Formulating Coating Powders for the Passive Electronic Component Industry

Pulled from the Dex­ter Elec­tron­ic Mate­ri­als (now SolE­poxy) archives, this tech­ni­cal paper was writ­ten in April 1996 by Jim Glover, Project Leader

Abstract

Formulating Coating Powders for the Passive Electronic Component IndustryMany capac­i­tors and resis­tors require an insu­lat­ing coat­ing after man­u­fac­ture to pro­tect against phys­i­cal, chem­i­cal and elec­tri­cal stress­es. These com­po­nents are often mount­ed on cir­cuit boards and may be her­met­i­cal­ly sealed in met­al, glass or ceram­ic cas­es. A major­i­ty of the units are insu­lat­ed with an organ­ic poly­mer which is eas­i­er to apply and can be more cost effec­tive. The insu­la­tion may be sup­plied as a one or two com­po­nent liq­uid poly­mer, or in many cas­es, mold­ed by either a trans­fer or injec­tion process. Anoth­er fast and eco­nom­i­cal process is to apply a pow­der coat­ing to the fin­ished units.

Pow­der coat­ings may be applied via flu­id bed dip, elec­tro­sta­t­ic depo­si­tion, or cas­cad­ing. The insu­lat­ing prop­er­ties obtained are a direct result of the mate­ri­als used in the for­mu­la­tion com­bined with the prop­er man­u­fac­tur­ing pro­ce­dures. The for­mu­la­tor has many raw mate­ri­als at his dis­pos­al, which in turn offer a mul­ti­tude of pos­si­bil­i­ties in a for­mu­la­tion. Some of these mate­ri­als are rel­a­tive­ly low in cost, while the prices of oth­ers con­tin­ue to esca­late.

This paper will describe the advan­tages of hav­ing the com­po­nent design­er work­ing in con­sort with the coat­ing pow­der man­u­fac­tur­er in the design phase. The opti­mal­ly designed coat­ing pow­der will have the best price vs. per­for­mance char­ac­ter­is­tics.

INSULATING MATERIALS

There are many types of insu­lat­ing mate­ri­als avail­able for pas­sive com­po­nents. These include glass, met­al, sil­i­cone, and organ­ic coat­ings. Glass and met­al encap­su­la­tion pro­vides a her­met­ic seal but are very expen­sive. Sil­i­cones pro­vide high temper¬ature ser­vice, but are also quite cost­ly. The remain­ing insu­lat­ed com­po­nents are gen­er­al­ly coat­ed with organ­ic mate­ri­als such as wax­es, paints, ther­mo­plas­tics, or epox­ies. Epox­ies are thermo¬setting mate­ri­als which can be applied as a liq­uid, mold­ing pow­der, or pow­der coat­ing. This paper will deal pri­mar­i­ly with the design, man­u­fac­ture, and usage of epoxy coat­ing pow­ders.

Epoxy coat­ing pow­ders are sol­id homo­ge­neous mix­tures which are fine­ly ground to behave sim­i­lar to a liq­uid dur­ing appli­ca­tion. The pow­dered epoxy will fuse (melt) with appli­ca­tion of heat and cure to a ther­moset state. The cured coat­ing pro­vides elec­tri­cal insu­la­tion and pro­tec­tion against many envi­ron­ments. Pro­duc­tion rates in excess of 700 pieces per minute offer a low cost pack­age. The economies real­ized are from a com­bi­na­tion of coat­ing pow­der costs, and the pro­duc­tiv­i­ty and yields obtained.

A sol­id ISO 9001 qual­i­ty sys­tem is a def­i­nite advan­tage in for­mu­lat­ing a coat­ing pow­der from the ini­tial con­cept to the final com­mer­cial­ized prod­uct. These pro­ce­dures guide the work to reduce the prob­a­bil­i­ty of future prob­lems.

MATERIAL DESIGN

The com­po­nent design­er deter­mines the spec­i­fi­ca­tion his part requires and what the insu­lat­ing mate­r­i­al has to do. The fol­low­ing is a gen­er­al for­mu­la or recipe for a coat­ing pow­der.

Resin 50
Hard­en­er 4
Cat­a­lyst 1
Filler 40
Pig­ment 1
Addi­tives 4
TOTAL 100

There may be a need for mois­ture pro­tec­tion, ther­mal shock resis­tance, or flame retar­dan­cy. These are require­ments for the cured film, but the mate­r­i­al must also be applied to the parts. Con­sid­er­a­tion has to be made for the type of pow­der appli­ca­tion equip­ment, line speeds and tem­per­a­tures, pow­der stor­age, and waste dis­pos­al. With the com­po­nent and pow­der design­er work­ing togeth­er, a coat­ing pow­der with the best prop­er­ties, price, and per­for­mance can be obtained. Table 1 is a list of prop­er­ties that could be desired in an elec­tron­ic coat­ing pow­der.

RAW MATERIALS

Resin

The start­ing point for any coat­ing pow­der for­mu­la­tion is the resin or resin blend. The most com­mon, and low­est cost, is a reac­tion of epichloro­hy­drin and bisphe­nol A (bis A resin). This type of resin is avail­able in dif­fer­ent mol­e­c­u­lar weight ranges. Oth­er resins are bisphe­nol F, novolac mod­i­fied bisphe­nol A, epoxy cresol novo¬lac, epoxy phe­no­lic, biphenyl, etc.

