Improving Aesthetics and Reflectivity in LED

Posted 12/7/2015 by Amit Patel

Joints soldered with cored wire are consumed in a variety of lighting applications. Most commonly, these spools of cored solder wire are used to make the connections between the control / power driver board and the LED engine board shown in Figure 1, to make board to board connections for linear lighting and for soldering the end cap contact which serves as the electrical gateway for retrofit based bulbs as shown in Figure 2.

Figure 1: LED Bulb Anatomy

 

Figure 2: Side and top view of a soldered end-cap

In general, flux residues created during the cored wire soldering process can be cleaned. For the traditional electronics industry, cleaning is driven largely by the need to maintain or increase functional performance of the total system. One example is that certain flux chemistries need to be removed post assembly because they are not benign. Additionally, aesthetics may further drive the need to clean the residues to maintain a certain level of aesthetics despite providing little to no functional performance improvement for the assembled system.

               However, for the LED lighting assemblies the implications for aesthetics of a soldered joint can have a much larger impact to the point where the functional performance and aesthetics converge. The need to remove the subjective nature of good versus bad cosmetics by quantifying this attribute can provide value for a LED lighting assembly manufacturer when qualifying cored solder wires.

Below are just a few examples where aesthetics and performance converge.

  • A significant percentage of LED assemblies use a white solder mask.
    • White color grade solder mask provides brightness maintenance.   
    • Lighting industry typically requires >90% white solder mask reflection.
    • Maintaining this high reflectivity by minimizing any browning of the solder mask is highly desirable.
  • LED light engines are sensitive optical modules.
    • Excess flux residues from the cored wire soldering on the LED board (while making the connection between the LED board and power driver board) are not desirable, since they can impact light output by interfering with the reflectivity of the white solder mask.
  • Visually Exposed Solder – Edison End Cap (Shown in in Figure 2)
    • Uniquely, LED based retrofit lamps have a visually exposed solder joint – at the base of the end cap.
    • Using cored wire solder with minimal and clear flux residue helps in minimizing cleaning, provides good aesthetics and lowers potential for corrosion.
    • The end cap is a pivotal node in the anatomy of the LED lamp. It serves as the electrical gateway for the lamp
    • LED lamps can last 15+ years and thus, issues such as corrosion influenced by excess flux residue need to be minimized.

 

Inspite of the fact that aesthetics and performance do converge in case of LED assemblies, these attributes are still visually and qualitatively evaluated.   Such subjective evaluation may yield inconsistent characterizations.  One way to quantify and objectively determine good versus bad cosmetics is by using the metric of reflectivity.

 

A novel method was developed by Alpha® to quantify the aesthetics (typically referred to as “shininess” of the joint).  Two cored wires within the family of tin-copper alloys with dissimilar fluxes were measured for their reflective values against a calibrated NIST grade mirror.

  • Alpha® Telecore HF850 Tin-Copper Alloy
  • Reference Cored Wire Tin-Copper Alloy

 

The results are shown in Figure 3.

 

Figure 3: Comparison of Alpha® HF850 and Incumbent Cored Wire

 

The results show that Alpha® Telecore HF850 SnCu cored yielded 180% higher reflectivity when compared to the reference cored solder wire for three different solder iron tip temperatures.  Further, increasing the tip temperatures resulted in a drop in reflectivity for the reference cored wire (which is mostly explained by the burning/browning of the flux at higher temperatures) whereas Alpha Telecore HF850 SnCu produced consistent levels of >20% reflectivity at increasing iron tip temperatures.

In summary, it is possible to develop a system of measurement that helps us objectively characterize aesthetics with reflectivity as a measure, specifically for solder at a resolution that can detect differences from the flux residue alone.  The implications for this can be significant for the LED lighting community.  The added-value of quantifying the reflective property of a soldered joint using cored wire allows manufacturers to:    

 

  • Compare cored solder wire performance by simply by ranking their cosmetic output under a stricter quantitative metric.
  • Determine if end-caps need to be cleaned or not.
  • Measure the impact a given cored solder wire has on light output from a given LED light engine module soldered to a power driver board.

 

Find out more: “Link to white paper


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