Soldering onto flexible substrates has been a challenge because even standard bismuth-based low temperature solders are not compatible with substrates like PET or even heat stabilized PET which cannot tolerate high temperatures.
To overcome this challenge, many deploy conductive adhesives. This is a good solution but has several shortcoming: (1) one misses the automatic self-alignment feature of solder which is an essential feature in SMT processes; (2) conductive adhesives can contribute to overall resistivity, putting flexible hybrid electronic further beyond standard PI-based FPCB techniques (lower conductivity of printed ink vs bulk copper plus lower conductivity of conductive adhesive interconnects vs standard solder); and (3) narrow pitch sizes will be difficult to support.
Digital thermal processing developed by Pulse Forge Inc (spun off from NovaCentrix) offers a solution. As shown below, a rapid pulse of light raises the temperature of the surface of the substrate very fast, whilst the substrate itself remains relatively cool, allowing one to sinter inks on low-T temperatures such as PET and paper.
This feature has been extensively used in connection with printed inks. Incredibly, it has recently been demonstrated to also work with solder. The second slide shows how the PulseForge technology can in less than a second reflow standard SAC305 solder, creating good joints and also benefiting from solder's automatic realignment feature.
Next slide shows how the PulseForge technology can be deployed to solder on Al on PET, enabling, for example, R2R production of LED foils on Al metallized PET substrate.
Interestingly, this technology can also be applied onto FR4 substrates. Here, there are two crucial benefits: (1) rapid reflow in just a few second (1-3s), saving time (standard reflow process can be 235C for 120s, for example), and (2) low energy reflow at 10% of the energy required for standard reflow ovens, making the process 'greener'.
The slide below shows that the shear strength of the solder joins made with intense pulse light technology and the standard reflow oven technology are comparable. The next slides shows that the joints are of a high quality with very low void content and that a good thin intermetallic layer is solder after pulse light reflow.
Can one can solder joints where no direct line of sight exists? Results on QFN and other packages where joins are not directly visible demonstrate that it is possible, although it will, in our guess, require notable optimization.
In fact, our guess is that significant operator know-how is required to optimize exposure parameters based on solder, substrate, and packages on a board to enable intense pulse light soldering, as it is still a non-standard SMT reflow technology with a new learning curve.
Note that the inline versions of the PulseForge machines can handle 300m wide substrates.
These are fantastic results. The tool solves an important problem in flexible hybrid electronics. It can also certainly make a meaningful impact in general SMT business on standard substrates like FR4 given its rapidity and low-energy nature. The impact in the SMT world will not be overnight though as the technology still has to prove and develop itself further to become a standard process, especially if it ever wishes to be a drop-in replacement for the well-established incumbent reflow which can handle all solders on complex large-sized boards containing a vareity of IC and joint types.
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