Desired Properties in an Electronic Coating Powder
Table 1: Desired Prop­er­ties in an Elec­tron­ic Coat­ing Pow­der

Coat­ing pow­ders have a high resin demand to achieve good appear­ance. This may pre­clude the use of resins oth­er than a bis A epoxy type for eco­nom­ic rea­sons, except as an addi­tive or for high per­for­mance. Spe­cial­ty resins will pro­vide pow­ders with a high­er glass tran­si­tion tem­per­a­ture, bet­ter high tem­per­a­ture prop­er­ties, and bet­ter chem­i­cal resis­tance. After choos­ing the resin type need­ed, con­sid­er­a­tion has to be giv­en to appli­ca­tion of the pow­der. Resins come in a vari­ety of soft­en­ing points and melt vis­cosi­ties and the resin or resin blend is cho­sen based on the max­i­mum tem­per­a­ture the part or sub­strate can tol­er­ate dur­ing the coat­ing oper­a­tion. Faster cure times and pro­duc­tiv­i­ty is achieved by work­ing at the max­i­mum allow­able tem­per­a­ture.

Figure 2 Shelf Stability of a Coating Powder with Two Different Catalysts Plate Flow Loss
Fig­ure 2: Shelf Sta­bil­i­ty of a Coat­ing Pow­der with Two Dif­fer­ent Cat­a­lysts Plate Flow Loss

Hardener

The selec­tion of a hard­en­er for crosslink­ing the resin is based on the final prop­er­ties desired in the cured coat­ing. Hard­en­ers such as amines, amides, anhy­drides, imi­da­zoles, Lewis acids, mod­i­fied phe­no­lics, and phe­no­lics are all poten­tial can­di­dates. Anhy­drides are gen­er­al­ly used where high tem­per­a­ture prop­er­ties are desired, while phe­no­lics have supe­ri­or mois­ture resis­tance. There are advan­tages and dis­ad­van­tages to each type, whether it be price or per­for­mance, and no one hard­en­er is best for all types of appli­ca­tions.

Catalyst

Cat­a­lysts are used to pro­mote cure and there are many from which to choose. The prop­er selec­tion not only has a bear­ing on the cure rate but on the final prop­er­ties obtained. A low cost cat­a­lyst may give the desired prop­er­ties, but may also pro­vide poor shelf sta­bil­i­ty as seen in the fig­ure 2.

Filler

The next main ingre­di­ent to con­sid­er is the filler or exten­der. Fillers are incor­po­rat­ed into coat­ing pow­ders to pro­vide prop­er­ties such as high­er ther­mal con­duc­tiv­i­ty, low­er coef­fi­cient of ther­mal expan­sion (CTE), low­er gloss, improved flam­ma­bil­i­ty rat­ings, low­er cost, etc. There are a num­ber of dif­fer­ent types of fillers that can be suc­cess­ful­ly used in coat­ing pow­ders, and there are advan­tages and dis­ad­van­tages with each of them. It is impor­tant to use the prop­er amount of filler in a coat­ing pow­der. Many prob­lems arise when too much filler is used, such
as poor flow out, poor flu­idiza­tion, and poor strength. Table 2 lists advan­tages and dis­ad­van­tages with sev­er­al types of fillers used in coat­ing pow­ders.

Table 2 Filler Comparisons

Pigment

Most coat­ing pow­ders are pig­ment­ed to give them an iden­ti­fy­ing col­or. Pig­men­ta­tion is also used to enhance laser mark­ing where desired. Pig­ments used in elec­tron­ic grade coat­ing pow­ders are cho­sen with great care. Some of the best pig­ments from the recent past are no longer being used because they con­tain heavy met­als. Pig­ments with­out heavy met­als can cost more than ten times as much as their pre­de­ces­sors.

Additives

A num­ber of com­po­nent man­u­fac­tur­ers pre­fer to have coat­ings that have a degree of flame retar­dan­cy built in. Mate­ri­als gen­er­al­ly used con­sist of a halo­genat­ed poly­mer com­bined with anti­mo­ny oxide, alu­minum tri­hy­drates, and phos­pho­rous con­tain­ing com­pounds. These mate­ri­als while con­tribut­ing to flame retar­dan­cy do lit­tle else to enhance the prop­er­ties of a coat­ing pow­der and often times cre­ate prob­lems as shown in the fol­low­ing:

Effect of Anti­mo­ny Oxide on Mois­ture Per­for­mance
% Anti­mo­ny Oxide 1.27 0
Per­cent Fail­ures 48 10
Test­ed in pres­sure cook­er, 132 hrs

Last­ly, most coat­ing pow­ders will con­tain a silane or wet­ting agent to pro­mote adhe­sion to the sub­strate and improve wet phys­i­cal strength. Only through design of exper­i­men­ta­tion can the prop­er type and amount of silane or wet­ting agent be deter­mined. Fig­ure 4 shows the enhanced effect of select­ing a bet­ter silane for mois­ture per­for­mance.

