Flexible Circuits vs. Printed Circuits  “Assembly Considerations”

Flexible Circuits vs. Printed Circuits “Assembly Considerations”

October 22, 2014 in Design Tips

As mentioned many times in the past in this blog, flexible circuits make up a small portion of the overall printed circuit market worldwide. The questions that typically arise quickly have to do with electrical and mechanical characteristics. Then if agreeable and a flexible circuit is determined to be the best answer for an application, the question of design practices comes up. After all of those deterrents have been dealt with, the circuit is introduced to the production department and a new set of concerns arise.

Traditional “rigid” printed circuit boards have been widely used in the United States since the 1940s. They showed up in consumer products the next decade and have been produced in very large scales ever since. The electronics industry is full of experts on the design, manufacture, and assembly of printed circuit boards (PCBs). So there isn’t a concern for our production groups when we introduce a new product and it’s made on a traditional printed circuit board. However, if the new product is a flexible circuit it arrives with many tedious questions and maybe even insecurity because of a lack of experience with this type of assembly. These concerns are understandable.

Printed circuit boards lend themselves well to automated assembly. They are after all rigid and stand up well to wave solder and IR reflow solder. Their flexible cousin however does not stand up as well. The inherent flexibility of the circuits requires that one think differently about how the assembly will travel through all the stages it must go through. Some of the process will be very similar to those we use with printed circuit boards and some will be very different.

Flexible circuit material absorbs moisture fairly well and must be baked out, as we do with traditional printed circuit boards, before being introduced to high temperatures. This process as well as the next (solder paste screening) will be very similar to those we use on our traditional printed circuit boards. However, after that point, we must take very careful steps to complete the assembly. Our next step would be to go to pick and place which puts some pressure on the circuit. Flexible circuits then must be protected during this process, temporarily, to provide the stability and even surface required to place the components reliably. They also must have that same stability as they travel through the soldering process. Whether that is wave or IR reflow. Then after all the components are soldered the circuits can be cleaned, inspected and re-worked. The stability required for travel through the assembly line is not needed any longer. We can remove the temporary stability fixture that was used throughout the assembly process and complete the task of packaging and shipment or placement into finished goods inventory.

Let’s look at some of the questions that are frequently asked.

  • Can flexible circuits have components soldered to it?

Yes! This is usually the first question that is asked. Because of its small footprint in the industry there are still a lot of designers that are unaware of the ability to solder components to flexible circuits.

  • Are there any components that cannot be assembled on flexible circuits?

No! Any component that can be soldered or adhered or wire bonded to a printed circuit board can be attached the same way to flexible circuits. There are some components that require very serious consideration by the assembly team, such as, BGAs and chip on board. However all components can be assembled to a flexible circuit.

  • Can flexible circuits be panelized and automated?

Yes! The flexible circuit panels will require special fixtures or tooling in order to run through the automated lines. But have no fear, they can be mass produced and with astounding results, just as their rigid counterparts are.

When faced with the assembly flexible circuits, it would do you well to consult with an experienced manufacturer during the design stage. They can assist with panel layout and any fixture designs necessary to ensure that the assembly process goes as smoothly as any printed circuit assembly.

Today in Electronics History

October 20, 2014 in Industry News

TI announces 1st transistor radio, October 18, 1954

Texas Instruments announced plans for the Regency TR-1, the first transistor radio to be commercially sold, on October 18, 1954.

The move was a major one in tech history that would help propel transistors into mainstream use and also give new definition to portable electronics.

TI was producing germanium transistors at the time, but the market had been slow to respond, comfortable with vacuum tubes.

However, the use of transistors instead of vacuum tubes as the amplifier elements meant that the device was much smaller, required less power to operate, and was more shock-resistant. Transistor use also allowed “instant-on” operation because there were no filaments to heat up. Read more:

Fitness and Health Drives Wearable Sensor Market

October 17, 2014 in Industry News

Driven by rising demand for fitness and health monitoring features as well as by improved user interfaces, shipments of sensors used in wearable electronic devices will rise by a factor of seven from 2013 through 2019, according to IHS Technology.

