Happy's Tech Talk #3: Photonic Soldering

Printed electronics (PE) continues to be a growing technology. But one of its advantages, as well as a drawback, is that low-cost substrates, like paper, cannot take the temperature of solder paste reflow. Also, the inks need to be cured. One current way to cure the printed inks is with ultraviolet radiation curing, such as that used with solder mask or legend inks.

One innovation is the curing or annealing of printed inks with flash tubes, which produce a high-intensity, broad-spectrum white light as seen in Figure 1.1 You might be familiar with the technology from photography, as electronic flash or strobes. But these, on the other hand, are much larger and much higher in power. You have probably noticed from photography that these electronic flashes produce heat. But this heat is limited to the surface only, so that the inks can be cured or annealed but the substrate remains cool.

Inks cured to the temperature at which they become conductive using IR ovens requires substrates that can withstand these temperatures, like polyimides, ceramics, and epoxy fiberglass. The flash tube’s white light is not drying with temperature; it is drying with the high energy density of electromagnetic radiation (ER). The ER rays penetrate deep into the layer to be dried and excite the molecules there.

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The absorption of the energy happens in a few tenths of a second. The molecular activation vaporizes the solvent or water. Because the radiation energy penetrates deep into the layer, it couples to the pigments and dries them from the inside out, and the undesirable effects on the surface such as bubble formation or blistering do not occur. In addition, ER light can be dosed and applied in a very targeted manner, right to the point.

With this innovative technology, highly conductive patterns, components (resistors/capacitors) and insulators can be cured on substrates like papers, fabrics, or plastics, all in less than a few hundred milliseconds.2 Printing technologies as well as ink types are detailed in Chapter 11 of Flexible Circuit Technology, 4th Edition.3

Photonic Soldering and Sintering
Table 1 provides an outline of different selective soldering techniques organized by how the thermal load is delivered. In these conventional cases, the exposure is confined to the heating area to ensure lower thermal load on the temperature-sensitive parts of the device. Since the heating medium is confined and will need to be moved from one area to another, the speed of processing with these techniques is typically slow.

Light-generated (ER) heating, as in laser soldering, can also be performed by flash tubes, but with the advantage of soldering components not in the line-of-sight. The first of these new soldering systems is PulseForge, developed by NovaCentrix.

With the use of 500-volt power supplies of 30 to 40 KW capability attached to banks of high-voltage storage capacitors and controlled by high-voltage circuits, specially designed flash tubes can now perform the standard lead-free reflow in just seconds and with power usage just 10% of standard reflow ovens.

Figure 2 shows that light-absorbing materials will heat during the light pulse, and cool immediately when the light is removed. Although this is possible to solder with just one pulse, the superior process is to use a pulse-train of pulses (with differing pulse duration and power) so as not to overheat the substrate.

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Happy_Dec_Table1_cap.jpgThermal Profile
The photonic soldering process builds on the process of using high-intensity flashes of broad-spectrum light to heat up a multi-layered stack in a non-equilibrium process. This process was developed as part of enhancing the manufacturability of flexible hybrid electronics (FHE) by sintering metal particle-based inks into conductive traces.

The photonic soldering tools rely on extremely high average power delivery for a xenon gas-filled flash lamp. As such, flash lamps must be water-cooled to prevent runaway heating and detrimental damage to the system under high-duty use. Additionally, the flash lamp system needs to have digital controls to adjust for soldering of different-sized components under various thermal conditions.

SAC-305 (Indium 8.9HF, type 4) solder paste was manually stencil-printed on the copper contact pads. Wet thickness of the applied solder paste was roughly 75 µm. Sulfur-tolerant chip resistors in 0603 packages from Rohm Semiconductors (part number SFR03) were used as the main component (Figure 3).

Areas which absorb parts of the spectrum more efficiently convert the light energy to thermal energy more effectively than other areas and result in a localized temperature increase. The temperature profile of the material being processed can be controlled by varying the timing of the pulses (pulse length and delay between subsequent pulses). The temperature reached can be above the rated temperature of constituent parts of the device stack without damaging them, in part because the heating is noticeably short, and the device stack will revert to ambient conditions soon after the light illumination has stopped.

The soldering process (Figure 3) shows the ideal trade-off between light power density and flash durations. The soldering is accomplished from one to four seconds depending on power density. Power settings from P1 to P9 will induce reflow from 4.5 to 8 seconds, with the shortest at P9 of 0.5 seconds.

