how to simpli

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T E C H S P O T L I G H T

How to Simplify Potting

Master Bond Inc. 154 Hobart Street, Hackensack, NJ 07601 USA

Phone +1.201.343.8983 | Fax +1.201.343.2132 | WhitePaper@masterbond.

A guide to getting the most out of epoxypotting/encapsulation compounds


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Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | [email protected]

How To Simplify PottingGet the most out of epoxy potting/encapsulationcompounds

Potting compounds are too often an afterthought in the

design of electronic devices. They shouldnt be. These

encapsulating systems play a crucial role in the assembly

and long-term protection of delicate electronic components.

When properly selected and applied, potting compounds

form the first line of defense against a wide range ofenvironmental, chemical, mechanical, thermal, and electrical

conditions that would otherwise ruin electronic components.

When not properly selected and applied, potting compounds

may not offer the desired level of protection. Even worse, a

poorly chosen epoxy may cause some damage of its own

by curing in such a way that subjects the potted electronic

component to unwanted stresses or heat.

Though there are encapsulants based on polyurethane,

silicone, and UV-cured acrylic, most potting applications

still rely on epoxy compounds. Hands down, epoxies have

the balance of mechanical, thermal, electrical, chemical andadhesion properties.

The design tips that follow can help you avoid the

more common pitfalls associated with the selection and

application of epoxy potting compounds:

Fist Ask, Why Ecapsuate?

Start by asking fundamental questions about the purposeof the encapsulation. Does the potting compound

perform a thermal management role? Should it protect

against aggressive chemicals or moisture? Does it protect

against shock loads? Are optical properties important?

Will the potting experience high temperatures during the

assembly process? Are there speciality requirements such

as low outgassing, cryogenic serviceability or medical

biocompatibility? Establishing the functional requirements

of the potting compound should be a given, but many

engineers still pick potting compounds through trial and

error. Asking the right questions early in the design process

will keep the errors to a minimum.

Uesta Tempeatue Cmpexities

Of all the mistakes related to encapsulation design, the

most common involves a limited understanding of thermal

conditions. Engineers will select a potting compound based

solely on the expected maximum and minimum application

temperatures. That approach, while seemingly correct, can

lead to the wrong potting compound for the job at hand

because it fails to account for dwell and ramp times.

Failing to account for dwell time, or how long the potting

compound remains at a given temperature, tends to result

in overspecifying. The reason why is that most potting

compounds, particularly epoxies, can withstand short

temperature spikes above their recommended continuous

use temperatures.

To take an example, an epoxy potting compound rated

for continuous use of 200C would have no difficulty

withstanding a short burst of 250C during a solderingMaster Bond potting/encapsulation compounds offer easy

application and high performance.


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operation. Engineers who focus only on the maximum

temperature and ignore its short dwell time will tend to

end up with a more expensive potting compound than they

actually need.

Failing to account for ramp times, or the speed of

temperature changes, can likewise lead to the wrong potting

compound. Fast ramp times and thermal shock go hand

in hand. Engineers who ignore ramp times may end up

with a potting compound that can meet the high and low

temperature requirements but not hold up to thermal cycling

without cracking.

Be Awae f Ppety Cmpmise

Most materials systems involve trade-offs, and potting

compounds are no exception. However, with the right

chemistry and additives and fillers, its now possible to

formulate potting compounds whose mechanical, chemical,

electrical and thermal properties have been tailored to

specific applications. Master Bond EP33 is a good example.

Its been formulated to avoid the usual trade-off between

high temperature performance and modulus, which gives

it the unusual ability to withstand both thermal cycling and

high temperatures.

There are also products available that meet speciality

requirements such as low outgassing, thermal conductivity,

cryogenic serviceability, and more.

Still, there are currently some property trade-offs that are

difficult to reconcile given todays technology. One exampl

is thermal conductivity and optical clarity because the filler

that make the compound conductive also interfere with

clarity.

Csie The Cue

Engineers with an eye on assembly costs normally want to

use potting compounds with a fast cure schedule. While

there are plenty of good products that will meet this

requirement, including Master Bond EP41S, keep in mind th

fast cure reactions tend to generate a larger exotherm than

slower reactions, raising the potential for thermal damage.

Fast cure systems also have a higher potential for entrappe

bubbles, which can reduce the potting compounds expect

electrical and mechanical properties.

The distinction between one- and two-part formulations

also matter a great deal in potting applications. Engineers

wanting to keep things simple on the assembly floor may

favor one-part products since they involve no mixing. There

Pttig Appicatis

Potting and encapsulation

compounds are widely used

whenever electronic components

require protection from damaging

thermal, environmental or

mechanical conditions.

These applications include:

Printed circuit boards

Medical electronics

Power electronics

LED lighting

Transformers

Transducers and other sensors

Capacitors

Surge protection devices

Electronic assemblies

Cable components

Solenoids


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are many good one-part potting compounds, includingMaster Bond EP2RRLV. Just keep in mind that one-part

products require a heat cure that may push extremely heat

sensitive components past their thermal limits.

One-component potting compounds also have a more

limited range of properties available, so applications that

need maximum performance properties should consider

two-part systems.

Accut F Shikage

Like other polymers, potting compounds shrink as theycureas much as 2.3% for an unfilled epoxy compound.

If not accounted for, this shrinkage can impart stresses to

the electronic components, open up leak paths, and create

visual defects. The good news, however, is that shrinkage

can be controlled by selecting the right potting compound.

Filled potting compounds and slower curing epoxies tend

to exhibit less shrinkage than their unfilled and fast-cure

counterparts.

