line width pattern of 70 to 80 μm as long as the equipment and conditions are complete. Most of the precision patterns below 0.3mm can be formed by the dry film method. Using a dry film, the thickness is 15 ~ 25μm, conditions permitting, batch level can produce 30 ~ 40μm line width graphics.

When the dry film is selected, it must be determined according to the matching with the copper foil board and the process and by experiments. Even if the level of experimentation has good resolution, it does not necessarily have a high pass rate when used in mass production. The flexible printed board is thin and easy to bend.
If a hard dry film is selected, it is brittle and has poor followability, so cracks or peeling may occur, and the pass rate of etching may be reduced.

The dry film is roll-shaped and the production equipment and operations are relatively simple.
The dry film consists of a three-layer structure consisting of a thin polyester protective film,
a photoresist film, and a thick polyester release film.
Before peeling off the film, first peel off the release film (also called afpc film),

and then press it on the surface of the copper foil with a hot roller,
and then remove the protective film before the development
(also called carrier The film or cover film) generally has guide positioning holes on
both sides of the flexible printed board,
and the dry film can be slightly narrower than the flexible copper foil to be filmed.

The automatic film mounting device for rigid printed boards is not suitable for the application of flexible printed boards, and some design changes must be made. Due to the high linear velocity of the dry film film compared to other processes, many plants do not use automated film but use manual film.

After the dry film is attached, exposure should be performed after 15 to 20 minutes in order to stabilize it.

If the line width of the circuit pattern is less than 30μm,
patterning with a dry film will result in a significant decrease in the yield.
Dry film is generally not used in mass production but liquid photoresist is used.
Different coating conditions, the thickness of the coating will be changed,
if you apply a liquid photoresist with a thickness of 5 ~ 15μm on a 5μm
thick copper foil, the laboratory level can etch the line width of 10μm or less.

The liquid photoresist must be dried and baked after coating.
Since this heat treatment will greatly affect the performance of the film,
the drying conditions must be strictly controlled.

Grace Zheng

Email: sales06@andwinpcb.com

 

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