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Spatial Selectivity
The photonic soldering process is unique and provides a soldering process for a given material system (substrate, conducting track, solder, and component) that cannot be duplicated in a normal reflow oven. The average power is a function of the energy of a single light pulse (depends on the voltage to which capacitor banks are charged and the length of time for which they are discharged) and the frequency at which light pulses are incident on the material system. Average power is the critical control over the temperature ramp rate achievable for the device stack. While certain device structures can be soldered with extremely high ramp rate, others require a slower ramp rate to preserve device structure and prevent uncontrollable outgassing. The temperature range can be controlled by spatial selectivity for:

  • Thermally-sensitive substrates like LED arrays on PET
  • Thermally-sensitive components like batteries or displays using an aluminum mask with laser-cut openings
  • Thermally-sensitive regions like the reflow under a BGA with conductive heating
  • Thermally-sensitive solder joints like SAC305 reflow in 0.375 seconds
  • Reliability-sensitive lead-free solder joints where the short heating time minimizes intermetallics
  • When low voids are required,

Happy_Dec_Fig4_cap.jpg

For the same structure, the peak temperatures can be manipulated either by increasing the ramp rate, exposure time, or a combination of both (Figure 4). Like standard reflow mechanism, each change provides a different opportunity in optimizing the solder joint quality. For the explored device structure, at an average incident power density of 16 W/cm2, reflow of the solder can be observed starting at 1.5 seconds but will improve the joint quality up to three seconds as reflected through improved fillet shape and intermetallic formation. At five seconds of exposure, we start to observe mechanical failure and buckling of the flex circuit. Figure 5 shows an interior of the photonic soldering equipment. This equipment (Figure 6) comes in a batch and conveyorized unit. A complete automated assembly line of paste-inspection-placement-soldering would be only 21 feet long and have a processing time of only three to four minutes.

Happy_Dec_Fig5_cap.jpgSummary

This new soldering process can accommodate a substrate up to 300 x 400 mm with R2R possible. The new opportunities now possible are:

  • Use high-temperature solders for comparable quality
  • SAC-305, SnSb, etc.
  • Use of temperature-sensitive substrates for lower costsSolder multiple sized components at once
  • PET, TPU, PVC, PPE, PEI, PVF, PEN, etc.
  • Potential for R2R handling
  • Achieve comparable results to reflow ovens but much faster
  • Works equally as well with FR-4 and other traditional boards but with a smaller footprint
  • Allows soldering on aluminum
  • Will work with no direct line-of-sight
  • Provides soldering on curved surfaces
  • Provides flexible/alternative product design options
  • No thermal stress on stacked microvias
  • Lower energy requirements
  • Selective control of soldering parameters
  • Optional N2 processing area

Happy_Dec_Fig6_cap.jpg 

References

  1. “Use of Flash Lamps to Achieve Non-Equilibrium Soldering and Assembly Utilizing Conventional SAC Alloys,” by Vahid Akhavan, SMTA-International 2020 Proceedings #567, Chicago, Sept. 2020.
  2. “PulseForge Flash Lamp Soldering using Conventional Soldering Alloys,” by Rick Larson, SMTA Boston/Nutmeg Webinar, Sept. 14, 2021.
  3. Flexible Circuit Technology, 4th Edition, by Joseph Fjelstad.
  4.  “Photonic Soldering Temperature Sensitive Components with High Temperature Solder Alloys,” Vahid Akhavan, #950, Proceedings of the 2021 SMTAI Conference, Minneapolis, Minnesota.

  This column originally appeared in the December 2021 issue of PCB007 Magazine.

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2021

Happy's Tech Talk #3: Photonic Soldering

12-20-2021

Printed Electronics (PE) continues to be a growing technology. But one of the advantages, as well as a drawback is using low-cost substrates, like paper, that cannot take the temperature of solder paste reflow. Also, the inks need to be cured. One current way to cure the printed inks is with ultraviolet radiation curing, such as used with solder mask or legend inks.

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Happy's Tech Talk #2: Induction Lamination

11-23-2021

Multilayers have been around about as long as the printed circuit. The industry has always used heated hydraulic lamination presses to produce these multilayers, with the introduction of vacuum assist in the 1980s. But recently, with the encouragement of GreenSource Fabrication, induction lamination has been perfected by Chemplate Materials of Spain. Chemplate had introduced the use of induction-pinning by optical alignment of innerlayers for multilayer stackup in the early 2000s. This was to go with another innovative way to laminate innerlayers together—the Italian CEDAL resistance-foil vacuum-press, which had some early adopters.

