Application of Powder Coatings for Electronic Component Protection

Pulled from the Dex­ter Elec­tron­ic Mate­ri­als (now SolE­poxy) archives, this tech­ni­cal paper was writ­ten in April 1995 by Joseph Kubic, Senior Lab­o­ra­to­ry Tech­ni­cian

Under­stand­ing the cor­re­la­tion between com­po­nents, coat­ing pow­der and coat­ing equip­ment has become increas­ing­ly impor­tant in recent years. With pres­sure to improve pro­duc­tiv­i­ty, increase yields and min­i­mize down­time, it will be essen­tial to main­tain a work­ing knowl­edge of appli­ca­tion tech­niques, pos­si­ble prob­lems and, of course, solu­tions to those prob­lems.

Coat­ing pow­ders for the insu­la­tion and pro­tec­tion of elec­tron­ic devices are designed to be com­pat­i­ble for use with a full range of equip­ment includ­ing flu­id bed machines, cas­cade coat­ing machines and inline wheel coat­ing equip­ment.

Pow­der coat­ing of elec­tron­ic devices and com­po­nents is a process that depends heav­i­ly on the con­fig­u­ra­tion of the device.

Appli­ca­tion equip­ment and the coat­ing pow­der should be designed to work togeth­er. Com­po­nent tem­per­a­ture lim­its should also be con­sid­ered when choos­ing coat­ing pow­der for a spe­cif­ic appli­ca­tion.

The fol­low­ing are descrip­tions of var­i­ous pow­der appli­ca­tion equip­ment avail­able as well as some dif­fi­cul­ties which may be encoun­tered.

LARGE FLUID BED MACHINES

The large flu­id bed equip­ment method for coat­ing radi­al lead­ed com­po­nents gives excel­lent con­trol over the amount of pant­leg. Pant­leg is used to describe the coat­ing that occurs on the leads. Excess pant­leg cre­ates a prob­lem when the part is to be mount­ed to the cir­cuit board. Excess pant­leg must be removed and in turn cre­ates an extra clean­ing step. By using a method referred to as “dead bed dip­ping” pant­leg can be con­trolled.

Dead bed dip­ping involves flu­idiz­ing the pow­der, shut­ting off the air and vibra­tion, scrap­ing the sur­face of the pow­der smooth and flat, dip­ping and remov­ing the pre­heat­ed parts. The pow­der is flu­idized again while the devices are reheat­ed. The process is repeat­ed until the desired coat­ing thick­ness is achieved.

Some appli­ca­tion dif­fi­cul­ties that may be encoun­tered with large flu­id bed machines are as fol­lows:

1. Fluidization

The term flu­idiza­tion is used to describe what hap­pens when a fine­ly ground pow­der is sub­ject­ed to air and vibra­tion in pre­scribed quan­ti­ties. With accu­rate adjust­ments, the pow­der per­forms in the flu­id bed equip­ment sim­i­lar­ly to a liq­uid or flu­id.

Fac­tors con­tribut­ing to opti­mum flu­idiza­tion are:

  1. 1.1 Vibra­tion: Suf­fi­cient vibra­tion is need­ed to break up gey­sers (columns of air) that may build up in the pow­der.
  2. 1.2 Air: Insuf­fi­cient air results in a dense bed of pow­der. Too much air caus­es gey­sers, exces­sive dust and errat­ic bed set­tling times.
  3. 1.3 Par­ti­cle Size Dis­tri­b­u­tion: To assure con­sis­tent coat­ing, par­ti­cle size dis­tri­b­u­tion must be main­tained. Fines deple­tion can cause prob­lems in con­sis­ten­cy from dip to dip. Bed den­si­ty will increase as the fines are lost, either through non-recir­cu­lat­ing dust col­lec­tion sys­tems or selec­tive depo­si­tion which is caused by the vibra­tion not being set cor­rect­ly. This caus­es the pow­der to seg­re­gate, with the coars­er par­ti­cles going to the out­side and the fines mov­ing toward the mid­dle where they are picked up by the heat­ed part. As the fines are lost the bed becomes more dense requir­ing more air and caus­ing incon­sis­tent coat­ings.

2. Uneven Pantleg

Uneven pant­leg can occur when equip­ment is out of align­ment. For exam­ple the fix­tures that hold the parts may be in need of adjust­ment. Oth­er pos­si­bil­i­ties are a mis­ad­just­ed scraper blade, uneven bed stops or exces­sive air flow dur­ing flu­idiza­tion. Exces­sive air could cause the pow­der to set­tle too slow­ly allow­ing fur­ther set­tling after the scraper blade goes by. Short pant­leg on parts in the mid­dle of the rack can be caused by scrap­ing the pow­der too soon after turn­ing the air off. If the pow­der is scraped too soon, it will con­tin­ue to set­tle in the mid­dle of the bed result­ing in short­er pant­leg there.

3. Large Parts

Large parts may require devi­at­ing from the stan­dard dead bed coat­ing tech­nique. Low bed air and vibra­tion will gen­er­al­ly improve the coat­ing of large com­po­nents.

4. Lead Spatter

Lead spat­ter or small par­ti­cles of unwant­ed mate­r­i­al on the leads above the coat­ed area can be caused by insert­ing the devices into the pow­der too quick­ly. This may cause the coat­ing pow­der to splash onto the leads.

5. Pinholes

Pin­holes can be caused by many con­di­tions includ­ing con­t­a­m­i­na­tion from flux residue, dust, or inad­e­quate clean­ing of the parts. Porous and wrapped devices can be dif­fi­cult to coat. If the pre­heat, reheat and cure tem­per­a­tures are not kept close to each oth­er, the air in the part will either expand, blow­ing out through the sur­face, or con­tract, caus­ing air from the out­side to col­lapse through the coat­ing. Quick gel and increased coat­ing thick­ness­es may help elim­i­nate pin­holes.