Figure 4 Effect of Silanes on Moisture Resistance

APPLICATION EQUIPMENT

There are many dif­fer­ent meth­ods for the appli­ca­tion of coat­ing pow­ders to elec­tron­ic sub­strates includ­ing dip­ping, cas­cad­ing, and elec­tro­sta­t­ic depo­si­tion. Each method may require a mod­i­fi­ca­tion to the coat­ing pow­der to han­dle prop­er­ly. One of the most com­mon machines for coat­ing elec­tron­ic parts is a large auto­mat­ed flu­idized bed where the parts are placed on car­ri­er strips and pre­heat­ed to 100–175°C. After pre­heat­ing the parts, the air to the flu­idized bed is reduced and the pow­der is scraped flat to pro­duce a lev­el sur­face. The parts are dipped in the pow­der for 1–3 sec­onds and removed for reheat­ing to melt the pow­der. Sev­er­al dips may be required to achieve the prop­er coat­ing thick­ness. A final post cure is giv­en to the coat­ed parts to achieve the desired prop­er­ties. The lev­el­ness of the pow­der in the machine will deter­mine the pant leg height which can be held to 15 mils in many cas­es. Pant leg is the amount of pow­der over and above the com­po­nent on the leads. This has to be kept at a min­i­mum to pre­vent prob­lems with sol­der­ing.

Coat­ing pow­ders may also be applied by elec­tro­sta­t­ic depo­si­tion. The sub­strate is attached to a ground or neu­tral poten­tial while ion­ized air charged to 40–80kV is used to flu­idize the coat­ing pow­der and form a cloud over the bed. The sub­strate is passed through the cloud attract­ing the charged par­ti­cles. Uni­form coat­ing thick­ness­es from 2–20 mils are obtained in this man­ner. After
coat­ing, the pow­der can be selec­tive­ly removed from unwant­ed areas. A final postcure for 30 min­utes at 100–200°C fin­ish­es the process.

A third method of apply­ing coat­ing pow­ders to com­po­nents is the cas­cade or wheel coat­ing method, where axi­al or radi­al devices are han­dled equal­ly well. Reeled parts trav­el along a con­vey­or and pass through a pre­heat stage to raise the part tem­per­a­ture suf­fi­cient­ly for coat­ing. Pow­der is cas­cad­ed or poured on a wheel where the parts pass through. A typ­i­cal set up will have from one to four wheels for pow­der appli­ca­tion with reheat­ing zones in between. The addi­tion­al wheels are for increased coat­ing thick­ness or pos­si­bly a dif­fer­ent pow­der, depend­ing on the appli­ca­tion.
Addi­tion­al heat­ing takes place at this point to pro­vide an ade­quate cure, either on line or in a batch type oven.

COATING POWDER PROCESSING

Coat­ing pow­ders are gen­er­al­ly made in a batch type process. Qual­i­ty approved raw mate­ri­als are weighed accord­ing to the for­mu­la and placed in a blender to achieve a uni­form mix­ture. The blend­ed mate­r­i­al is passed through an extrud­er or melt mix­er to obtain a homo­ge­neous com­pound. The resins used in com­pound­ing are sol­id at room tem­per­a­ture requir­ing the melt mix­er to be at an ele­vat­ed
tem­per­a­ture (75–125°C) for ade­quate pro­cess­ing. The extru­date is cooled to room tem­per­a­ture, pul­ver­ized, sieved to desired par­ti­cle size, qual­i­ty test­ed, and pack­aged for ship­ment.

An impor­tant design cri­te­ria is the par­ti­cle size dis­tri­b­u­tion of the coat­ing pow­der. The method of appli­ca­tion of the pow­der dic­tates the grind pro­file require­ments. Flu­idized bed and cas­cade
coat­ings need a rel­a­tive­ly coarse grind size, i.e. 100% fin­er than 177 microns (80 mesh) and 35% fin­er than 44 microns (325 mesh). Ide­al­ly, all par­ti­cles fin­er than 5–10 microns are removed for two
rea­sons. Fines in a pow­der can coat out on leads of devices mak­ing sol­der­ing dif­fi­cult. Sec­ond­ly, fines in a pow­der are a pos­si­ble res­pi­ra­to­ry and house­keep­ing prob­lem.

CONCLUSION

Coat­ing pow­ders per­form an impor­tant func­tion in the insu­la­tion of pas­sive elec­tron­ic com­po­nents. The ide­al insu­la­tion is one that is designed with the com­po­nent man­u­fac­tur­er and the coat­ing
pow­der sup­pli­er work­ing togeth­er to for­mu­late a prod­uct that meets all of the nec­es­sary require­ments. A good part­ner­ship will pro­vide the opti­mum prod­uct for the least cost where all par­ties can be win­ners.

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About Chris Perabo

Chris is an energetic and enthusiastic engineer and entrepreneur. He is always interested in taking highly technical subjects and distilling these to their essence so that even the layman can understand. He loves to get into the technical details of an issue and then understand how it can be useful for specific customers and applications. Chris is currently the Director of Business Development at CAPLINQ.

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