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Happy’s Tech Talk #1: Vertical Conductive Structures (VeCS)

10-22-2021

The industry has not had many new structures in the last 60 years. Multilayers have continued to evolve with thinner materials, smaller traces / spaces as well as drilled vias. It’s been nearly 40 years since Hewlett-Packard put their first laser-drilled microvia boards into production for their innovative Finstrate process.

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2017

Happy’s Essential Skills: Tip of the Month—The NIST/SEMATECH e-Handbook of Statistical Methods

07-05-2017

In the 1990s, the National Bureau of Standards was distributing a popular statistical document, the Handbook 91, written by Mary Natrella of the NBS Statistical Engineering Laboratory. A request by Patrick Spagon of the Statistical Methods Group of SEMATECH to update the NBS Handbook 91, Experimental Statistics, led to the creation of a project team from NIST and SEMATECH to create a new web-based statistical handbook including statistical software.

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2016

Happy's Essential Skills: Understanding Predictive Engineering

12-16-2016

New product realization and design for manufacturing and assembly (DFM/A) have now started to become more visible as programs that can improve a company’s time-to-market and lower product costs. Many programs are underway by many companies and what is now needed is a framework to coordinate the application of these programs. This column will cover the interactions of DFM/A and the need for development of a new framework to coordinate the trade-offs.

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Happy’s Essential Skills: Technology Awareness and Change

11-22-2016

From Happy Holden: A long-time printed circuit-industry friend of mine, Martin Tarr, an instructor at University of Bolton, UK, is a leading expert on change. He wrote an excellent tutorial for his university course on electronics manufacturing. With permission from Tarr, I am including a portion of it here as the basis of this column, starting after the graph in Figure 2. But first, a few thoughts of my own.

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Happy's Essential Skills: 10-Step Business Plan Process

11-03-2016

It takes more than just a good idea to exploit that brainstorm of yours. Hewlett Packard’s “10-Step Business Plan Process” is the format to present an idea or product in a fashion that will answer most questions that management may have about a product or idea.

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Happy's Essential Skills: Lean Manufacturing

10-19-2016

Lean doesn’t have to exist in manufacturing alone. Lean is a fairly recent principle that can apply to all of our goods and services. For those of you not familiar with Lean, I recommend the free E-book "Survival Is Not Mandatory: 10 Things Every CEO Should Know about Lean" by Steve Williams, a regular columnist for I-Connect007.

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Happy's Essential Skills: Metrics and Dimensional Analysis

10-05-2016

After 20 of my columns, readers probably realize that I am an analytical person. Thus, I dedicate this column to metrics—the method of measuring something. I mentioned the four levels of metrics in my June column "Producibility and Other Figures of Merit." I also introduced the five stages of metrics in the second part of the column "Design for Manufacturing and Assembly, Part 2." This column completes the discussion with a look at dimensionless quantities.

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Happy’s Essential Skills: Recruiting and Interviewing

09-29-2016

Hopefully, your career has progressed to the point that you are empowered to recruit your own team or a key person for your team. There are always technical people looking for better jobs, but many times, the most talented are busy doing their work and not looking for a new opportunity.

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Happy’s Essential Skills: Computer-Aided Manufacturing, Part 2 - Automation Examples

09-22-2016

Semiconductor fabs like to avoid writing custom software to fit all of the idiosyncrasies of individual processing systems. So HP developed PC-10 to handle IC process equipment by separating it into general classes. SECS II was a mandatory prerequisite of the equipment before an interface to PC-10 could be developed.

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Producibility and Other Figures of Merit

06-10-2016

Metrics are data and statistically backed measures. It is always expedient to base decisions on data and metrics, for example, in PCB design. These measures can be density, first-pass yield connectivity or in this context, producibility. These measures are the basis for predicting and planning a printed circuit design. But what if a metric doesn’t exist? Then you can create the next best measure, the Figure of Merit

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Learning Theory/Learning Curves

06-01-2016

Learning is not instantaneous! Nor is progress made in a steady manner, but at a rate that is typified by one of two basic patterns. In some cases, plateaus will be seen in learning curves. These are caused by factors such as fatigue, poor motivation, loss of interest, or needing time to absorb all the material before progressing to new. This column will not go into details of how learning is achieved, but will summarize some of these theories.

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Happy’s Essential Skills: Project/Product Life Cycle

05-18-2016

The product, and or project (process) life cycle (PLC) is fundamental to a corporation intent on developing new products or processes. It sometimes is called the new product introduction (NPI) process but that is only half of the life cycle. There is product support, enhancement and eventually, obsolescence.

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