Analysis of Fog in LED Bulb Shell

Introduction to LED bulb lamp

As one of the more mature LED lighting applications, LED bulb lamp has entered the stage of large-scale production. LED bulb lamp is mainly composed of lamp bead and base plate, heat dissipation shell, bulb shell, driver, lamp cap, etc. In the assembly process of LED bulb lamp, the silicone materials that can be used are shown in the following figure:

One component RTV adhesive is generally used for bonding aluminum shell and foam shell; The safety product model is AP-602L; For heat conduction between lamp beads and aluminum substrate, heat conductive silicone grease, heat conductive film and heat conductive silica gel are generally used for heat conduction between PCB and aluminum substrate; The safety product models are AP-505, Tc Pad AP-15XX, AP-607, etc; Generally, the lamp body is filled with additive heat conduction sealant, and the safety product model is AP-905.

With the expansion of production scale, product quality is particularly important. Here is a brief analysis of the fog phenomenon that often occurs in LED production, hoping to be useful to everyone.

Cause Analysis of Water Mist Formation in LED Bulb Lamp

If the "water mist" is water, it is likely that the temperature of the assembly environment is too high. During assembly, the moisture is trapped in the housing. After the LED is lit, the temperature difference between the inside and outside is different. The moisture condenses in the housing and forms water mist attached to the mask (the same reason as there is water mist on the windows in winter). To solve this problem, it is necessary to dehumidify the assembly workshop, control the humidity of the workshop, and dehumidify the workpiece if possible. In the absence of dehumidification equipment, the simplest method is to blow dry the housing/LED lamp holder with a hot air gun, and then assemble it. Also, if the use environment of the lamps is heavy with moisture, it is necessary to consider the structural waterproof.

If the "water mist" is other chemical substances, it is the volatile matter of thermal conductive silicone ester or sealant, which is formed by heating and evaporation attached to the mask. The "oil yield" and "volatility" of the thermal conductive silicone ester should be considered when selecting the model. The thermal conductive silicone ester with poor quality will precipitate silicone oil at the operating temperature of the LED (some worse, a layer of transparent silicone oil can be seen on the surface when the can is opened); Use glue (silica gel/epoxy/acrylic acid structural adhesive can be used) to seal and bond. It is important to wait until the glue is completely cured before aging. The glue may volatilize when heated before it is completely cured, and the blister may crack. In the LED lighting industry, silica gel is the most commonly used for the cover of sticky bulb bulb lamp, candle lamp and panel lamp. Its curing principle is to reverse generate vulcanization crosslinking reaction by absorbing water in the air, and produce by-products at the same time. According to the different by-products, it can be divided into deacidification type, deaximation type and dealcoholization type. The larger the molecular weight of the small molecules, the easier it is to come out in liquid form and attach to the inner wall of the outer cover to form water mist. The molecular weight: acid>oxime>alcohol, so the dealcoholized type should be selected during the selection to control the optimal dispensing amount through quantitative dispensing, so as to minimize the impact of volatile matter on the lamps.

The small molecules in the cured glue will also volatilize under high temperature, forming a layer of mist like substance on the mask, which will affect the appearance, brightness and other properties of the lamp. Common small molecular substances are mainly divided into low molecular cyclosiloxanes and low molecular linear substances. Generally, the total amount of D3-D20 is used to represent the content level of small molecules.

The test method of low molecular polysiloxane is gas chromatography-mass spectrometry. Generally, the content of low molecular polysiloxane in silicone rubber must be strictly controlled for the encapsulation and sealing of electrical, electronic, communication equipment, instruments and optical components used in aviation and aerospace, as well as the packaging materials used as insulation, shockproof, moisture-proof and conductive links, and the coating glue on PCB boards, so as to avoid equipment failure or pollution.