PCB Technical - FPC and PCB connection and welding method

With the development of electronic products, the technical requirements for using FPC soft boards to connect to PCB are getting higher and higher. This article tries to introduce several commonly used FPC and PCB connection and soldering methods in the industry.

FPC (Flexible Printed Circuit, "flexible printed circuit board" or "flexible circuit board") is currently used in a variety of electronic products, especially wearable devices and lightweight and short handheld devices. Although the PCBA process has been developed to be used in Electronic parts are welded directly on the FPC, but its solderability and quality reliability are still poor, such as the "I/O connectors" that are plugged and unplugged for end users, such as micro-USB, USBC connectors for charging and data transmission, It is not recommended to solder large BGA and QFN to FPC.

Generally, the FPC still needs to be connected to the rigid PCB, and the process of connecting the FPC to the PCB is used:

▪ FPC Connector

▪ Rigid-Flex Board

▪ Soldering. The main reason for using welding is to save the cost of the "connector" and reduce the height.

This article only introduces the welding process between FPC and PCB, and the soft board welding methods are also various, but in general, the following methods can be used to weld:

1. Manual soldering

Manual soldering of flexible boards to circuit boards is the cheapest process of all flexible board soldering processes. Some of them can be operated without the use of jigs at all. However, the soldering quality is also the most unreliable process, because manual soldering is easy to cause voids. Welding, false welding, false welding, short-circuit bridge and other quality problems. This is because the soft board tends to move or lift up when the soldering iron tip is removed and the solder is not solidified during manual soldering. After the solder is solidified, false soldering, empty soldering, etc. are formed.

Therefore, it is best to use a heavy object to press on the soft board until the welding is completed, which can greatly improve the welding yield. In addition, it is better to have through holes (PTH) on the gold fingers for soft board welding, which can increase the visual confirmation of whether the welding is good or not, and can also reduce the risk of overflowing tin and short circuit.

Generally speaking, if the design is not locked (Lock-down), in order to reduce the cost of fixture design and production, a small amount of manual soldering iron can be used to solder the soft board to verify the function of the RD. It is strongly recommended not to manually weld in mass production.

Two, HotBar (Haba) welding

HotBar (Haba) welding

The principle of HotBar (Haba) welding is basically to use the so-called [pulse current (pulse)] to flow through molybdenum, titanium and other metal materials with high resistance characteristics of [Joule heat] to heat [thermodes/heater -tip)】, and then use the thermal head to heat and melt the printed solder paste on the PCB to connect with the FPC to achieve the purpose of soldering.

Therefore, HotBar must have a HotBar machine and a carrier to fix the FPC on the PCB. As long as the FPC and PCB are well designed, the goal of mass production and a certain yield can basically be achieved.

▪ The quality of HotBar welding basically depends on the design. For detailed design requirements, please refer to Shenzhen Honglijie’s previous article on HotBar design.

Restrictions on parts near HotBar FPCB

HotBar FPCB soft board design considerations-PTHs

Suggestions on the relative position of HotBar hot-pressed FPCB and PCB soldering pad

HotBar FPCB soft board design-to avoid stress concentration and circuit breakage

▪ Next is the size of the FPC gold finger spacing. The larger the spacing, the easier the production and the higher the yield rate. However, the design requirements are getting smaller and smaller. This also makes HotBar more and more difficult to make, and the yield rate is straightforward.

▪ The last issue is process control. For example, the control of the amount of solder paste, whether the application of the flux is in place during the HotBar, the temperature, pressure, and time settings of the HotBar, and the capability of the HotBar machine (whether the pressure of the hot pressing head can be programmed).

In addition, some HotBar flexible board designs cannot achieve effective thermal conductivity. At this time, it may be necessary to consider whether to use tin-bismuth (SnBi) low-temperature solder paste, but the low-temperature solder paste is brittle, so it is recommended to strengthen the auxiliary structure.

It is recommended to read related articles:

Feasibility Evaluation of Low-Temperature Solder Paste for HotBar Soldering

3. Reflow soldering

Affected PCB companies have not tried the reflow soldering process for flexible board attachment before, but it should be feasible in theory, and some discussion boards can also be seen on the Internet to perform the reflow soldering process. Its method is basically to print solder paste on the PCB now, then place the FPC in front of the reflow furnace, and use the upper cover and lower seat reflow carrier to pass through the reflow carrier, and the upload and download tools use magnetic fasteners to ensure The FPC position will not move, and the FPC solder joints will not lift during reflow.

There are several precautions for this FPC reflow process:

▪ Whether the FPC material can withstand the high temperature of lead-free reflow. If the high temperature of the FPC material is limited, the feasibility of low temperature solder paste may need to be considered.

▪ The placement of FPC is usually manual work. At this time, the solder paste is still in paste form. How to avoid manual work to touch the solder paste or other parts is a big issue. Therefore, this process is not suitable for parts placed under the FPC.

Shenzhen Honglijie personally believes that this process is suitable for products that do not have too many parts on the PCB and there are no parts on the FPC. Otherwise, the defect rate caused by touching other parts or solder paste should be quite high. Fine-pitch FPC should It's not suitable either.

4. Laser soldering

Laser soldering (Laser soldering) relies on the excitation of laser energy to convert it into heat to achieve the purpose of welding. Therefore, its welding is usually used to directly heat the position where the laser can be hit on the solder.

In addition, this kind of laser welding equipment is usually a special-purpose machine and not a general-purpose machine. Its energy is generally not suitable for performing other operations. Therefore, the equipment cost is not cheap. Unless the output is large, it is not recommended. Take a look at ROI!

Five, ACF (anisotropic conductive adhesive)

When it is not possible to use solder to solder FPC and PCB, you may consider using "ACF (Anisotropic Conductive Film)" to conduct electronic signals between FPC and PCB. The processing temperature of ACF is lower than that of HotBar. Worrying about the problem of FPC burnout, ACF's operation method is actually similar to HotBar, but ACF is simpler. It is actually a bit like glue. It is placed between the golden fingers of PCB and FPC, and then heated and pressurized to complete the connection. The station can even set different temperatures and pressures for use in HotBar and ACF processes at the same time.

However, the biggest disadvantage of ACF is that its adhesive is prone to failure and peeling after using it for a period of time, so additional mechanisms must be added to ensure that the FPC will not loosen from the PCB.