Introduction of FPC Technological Process

The most common surface treatments

To protect our living environment requirement, human continue to improve it. At present, the environment of PCB production is especially true. Lead free and halogen-free will effected PCB development in many ways. Although PCB surface treatment seems little change at this moment, we should focus on that: long term slowly change will result in big change. In the case of increasingly high environmental protection, PCB surface treatment process will certainly change in future.

There are many different surface treatments, the normal are: HAL (hot air level), OSP (Organic Solderability Preservatives), ENIG (Electro less Nickel/Immersion Gold), Immersion silver, immersion tin, I will introduce individually.

1. HAL(hot air level)
Hot air level (HAL), it also called hot air solder level (HASL), the technology is: coating tin (lead) solder on PCB surface, and level it by heating compressed air, it will forming one layer which can anti-oxidation and good soldering.

2. Organic Solderability Preservatives(OSP)
OSP is one of surface treatment which comply with ROHS standard. OSP means Organic Solderability Preservatives, it also calls Preflux. Simply speaking, by chemical method, it will send up one organic film on clean bare copper surface. For this film, it can anti-oxidation, heat shock/humidity resistance, it protect copper surface away from rusty. In the following soldering with high temperature, this film will easily clean by flux, and also the bare copper will combine with tin solder shortly.

3. Ni/Au Plating
Ni/Au plating: plating one layer Ni and then one layer gold on surface of PCB conductor. The main purpose of plating nickel is preventing diffuse between gold and copper. There are two different gold plating at present, plating soft gold (pure gold, the surface not very bright) and plating hard gold (the surface is smooth and hard, wear, which include Co and other elements that will make surface brightly). Soft gold usually used for chip mounting, and hard gold used for electrical interconnection on non-soldering area.

4. ENIG(Immersion gold)
About ENIG surface treatment, which is coating one thicker and good electrical nickel and gold alloy, it can protect PCB with long period. And also the tolerance to environment, that is other surface treatments cannot reach. Moreover, ENIG can prevent copper dissolution, which benefit for lead free assembly.

5. Immersion tin
At this moment, tin is all solder’s base, so tin layer can match with all any solders. Through immersion tin technology, it can form flat copper tin compound, which have same Solderability characters with hot air level, and free of flatness problem. Boards with immersion tin, it can’t stock for a long period.

6. Immersion silver
It is a simple and fast technology, it is between OSP and ENIG, even exposure on hot, moisture and contaminated environment, the silver still keep well Solderability, just tarnish. But immersion silver haven’t character like ENIG with good physical strength, there isn’t nickel below silver layer.

7. ENEPIG

Compared with ENIG, there is a Pd layer between nickel and gold layer. Pd can prevent corrosion, it is helpful for immersion gold. Gold can combine with Pd and provide good contact area.

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A complete & comprehensive PCB design file should according to below information.

Base material type: There are different base material type, as a rigid PCB, the normal material like FR4, FR1, CEM-1, CEM-3 and etc.Do you have Tg value and CTI requirements? Tg135, Tg150 or Tg 180? CTI600 ?

Finished board thickness: No one will know which thickness do you want? 1.00mm? 1.60mm? or more?

Copper thickness: The normal finished copper thickness is 1OZ, 2OZ, which one do you want? And also please don’t forget to state that this copper thickness is started thickness or finished copper thickness. Because the PCB costs will different for 1oz, 2oz, 3oz and etc copper thickness.

Surface treatment: There are many kinds of surface treatments, which one do you prefer? Lead free HASL? ENIG? Immersion silver? OSP/Entek? Gold flash?

Solder mask/ silk screen color: The normal solder mask color is green, and the silk screen is white, if you want to special color, please don’t forget to point out in your fabrication notes.

IPC Class I/II/III: Indicate in your notes which specification you require your board to meet. If your company has its own proprietary specification, then supply it to your vendor for their review.

Difference Between ED Copper and RA Copper

The base material for FPC is FCCL, which are Copper +PI or Copper + AD + PI.As below picture shown, you may clearly know the relation of these three products.

 So originally, copper is very key for the flex circuits material. Which kind of copper we could choose? The answer is ED Copper (Electrodeposited Copper Foil) and RA Cooper (Rolled Annealed Copper Foil). Then what the difference between these two? I will show it from below 4 aspects:

1. Definition & Manufacturing Method
Different copper is defined by its manufacturing method. ED Copper, which is made from CuSO4 solution, by using electrolysis method, made Cu²⁺ dip into spinning cathode rolls and stripping, then made it into ED copper. RA Copper, which is made from high purity copper(>99.98%), by using pressure process, made copper into different copper thickness.

