Addressing Temperature Challenges in Flex Circuit Rework


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During NEPCON China 2018, I met with Zen Lee, technical director, and Michael Gouldsmith, sales and marketing director of Thermaltronics, a manufacturer of soldering products and rework equipment. We discussed the challenges of flex circuit assemblies, especially during the rework process. They highlighted the power-on-demand feature of smarter hand soldering systems, as well as how Curie Point helps operators avoid temperature overshoots during rework.

Stephen Las Marias: What are the top challenges when working with flexible circuits?

Zen Lee: The most important consideration is that the parts are very small. You need to have a fixture, since you cannot hold the flex board. The tip needs to be of the correct geometry, for the application and you need address, (touch), and the joints in the correct manner – in other words – “not heavy handed”. Basically, you cannot fool around, when doing this kind of rework, because the real challenge is the potential for delamination of the flex circuit. Good solder joints are essential —you have to take away the energy from the tip and give it to the joint, but you don’t want to overshoot. Otherwise, you will also have solder splash issues, on top of delaminating the flex circuit. That’s the problem.

Las Marias: When reworking flex circuits, what are the key issues to consider?

Lee: I think it is the control issues. Because if soldering is done by hand, the operator needs training as well as the knowledge to select the right soldering tip.

Michael Gouldsmith: One of the key points Zen is making is concerning the tips we manufacture, which feature a unique technology called Curie Point. There is no overshoot in temperature; we maintain the temperature very well. It’s also a power-on-demand basis. So, if there’s enough stored energy in the tip, unlike a conventional soldering iron, which is providing constant power, there’s less chance of delamination, less chance of solder splash, less chance of rework problem, and losing the flexible circuit.

Las Marias: What are the best practices to consider when dealing with flexible circuits during rework?

Lee: Ideally, it should be one touch, and you make the joint. That means you cannot use the tip to do it twice, or many times. Too many touchups, and you will have a problem, because the joint is very small, and the tip geometry is very small, and of course, as I said previously, the operator needs good manual soldering skills.

The tool, the tip cartridge or soldering iron, is an equipment issue. You need to select correctly the equipment first. The second part into making a good rework is the operator. You need to have both. If you have a good operator, but you don’t have good equipment, you will get delamination. If you have good equipment, but not a good operator, you will still have a problem. So, it’s the combination of the human factor and good soldering iron.

Gouldsmith: It’s a combination of two skill sets: the skill sets of the operator and the quality and performance of the tip. Because we use the Curie Point, there is no chance for any of the problems that occur with conventional soldering irons. With conventional soldering irons, you have the potential for overshoot of temperature, constant power, instead of “power on demand”, and as a result delamination or solder splash.

Las Marias: Do you think there will always be an issue when dealing with flex circuit assemblies?

Lee: Yes, always. Once they use a flexible printed circuit, that means assembly issues, that means space issues—they need tight solutions, so they use flex circuits. That means the joints and components are very small.

Las Marias: In which markets do you see the use of flex circuits increasing?

Lee: Mobile phones, cameras, whatever device that’s getting smaller and thinner. They’re always going to need flexible circuits.

Las Marias: Great. Thank you both very much.

Zen: Thank you.

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