6. Orange Peel

The cos­met­ic coat­ing defect iden­ti­fied by its rough sur­face resem­bling the out­side sur­face of an orange can be caused by insuf­fi­cient heat, or short flow due to aged mate­r­i­al or excess mois­ture in the pow­der.

7. Bed Life

Ways to improve bed life of a pow­der include:

  1. 7.1 Main­tain­ing long enough reheat times to insure that the bed is well flu­idized. This will keep pow­der well blend­ed allow­ing for an even heat dis­tri­b­u­tion through­out the pow­der. Ulti­mate­ly this will low­er the risk of a crust form­ing on the top edges of the pow­der.
  2. 7.2 Reduc­ing fines loss will keep the pow­der uni­form and make for more con­sis­tent pro­duc­tion.
  3. 7.3 Store unused pow­der at cold tem­per­a­tures to insure flow sta­bil­i­ty. In order to min­i­mize mois­ture con­t­a­m­i­na­tion of the pow­der, allow the mate­r­i­al to reach room tem­per­a­ture before open­ing con­tain­ers. Mois­ture con­t­a­m­i­na­tion can affect the gel and cure rates of the pow­der.
  4. 7.4 Keep­ing the bed at cool­er tem­per­a­tures will slow the heat aging of the pow­der.

8. Insufficient Build Rate

Poor build can be attrib­uted to insuf­fi­cient pre­heat for the spe­cif­ic coat­ing pow­der. Oth­er fac­tors which con­tribute to insuf­fi­cient build rates are heat expo­sure or aging of the pow­der. Both these con­di­tions could cause the mate­r­i­al to lose flow and result in insuf­fi­cient build.

9. Heat Sources

Effi­cient pre­heat­ing and reheat­ing of devices are crit­i­cal fac­tors for con­sis­tent coat­ing appli­ca­tion.

Non-uni­form heat­ing can cause incon­sis­tent coat­ing thick­ness­es from com­po­nent to com­po­nent regard­less of the equip­ment used.

LINEAR COATING MACHINES

Typ­i­cal­ly the com­bi­na­tion of the long nar­row flu­id beds of lin­ear coat­ing equip­ment and fine pow­der grinds result in poor flu­idiza­tion. A coarse­ly ground pow­der usu­al­ly pro­vides more uni­form flu­idiza­tion in this type equip­ment which are lim­it­ed to coat­ing only one or two strips at a time.

AXIAL LEADED DEVICE COATERS

There are two types of equip­ment used for coat­ing axi­al lead­ed parts; cas­cade coaters and wheel coaters.

Cascade Coaters

In the cas­cade coat­ing process, after devices are arranged on a con­vey­er, and passed through the heat­ing zone, pow­der is deposit­ed on the rotat­ing device before pass­ing through a sec­ond heat zone to gel and cure.

Wheel Coaters

Sim­i­lar to cas­cade coat­ing, elec­tron­ic devices are arranged on a con­vey­er and car­ried through the first heat­ing zone to a rotat­ing wheel. A trough in the wheel con­tains the pow­der which is deposit­ed to the pre­heat­ed part. More than one pre­heat­ing zone and wheel may be used to achieve the desired coat­ing thick­ness.

The device is then reheat­ed, shaped by a rotat­ing wheel and a fixed shoe, and is cured in a final heat zone.

Some of the dif­fi­cul­ties that may be encoun­tered with both Cas­cade and Wheel Coaters are as fol­lows:

  1. 1. Insuf­fi­cient BuildInsuf­fi­cient pre­heat of the devices can result in poor build of the coat­ing pow­der. If tem­per­a­tures can­not be increased, a low­er melt coat­ing pow­der may be need­ed. Short flow may require replen­ish­ing the coat­ing pow­der.
  2. 2. Aged Mate­r­i­alProp­er care and han­dling of coat­ing pow­ders require cold stor­age until ready for appli­ca­tion. Keep­ing pow­ders away from heat sources will extend their use­ful life.
  3. 3. Mois­ture Con­t­a­m­i­na­tionIt is impor­tant to allow the mate­r­i­al to reach ambi­ent tem­per­a­ture before open­ing the con­tain­er in order to avoid con­den­sa­tion on the pow­der. Mois­ture absorbed by the pow­der may affect gel, cure time and/or integri­ty of the coat­ing.
  4. 4. Shape Reten­tionThe inabil­i­ty of a device to retain its shape dur­ing the cure process is an indi­ca­tion the cure tem­per­a­ture may be too high or the flow of the com­pound may be too long.
  5. 5. Auger Jam­mingAuger jam­ming is caused by a large influx of fines into the reser­voir of coat­ing pow­der. The recy­cle from the dust col­lec­tor should be added grad­u­al­ly to avoid this prob­lem.

CONCLUSION

Prod­uct con­sis­ten­cy and improved per­for­mance dur­ing the appli­ca­tion will result in increased pro­duc­tiv­i­ty and enhanced per­for­mance char­ac­ter­is­tics of the com­po­nents.
Addi­tion­al tech­ni­cal and appli­ca­tions sup­port is avail­able by call­ing your local SolE­poxy sales rep­re­sen­ta­tive.

ACKNOWLEDGEMENTS

The author wish­es to thank Dick Ger­szews­ki for his con­tri­bu­tions to this work.

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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.

One thought on “Application of Powder Coatings for Electronic Component Protection

  1. My cousin works for an mdf pow­der coat­ing com­pa­ny. Accord­ing to him it’s 100% envi­ron­men­tal­ly friend­ly, more afford­able and more durable than oth­er tra­di­tion­al fin­ish­es.

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