Medium and small power TOP LED silicone selection

[Source: "High-tech LED - Technology and Applications" October issue]

Surface-mounted LED status

The surface-mount LED, that is, the TOP LED has been greatly promoted in recent years due to its compact structure, diversified design and good stability. Silicone is consistently recognized on TOP LEDs due to its low stress, high stability, and high and low temperature resistance. However, in the past two years, feedback products on the market have often found problems with blackening, light decay, color temperature fading, and even dead lights.

The following is a detailed analysis of the reasons why one of the cases 3528 caused the LED to fail during the application process:

1. Analyze the blackened sample of the 3528 functional area and find that the blackening is located on the surface of the functional area and the silicone joint;

2. Carefully observe the bottom PPA and the silica gel, and the black surface of the silica gel and the PPA cup wall is not black;

3. Through the analysis of EDX and SEM elements, the part where the stent is not black does not contain sulfur, and the sulfur content of the blackening phenomenon of the stent accounts for 2.24%;

4. By EDX analysis, Si, Ag and Cd elements were detected in the unblackened parts of PPA and silica gel. In the black area of ​​the functional area, Si, S, Cu, Ag, and Cd were found, Si was deduced as silica gel, Cu, Ag, and Cd were used as scaffold materials and electroplated layers, and S was infiltrated into the package.

5. It is well known that LED materials, including brackets, PPA, wafers, phosphors, and glues are all environmentally friendly materials and contain no sulfur. It is judged that this blackening phenomenon is caused by S infiltration from the outside and reaction with Ag to produce black Ag2S.

The process structure of the TOP LED bracket is: C194 copper stamping - silver plating - injection molding PPA - bending forming. Therefore, the sealing of the TOP LED mainly relies on the anti-seepage design of the stent and the sealing performance of the silicone. PPA: The Chinese name is "poly-phthalamide/polyphthalate", the English name is "Polyphthalamide", which is a thermoplastic similar to nylon. PPA plastics have a hot profile temperature of over 300 ° C and a continuous use temperature of 170 ° C. They meet the short-term and long-term thermal properties required. They maintain their superior mechanical properties over a wide temperature range and in high humidity environments. -- Strength, hardness, fatigue resistance and fire resistance. However, the PPA is physically joined to the stent and does not achieve the desired sealing performance. Therefore, choosing the right silica gel is an important factor in determining the LED's ability to meet the corresponding quality requirements.

Silicone selection

According to its vulcanization temperature, silica gel can be divided into high temperature (heating) curing type and room temperature curing type; it can be divided into solid silicone rubber and liquid silicone rubber according to product form; it is classified into single component and two component according to packaging ( That is, AB gum can be divided into two types: condensation reaction type and addition reaction type according to the crosslinking reaction mechanism. According to the cross-linking reaction, the morphology can be further divided into three types: silicone rubber, silicone elastomer and silicone gel. Liquid silicones commonly used in LEDs are heat-cured, addition-molded silicone rubbers, silicones and silicone gels. Silicone has excellent resistance to high and low temperature, weathering, ozone, arc resistance, electrical insulation, resistance to certain chemicals and physiological inertness. It is suitable for LEDs, and its existence of breathable and moisture permeable properties brings hidden dangers to LEDs. So how do you choose silica gel for use in TOP LEDs?

We take the silica gel of ShinEtsu of Japan as an example to discuss the problem of solving TOPLED anti-vulcanization. The evaluation of Shin-Etsu Silicone in the market is a high-end brand. It is also aware of the anomalies caused by scaffold vulcanization. It has developed SCR series organically modified silicone resin for vulcanization design and verified the anti-vulcanization effect.


For more information, please refer to the October issue of "High-tech LED-Technology and Applications"

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