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Flexible PCB Fabrication Technology Development
As early as 2005, flexible PCB relying on 2-layer flexible CCL (copper clad laminate) for its application in cell phones witnessed more than 15,000m2 output. Usually via making and grooving on PI (polyimide) substrate material used in flexible CCL depends on mechanical manufacturing such as punching or drilling or new laser processing. With high density of flexible PCB (printed circuit board) circuit and thinner substrate considered, it's better to implement wet treatment technology to be compatible with volume production and cost reduction and to open via and groove on PI substrate material through etching.
Mechanism of PI (Kapton) etching will be discussed in this part. Kapton is generated by composite reaction between pyromellitic dianhydrite (PMDA) and DADPE plus imido. Hydroxy in etching solution will react on imido, making PMDA and DADPE dissolved, leading to successful etching.
During the process of etching, two types of etchant resists are available to protect images. One is copper etching resist that makes copper conductor image protect PI film while the other is imageable etching resist film that protects graphics through film, exposure and imaging. Comparison between the two types of etching resist is summarized in below table.
| Items | Copper Mask | Dry Film Mask |
| Graphics manufacturing | Ordinary | Easy |
| Microvia manufacturing | Good | Ordinary |
| Manufacturing accuracy | Good | Ordinary |
| Manufacturing taper angle | 50°C - 60°C | 40°C - 50°C |
Substractive Process, Fully-Additive Process and Semi-Additive Process
PCB, including flexible PCB tends to develop towards high density interconnect (HDI) featuring fine line, microvia and multiple layers. Fine line refers to that whose both tracing and spacing are below 0.1mm and fine pitch is less than 0.2mm.
When high-density fine line graphics generation is considered, technological methods contain following three methods.
• Substractivce Process
As an ordinarily-used copper foil etching method, substractive process has been regarded as a mature technology that has been used for decades' years. However, it is still being gradually upgraded due to its limitation as far as high-density fine line circuits are concerned. Among substractive processes, silkscreen printing to resist etching is most used due to its compatibility with mass-volume production, high convenience of operation and low cost. The most typical lies in imageable dry film etchant resist.
To achieve the target of fine line and to improve resolution of etchant resist, the first method lies in the application of thin dry film etchant resist layer, from 35μm, 25μm, 15μm to 10μm. The second method lies in the application of wet film (liquid imageable etchant resist) and thickness of coating film falls in the range from 10μm to 6μm. Moreover, etching capability should be improved. On one hand, it aims to maintain the stability of etching resolution and on the other hand, it aims to improve etching equipment. Moreover, thin copper foil should be applied whose thickness range is from 18μm, 12μm, 9μm to 5μm to reduce side etching and guarantee etching accuracy.
• Fully-Additive Process
Fully additive process contains the following items:
a. Insulating substrate;
b. Seed layer that is generated on substrate material surface;
c. Imageable etchant resist;
d. Electroplating graphics;
e. Insulating layer and via manufacturing;
f. Immersion copper
With PI film as substrate material for flexible PCB, insulating layer is photosensitive PI resin. Up to now, fine line has been generated with tracing and spacing being both 5μm and via aperture can be 20μm or even 10μm.
• Semi-Additive Process
Semi-additive process is between substractive process and fully-additive process, containing the following procedures and items:
a. Thin CCL or insulating substrate can be used;
b. Mechanical drilling or laser drilling can be implemented and then electroless copper leads it to be via;
c. Negative patterning is formed on the surface;
d. Copper plating is then carried out on graphics;
e. Imageable etchant resist is eliminated;
f. Copper should be etched away.
Comparison between substractive process, fully-additive process and semi-additive process can be summarized in below table.
| Items | Substractive Process | Semi-Additive Process | Fully-Additive Process |
| Finest pitch | 30μm | 20μm | 5μm |
| Ratio between wire height and width | <0.5 | >0.5 | >1.0 |
| Minimum aperture of microvia | 50μm | 20μm | 10μm |
| Layer count | >15 | >10 | >6 |
| Raw material | FCCL with no adhesive | Thin FCCL or PI film | PI film and liquid PI film |
| Product physical property | Substrate material determining | Substrate material determining | Substrate material and technology determining |
| Technical difficulty | Low | Relatively high | High |
| Equipment investment | Relatively low | Medium | Relatively high |
| Manufacturing cost | Low | Medium | High |
| Practical application | Multiple applications | Relatively applicable | Hardly applicable |
Build-Up Technology
Rigid HDI PCB fabrication technology mainly relies on build up that works equivalently suitable for multi-layer flexible PCB and flex-rigid PCB.
Leading methods of build-up technology include layer-by-layer build-up, bumed interconnect, full-microvia connection and PALAP (patterned prepreg lay up process). Via manufacturing methods can be classified into mechanical drilling, mechanical punching, laser drilling, plasma etching via, photosensitive via manufacturing and chemical etching.
Flexible PCB fabrication also relies on build-up technology, leading to the generation of blind via and buried via and stacked microvia with high density achieved. Flex-rigid PCB fabrication relies more on build-up technology and one typical one is called snap-off flex-rigid PCB. Traditional flex-rigid PCB is manufactured by placing flexible layer in the middle and then implementing build-up manufacturing, which is regarded to be inconvenient. Snap-off flex-rigid PCB, however, is generated by first manufacturing rigid multi-layer core board, then bendable surface circuit on build-up layer and finally rigid board elimination after component assembly.
Generation of Coverlay
When it comes to generation process of flexible circuit image coverlay, PIC (photo-imageable coverlay) is laminated onto board surface and conductor interconnect pad is then exposed by exposing and developing. This method doesn't need coverlay to be punched in advance or window opening of via drilling so that it leads graphic position to feature high accuracy. Another new technology is etching polyimide, making vias drilled on polyimide coverlay or substrate material.
Modifications of Flexible PCB Fabrication Equipment
Based on different manufacturing modes of flexible PCB, flexible PCB fabrication equipment is classified into two categories: single sheet and roll to roll. Single-sheet fabrication for flexible PCB works the same way as for rigid PCB. Base board is first cut into single pieces that will be manufactured one by one. To improve manufacturing efficiency, roll to roll manufacturing is more relied. Apart from single-sided flexible circuit board fully-automatic roll-to-roll manufacturing line, lots of roll-to-roll manufacturing devices containing a single phase become available to be compatible with the requirement of double-side PCB and multi-layer PCB fabrication.
As far as manufacturing freedom degree is concerned, single-sheet manufacturing is more convenient. Therefore, single-sheet flexible PCB manufacturing equipment should be focused on and optimized. The key task lies in transmission device improvement, which is more suitable for thinner flexible PCB manufacturing.
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Helpful Resources
• Flexible Printed Circuit Board Applications
• Streamline Assembly and Improve Reliability with Flexible and Flex-rigid PCBs
• Flexible PCB Manufacturing Services from PCBCart
• PCBCart Offers Flexible PCB Assembly Services
• Flexible Printed Circuit Board Material Selection Guide