The worldwide market for sensors in wearables will expand to 466 million units in 2019, up from 67 million in 2013, as presented below. Read more:

Flexible Circuits vs. Traditional Printed Circuit Boards  “Mechanical Considerations”

Flexible Circuits vs. Traditional Printed Circuit Boards “Mechanical Considerations”

October 15, 2014 in Design Tips

Flexible circuits are usually considered for a product when a mechanical designer encounters interference or connectivity issues using a traditional printed circuit board. Other reasons exist for the decision to use flexible circuits that include overall weight, the “smaller is better” requirements we all face today and the ability to replace two or more circuit boards with connection cables with one circuit. Once the decision is made we must address the mechanical characteristics of the flexible circuit compared with the traditional printed circuit board.

In order for a flexible circuit design to be fully compatible it must be able to carry all the pertinent electronic components and solve the mechanical issues encountered. The inherent flexible nature of these circuits, cause many to question the reliability and robustness of this type of design. It’s understandable given the flexible circuit’s small footprint in the overall printed circuit industry. There just isn’t as much information available as there is for traditional printed circuit boards. Let’s look at a few concerns and address them one at a time.

  • Can we attach all of the components associated with our design on a flexible circuit?

Yes! Any component that can be soldered reliably to a printed circuit board can also be soldered reliably to a flexible circuit. There are some exceptions that require special attention, for instance, a ball grid array or chip on board. These components rely on a very consistent flat surface to be soldered correctly. There are several options available to allow these tedious components to be soldered to a flexible circuit. Some are permanent and some are temporary and are in place only during the assembly process. But all components may be soldered to a flexible circuit.

  • Can a flexible circuit be bent or formed to fit into tight places with components attached?

Yes! The solder joint on a flexible circuit is surprisingly fragile. If the circuit is designed correctly to keep the solder joints away from the bend areas a fully functional circuit may be assembled, cleaned, tested and then bent or formed into place without any ill effects. Contract manufacturers experienced in flexible circuit assembly can guide you through the methods typically used to protect those fragile areas and create a robust electronic assembly that is flexible and able to conform to your unique mechanical design.

  • Can a flexible circuit assembly handle excessive vibration in an application?

Yes! Flexible circuits are used regularly in high vibration applications. They are commonly found in airplanes, automobiles, trains etc. These are applications that experience high levels of vibration and must function as intended. Again, when designed correctly flexible circuit assemblies will provide a robust and reliable product even when subjected to high vibration.

  • Can a flexible circuit be mounted with traditional hardware?

Yes! Typically a flexible circuit would be designed with a rigid stiffener that is made of FR4 or G10 (materials used to fabricate traditional printed circuit boards), laminated to it in the mounting areas to afford the use of screws, washers and nuts. In this way, they are mechanically similar to a traditional printed circuit board. You may permanently mount portions of the flexible circuit assembly and leave other portions as is so that it may be bent or formed in to the tight constraints of the application that warranted the flexible design to begin with.

There are many other scenarios that can be addressed, but the takeaway message should be that you can trust the mechanical nature of a flexible circuit assembly to perform well in any application. It requires some expert design and assembly assistance, but in the end, your flexible circuit assembly will be robust and reliable.

Simple trick to measure plane impedance with a VNA

October 13, 2014 in Uncategorized

The question of time vs. frequency, as the most useful measurement domain, has long been a controversial topic.  In some cases, it leads to rather heated discussions.  The argument in favor of a vector network analyzer (VNA), a frequency domain instrument, is that the dynamic range and signal to noise ratio (SNR) of a VNA are much better than they are for a time domain reflectometer (TDR).  The argument in favor of TDR measurements is that they tend to be lower cost and are taken from a direct reading, so there is little to interpret.  Fortunately, most new TDRs can also transform measurements to S-parameters (much like a VNA) and most new VNAs can also transform to time (providing TDR equivalent data).

Having said all of this, the measurement of a PCB plane using a VNA may not be as straightforward as you might expect.   One simple trick makes it easy. Read more:

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