“Will exposing areas of a PCB board that are not covered by an adhesive or conformal coating to UV curing energy damage the board in anyway?”

Any frequency of energy that gets impinged onto a substrate has two options: it can get reflected, in which case it is harmless, or it can get absorbed, in which case it will turn into heat. Some light-curing equipment, like the Dymax BlueWave^® spot lamps, have an intensity adjustment feature which allows for the ability to reduce unneeded energy. Adjusting the intensity reduces any excess energy that may turn into heat and damage temperature-sensitive substrates.

Adhesives, Curing Equipment, Electronic electronics assembly, Light Curable Adhesive, Light Curing Process, light-curable adhesives, PCB coating, uv curing, uv curing lamps, UV/Visible Light Curing Adhesive

“I need to find a UL-listed conformal coating. Can you tell
me a little bit about the UL designation and do you have any conformal coating materials with this rating?”

Underwriters Laboratories^® (UL) is a global independent safety science company offering expertise across five key strategic businesses: Product Safety, Environment, Life & Health, University, and Verification Services. UL has been working for a safer world since 1894 and partners with manufacturers to provide safe products to consumers through UL testing, certification, and follow-up audits.

DYMAX has many conformal coatings that are UL recognized; they can be found on the DYMAX website and are denoted with the UL marking. To retrieve the most up-to- date information on DYMAX conformal coatings, log onto the Underwriter Laboratories website www.ul.com, click on Certifications, and input DYMAX’s UL File Number, QMJU2.E140512.

Conformal Coatings, Electronic Conformal Coatings, UL Listed

“Can someone suggest a material to use for bonding a Ball Grid Array (BGA)? We currently use 3M™ Scotch-Weld™ Epoxy Adhesive EC-2216 A/B. Also, will corner bonding a BGA prevent failure from vibration? “

There are a number of different adhesive technologies available for corner bonding. One of the newer technologies is light-curable adhesives. The adhesive is applied to the circuit board post-reflow and cured in seconds. Flow is engineered to wet the edge of the component while minimizing flow underneath. This is important for BGAs where the outside edge of balls can be very close to the edge of the BGA. It’s also needed to ensure all material gets exposed to light and cures. If fast process speed is critical for the application, this technology may be worth a look. DYMAX 9422-SC was specifically designed for corner bonding. More information on this product is available in Lit 244 - Leadless Component Ruggedization.

As for your question regarding vibration, there is no published data specifically on this issue. The most common reason for using corner bond is to increase reliability for drop testing. We have seen instances where drop tests went from less than a 50% passing rate to 100% with the adhesive in place. The secondary benefit is higher reliability through thermal shock.

Electronic 9422-SC, Ball Grid Array, electronic adhesive, light-curable adhesives, ruggedizing adhesive

Tackiness or stickiness may be noticed on the surface of some ultraviolet (UV) light curable adhesives and coatings. This phenomenon, known as oxygen inhibition, is the result of atmospheric oxygen inhibiting the cure on the surface layer of the polymerizing material. This condition is present anytime free radical polymerization occurs. However, the ability of a UV resin to be cured "tack-free or to a slick, dry finish" is dependent on the composition of the adhesive or coating formulation and the intensity and wavelength of the UV light.

Overcoming the effects of oxygen inhibition and producing a tack-free surface cure is dependent upon several factors:

• Heat generated by the UV curing system
• Intensity generated over the entire UV spectrum (200-390 nm)
• Exposure time
• Specific formulation of the adhesive or coating

Typically, short and medium wavelength (220-320 nm) UV light generated by mercury vapor lamps achieve more efficient surface cures. Short and medium wavelength curing systems, however, do have depth of cure and safety issues associated with them making them an undesirable option for many UV curing applications. Longer wavelength (320-390 nm) systems, which usually emit a small fraction of UV light in the lower wavelengths, will provide fast, tack-free curing while achieving better depth of cure.

Time to cure “tack free” should not be confused with full cure time. It is only an indication of the material’s ability to overcome oxygen inhibition, at the surface of the curing material, when the material is exposed to a given level of light intensity for a specific period of time. It has been demonstrated that the higher the intensity of the UV light the lower the total energy level needed to achieve a "tack free" surface. For example, to produce a tack-free surface cure of a DYMAX conformal coating (984-LVUF) using a 200 mW/cm² light source, the coating should be exposed for 20-30 seconds. This equals approximately 7 J/cm² of energy. This same coating cures in 1-2 seconds when exposed to 2500 mW/cm² equaling 2.5 J/cm² of energy.

Even though removing oxygen from the surface will also work at achieving a tack-free surface, this tends to be the least desirable method since it can be logistically challenging to implement.

Adhesives, Coatings, Conformal Coatings, Dome Coatings, Electronic Adhesives, Conformal Coatings, Dome Coatings, Oxygen Inhibition, Tack-Free

As electronic assemblies get smaller and more widespread, conformal coating is becoming increasingly popular - because it can make the difference between premature failure and successful operation throughout required lifespan of the device. Conformal coatings are often regarded as a vital component of a circuit assembly. While they may not always perform a particular electrical function, they are the most effective way to increase circuit lifespan in harsh or rough service environments.

Use of conformal coatings is expanding. Growth areas include mobile devices, where smaller circuits have to withstand shock and elevated humidity levels, automobiles, where sensors are progressively positioned throughout new vehicles and medical devices, where functionality is critical.

Coatings, Conformal Coatings, Electronic Conformal Coatings