2.Form

ED Copper (Electrodeposited Copper Foil)

RA Cooper (Rolled Annealed Copper Foil)

3. Performance
Due to different manufacturing methods and different shapes, the performance of them is not same. ED copper has better conductivity than RA copper. While as for extensibility, RA copper is much better than ED copper. Details about performance comparison can be understood via below chart : 

4. Application
Because of different performances, RA copper and ED copper are used in different products which tend to different product requests. For example, the flex board needs to be bendable a lot, so obviously, RA copper will be a good choice. Otherwise, we could use ED copper with high electrical conductivity.  

Whatever, you should select copper according to your circuits' requests (final board performance, cost, function...)

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Year established: 2011

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Rework and Repair on Flex Circuits

Flex circuits are used in a variety of applications for the interconnection of conductors which need to be bent or exercised continuously. This interconnection technology has historically been used as a wire interconnection. There are a variety of versions of this type of circuit. One of them is the dual access flex circuit which is a single-sided flex circuit that is manufactured so that the conductive material can be accessed from both sides of the flex. A double-sided flex circuit is a circuit having two conductive layers, one on each side of the base layer within the circuit. Trace patterns, specific to your needs, can be created on both sides of the substrate film. They can be interconnected where desired with copper plated through-holes. A multilayer flex circuit combines several single-sided or double-sided circuits with complex interconnections, shielding and/or surface mounted technologies in a multilayer design. Rigid flex circuits combine the best of both rigid printed circuit boards and flexible circuits integrated. Circuits are typically interconnected between the rigid and flex circuits through plated through-holes.
There are a variety of benefits to the flex circuits. One of the major benefits of a flex assembly is the nearly error-free implementation of wiring in lieu of labor intensive hand wiring. They are also able to configured, unlike their rigid counterparts, as complex 3-dimensional configurations as they can be contorted in to a variety of shapes. As the name implies, the materials used in flex circuits can be bent back and forth numerous times meaning they can be used in in highly repetitive applications such as on print heads. When weight is an issue, flex circuits are a good alternative to rigid boards and wires as both the dielectric material and conductor runs are very thin.
Over the last several years the flex industry has seen a growth in demand. It is now a 10 billion WWD industry with growth rates in the 7–10% year range.

Figure 1: Rigid-flex circuit board assembly.
With this surge in usage of flex, the standards for rework (replacing devices while still meeting the initial specification and functionality) and repair (repairing the physical damage on a flex circuit) of these type of electronic interconnection circuits has not kept pace.

There are some rework challenges that come from the very nature of flex circuits. First of all, it is difficult to hold the flex circuit flat. The flexible nature of the Kapton or other base flex material, which makes it so attractive in the application, makes it challenging from a rework perspective. In order to retain the flatness of the assembly, it needs to be taped to be held down. In some cases, a vacuum fixture, a relatively pricey endeavor, is fabricated for flex circuit rework. When placing fine pitch components, the vacuum structure of such fixtures has a significant influence. If the vacuum is directly under some of the leads of a fine pitch component, there is a likelihood any vacuum will “pull” the flex into the hole, preventing the component from contacting the flex circuit lead, thereby resulting in an electrical “open.” For rework paste printing, co-planarity is a challenge when the stencil and surface to be printed are not coplanar. Therefore, paste print deposition using a syringe is often used instead. Sometimes, conductive epoxies are used in interconnecting devices to the flex material. While the curing temperature of these joining materials is much lower than the reflow temperature of more standard solder, it can make a mess. Even when the rework process is engineered properly, many times the limitation on rework is that the marginal cost of the assembly is far less than the burdened rework cost, making the scrap pile a more attractive economic alternative.

Figure 2: Rigid flex ripped circuit.
There are some advantages to reworking flex circuits from a process standpoint. The lower thermal mass compared to a rigid PCB shortens the duration time to reach liquidus when soldering to a flex board. This speeds up the rework process for replacement.

In addition, many times this lowers the air temperature required from the hot air system thereby resulting in less potential component damage. The high temperature withstand properties of flex materials such as Kapton, Peek and high-temperature polyimide all give the flex rework process a larger process window.
In terms of industry standards for PCB repair, the IPC 7711/21 Repair and Modification of Printed Boards and Electronic Assemblies covers the rework and repair processes for flex circuits. Each of the processes in the standard are listed in terms of their applicability to flex rework or repair with an “F” in the upper right-hand part of the process documents under the “Board Type” section heading. There is even a flex-specific standard of conductor repair. Conductor repairs on flex is covered in procedure 7.1.1.

Figure 3: Repaired base Kapton material along with conductors.
By way of an example of a conductor repair, Figure 2 illustrates a torn flex circuit as part of a rigid-flex board. The standard process found in IPC 7721 3.5.1 was used to repair the material. The conductor runs had a copper foil jumper installed to replace the damaged conductors and then they were soldered together for further rigidity. The outcome of this repair can be found in Figure 3.

Rework and repair of flex circuit assemblies is evolving and remains a challenge as the industry continues to adopt best practices from the rigid assembly world.