Make It Stick

Like any bonding product, potting compounds work best ifthey have good adhesion to their substrates. The problem is

that many of the polymers used for electronics housings and

potted components frequently have low surface energies

and do not bond easily. Poor adhesion with the substrate

materials can be fixed early in the design process through

the use of surface treatments and primers. Engineers

can combat poor adhesion with part features, such as

undercuts that let the cured potting compound lock itself

into the electronic housing.

G F The Fw

Potting compounds must flow well to encapsulate the

electronic components fully and leave no voids in the

housing. Achieving this flow is easier said than done. It

requires careful attention to the viscosity of the potting

compound, which tend to fall in a range between 400 cps

to 50,000 cps depending on the application. The geometry

of the housing or potting shell in relation to the electric

component can also play a role in impeding or promotingflow. One problem to watch out for is pottings with large

horizontal surfaces. When top filled, they can entrap air and

moisture that can damage electrical components.

Fav Smae Pttigs Whe Pssibe

As the size of a potting increases, so too does the risk

of thermal damage during the potting process. Potting

compounds cure exothermically, and these cure reactions

can generate heat to damage electronic components. Its

not uncommon, for example, for potting compounds to

heat up by 200C or higher, as they cure. The heat from thexothermic cure is a major issue and will be a primary facto

when choosing a potting material, particularly in coatings

exceeding thicknesses of 1/4 to 1/2 inch.

Potting size has cost implications too. Larger pottings

consume more potting compound and take longer to cure,

which can add cost to the assembly process.

For further information on this article, for answers to any

adhesives applications questions, or for information on

any Master Bond products, please contact our technical

experts at Tel: +1 (201) 343-8983.

Shrinkage isnt all bad. A bit of controlled shrink can

actually be helpful in thermal cycling applications. When

the potting compounds coefficient of thermal expansion

(CTE) does not match that of the substrate, a bit of

shrinkage can create just enough clearance to relieve

stresses caused by differential expansion and contraction.


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PoPUlAr MASTEr Bond PoTTInG & EnCAPSUlATInG ProdUCTS

TWO-PARTSYSTEMS

MasterBond Grade

MixRatio byWeight

Color Code

Set-UpTime

Minutes,RT

Cure ScheduleTemp/Time, F

ServiceTemp

Range, FApplications

EP21Fl 100:25A amberB amber

120-18024-48 hrs @ RT2-3 hrs @ 200F

-60 to+250F

Flexibilized version of EP21. Superior thermal cyclabiity, shock and impact resistance. Excellent potting/encapsulation compound. Lower exotherm, suitable

for moderate size castings.

EP21Fr-

SPlV100:100

A redB white

30-6024-48 hrs @ RT2-3 hrs @ 200F

-60 to+250F

Fully meets UL94V.0 for ame retardancy. Excellentphysical and electrical insulation properties. Superbchemical resistance.

EP30dP 100:10A lightamber

B clear60-90

48 hrs @ RT2-3 hrs @ 200F

4K to+250F

Toughened system. Withstands rigorous thermalcycling. Superior thermal shock resistance. Allows forepairability. Cryogenically serviceable.

EP30Fl 100:25A amberB clear

25-3024 hrs @ RT

1-2 hrs @ 200F4K to

+250F

Marvelous thermal & mechanical shock resistance.Excellent thermal cycling durability. Cryogenically seviceable. Lower viscosity than EP30DP. Less rigid.

EP30M3lV 100:50A black

B brown130-150

48-72 hrs @ RT3-4 hrs @ 200F

-60 to+250F

Low viscosity, low exotherm, long working life systemExcellent for potting capacitors. Superior dielectric

properties. Rigid.

EP37-3Fl-

FAo100:100

A whiteB white

3 hrs4-5 days @ RT

4-6 hrs @ 200F4K to

+250F

Thermally conductive, electrically insulating versionof EP37-3FLF. High exibility and low viscosity. MeeNASA low outgassing specications.

EP30-2 100:10A clearB clear

20-2524 hrs @ RT

1-2 hrs @ 200F-60 to+250F

Clear system for optical and ber optic bonding &sealing. Superb chemical resistance. Transparent.Rigid. Meets NASA low outgassing requirements.Cryogenically serviceable.

MasteSi

151100:10

A clear

B clear30-60

24-48 hrs @ RT

1-2 hrs @ 200F

-75 to

+400F

Unsurpassed combination of temperature resistance,exibility and electrical insulation properties. Easilyrepairable. Versatile silicone system.

EP29lPSP 100:65A clear

B translucent>6 hrs

8-10 hours at

130-150F

4K to

+250F

Special version of EP29LP. Resists cryogenic tem-peratures & shocks. NASA low outgassing approved.

Transparent.

B-STAGE SYSTEMS

Master Bond GradeColor Code

Cure Schedule

Temp/Time, F

Service Temp


EP36 tan2 hrs @ 300F

uncured material reusable-100 to 500F

Unique B-stage system. Combines superb tempera-ture resistance with high exibility & elongation.Capable of resisting rigorous thermal cycling.

EP36Ao light tan2 hrs @ 300F

uncured material reusable-100 to 500F

Thermally conductive, electrically insulating versionof EP36. Semiexible. Good mechanical and thermashock resistance.

ONE-PART UV DUALCURE SYSTEMS

Master Bond Grade

Color CodeCure Schedule

Temp/Time, F

Service Temp


UV15-7dCcolorless,

transparent

10-30 min at

250-260F-60 to +300F

UV curable. Low viscosity. Rigid. Requires 125C forsecondary cure in shadowed out areas.

UV15dC80clear,

transparent20-30 minutes at 80C -60F to 300F

High viscosity, dual cure UV. Requires 80C for sec-ondary cure in shadowed out areas.

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