|Ask the Experts|
September 24, 2020
Dam & Fill vs. Conformal Coating
Are there any special reasons or advantages to use Dam & Fill compared to conformal coating? Is Dam & Fill primarily used to provide additional mechanical support?
|Expert Panel Responses|
Dam and Fill is generally an encapsulant that utilizes epoxy. This is generally used when product has mechanical or thermodynamic properties, that utilize a high viscosity material or you are needing to mechanically hold a dimensional size.
The conformal coating process is barrier to protect products. This is selected as result of what the assembly is subjected to. This is generally function of external environmental elements and generally the coating is selected on its cost and ability to protect.
Our systems can perform Dam n Fill and/or Conformal Coatings, pending the customer requirements with simple review of the product we can offer recommendations to meet the designer and manufacturing objective.
Capital Equipment Operations Manager
Specialty Coating Systems
Yes the Dam & Fill provides mechanical support to ensure proper acceptable fillets. Conformal coating is use as moisture resistant over the entire board, covering the components and leads. They are different, the first is process application and the second is design.
President and Founder
Contract Manufacturing Consultants, Inc.
A dam & fill strategy may be driven by a need for stronger environmental protection, for high voltage isolation, for mechanical stabilization of components, security, among other possible reasons.
From an environmental standpoint, normal conformal coatings provide only moderate,temporary protection against moisture. Potting provides much more complete coverage (especially if vacuum-encapsulation is used), and can provide superior environmental protection.
Depending on the material chosen, potting can also provide mechanical protection, especially from shock, however it can also be a problem if it creates forces on components during thermal cycling.
Dam & fill application is used for a variety of reasons.
Some are purely aesthetic in nature, ie to give the area a molded finish when volume doesn't justify mold equipment. As an encapsulant, dam & fill products are generally more viscous than many/most conformal coatings. This gives two advantages.
The first is that it is generally stronger and more forgiving to mechanical shock than conformal coatings. The other advantage is that unlike many coatings,these products don't capillary or weep under the components. In fact, simple damming is sometimes used around specific components when applying coating to the assembly for just this reason.
All in all, the nature of these encapsulants inherently protects the fragile areas from mechanical shock better, in a general sense, than typical conformal coatings. As a rule, most conformal coatings are not tested to a set mechanical shock standard (this may change in the near future I've heard) during typical property test protocols such as IPC-CC-830B.
Unless the coating manufacturer can provide some assurance of coating protection against vibration& shock, it's dicey at best to pick one.
Silicones coatings are known for their flexibility and resistance to mechanical shock, so that may be an area to consider other than true encapsulants. There is an excellent publication on dam & fill applications available - here's the link. http://www.nordson.com/en-us/divisions/asymtek/Documents/Papers/1999_02_damfillencap_NepconWest_NordsonASYMTEK.pdf
Dam-and-Fill materials and techniques are normally used to encapsulate a wire bonded device as an electrically insulating material. Examples include dispensing a high-viscosity dam followed by a low-viscosity fill. This will create a completed encapsulated package such as CSP and BGA which protects the device from damage to fragile wires etc.
However, the use of a conformal coating (liquid or parylene) to protect this device is possible. I would guess that there would be two reasons not to encapsulate. The first is cost, since there may be some savings and the second is rework, since it would be easier to get to the device to repair.
But you will no longer have the mechanical strength of the encapsulate and the fragile wire bonding's could be subject to damage. There is some evidence somewhere that coatings do add some mechanical strength to PCBs coated in terms of thermal cycling tests but it is nowhere near the same level as encapsulating the device.
Also, there will be poorer coverage protection against water,condensation and moisture, however well the coating is applied (in the case of liquids), since it is only a thin film application versus total encapsulation. That said, if you can satisfy yourself that mechanical damage due to factors such as impact or flexing can be avoided, and the coating does provide the protection you need, then it could work fine.
Dam & Fill applications were designed for "Chip on Board" applications. Chip on Board technology is where a semiconductor wafer chip is placed on a PCB directly and wire bonded to pads on the circuit board.
In this type of application the Dam material is applied around the chip with the Fill used to flow around the wire bonds for protection. The Dam material is necessary to hold the low viscosity Fill material in place so that the wires are completely covered.
Conformal Coating applications are more used to protect standard PCB assemblies from moisture and not used as a mechanical attachment or protection. Low viscosity Fill materials are necessary to flow around the wire bonds without moving or damaging the interconnecting wires.
Conformal Coating materials would likely be two high in viscosity for wirebond type applications, thus damaging the wires while being dispensed. Conformal Coating comes in several different type of chemistries, most commonly Silicones, Epoxies, and Urethanes with the main purpose to protect against moisture and humidity.
Dam& Fill is generally used for mechanical protection of flip chip, BGA or wire bonded chips. It does also give some protection from moisture. They are also generally epoxy based so tough not easily removed.
Conformal coatings will give protection from moisture and chemical environments depending on the type selected.
HumiSeal produce a range of conformal coating gels that are used to seal the base of connectors before coating or to put a bead of material on the board surface as a barrier to prevent flowing of coating into keep out areas. Some people also use the gels to create dam and fill areas of conformal coating!
The advantage over dam and fill with epoxy is that you get better moisture protection and the coating can be easily removed.
Exactly Chris! We have been using a Dam at the bottom of Mil Connectors to prevent the wicking of conformal coating from the bottom of the connector. The dam enables us to get coating right up to the sides of the connectors without any encroachment to the pin surface from underneath.Joe Racz CQE, CQA, CIT, Precision Graphics Inc., USA
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