Electroplating: What Every Engineer Needs to Know
Metal finishing has transformed itself within a few decades from what wasonce an empirical craft into a key technology grounded on scientificprinciples.1
Modern electroplating is a form of metal finishing used invarious industries, including aerospace, automotive, military, medical, RFmicrowave, space, electronics and battery manufacturing. It is theelectrochemical process whereby metal ions in solution are bonded to a metalsubstrate via electrodeposition.
Prior to electroplating, parts must be cleaned and follow a process ofchemical baths to prepare, or activate, them such that a strong bond, andtherefore strong adhesion, is created during the electrodeposition process.
The electroplating bath involves many variables and components which mustbe closely monitored. A power supply provides a flow of direct current tothe parts and the electrical connections at the plating bath. This flow ofcurrent initiates the attraction of ions in the solution to the surface of themetallic part.
For every mole of electrons that is transferred to the part, one mole ofmetallic ions in solution will adhere to the part. In addition, a chemicalreaction occurs at the surface of the part, involving the reduction andoxidation of ions.
(Photo courtesy ofThe Time Preserve/watchplating.com)
What an Engineer or Designer Should Consider Before Electroplating
Nesting of parts during the electroplating process. Since electroplating involves both an electrical and a chemical reaction at the surface of the part, exposure to the plating chemistry is critical to the overall performance of the finished product. Nesting of parts will result in a lack of adhesion or coverage on the surface of the finished part.
The tolerance on critical part dimensions should be designated with the plating thickness in mind. This also means that the fit into the overall assembly should be accounted for.
The environment that the finished parts will be exposed to. This will help to determine the plating thickness required for a part to be resistant to corrosion or repeated cycles of wear, for example.
Because electroplating involves the use of current to initiate a reaction at the surface of a part, the overall geometry of the part will influence the current distribution, often called current density, across the surface of a part. Plating tends to build up on features such as sharp corners, bends or threads. Advanced plating processes exist that can prevent this issue from occurring.
Drainage of plating solution (surface preparation or plating bath chemistries) such that the inside surfaces of parts will be sufficiently covered and the plating will have adequate bond strength. For some parts, this means the addition of a weep hole during the design phase.
The intended use and required characteristics (e.g., conductivity, low friction, high strength and resistance to corrosion, wear, etc.). These criteria should be sufficient in designating the type of metal that should be used for finishing each specific part.
What Are the Benefits of Electroplating?
Electroplating enhances or alters the properties of a metallic part.
Depending on the use of the part, a manufacturer may want better wear andabrasion resistance, protection from corrosion, greater lubricity and lowerfriction, improved EMI/RFI shielding, temperature and impact resistance,improved conductivity, improved solderability, reduced porosity, added hardnessor strength or to build up thickness on small or undersized parts.
In addition to the mechanical or functional properties which may bealtered during the electroplating process, often the overall aesthetics of thefinished part are also important.
Types and Methods of Electroplating
Specialty plating facilities can plate different base materials utilizingvarious surface finishes:
Common Base Materials
Beryllium Copper
Brass
Cold Rolled Steel
Copper
Nickel
Phosphorus Bronze
Stainless Steel
Tellurium Copper
Nickel Silver
Common Surface Finishes
Gold
Silver
Electroless Nickel
The plating material, plating method and parts to be plated will all varydepending on the application.
Gold plating providesexcellent electrical conductivity, making it one of the best choices forelectrodes, current carrying pins and circuit board components. Gold is ideallysuited for protection against intense heat and corrosion in a wide range ofenvironments and climates.
Silver plating is also often used for electronics (over a copper 「flash」)due to its lower electrical resistance.
Nickel plating is commonbecause it offers superior chemical and corrosion resistance along with greaterwear resistance, which increases product lifecycles. Nickel can be a substitutefor silver in electronics or can be used as a coating on steel for analternative to products made of more expensive stainless steel. Nickelalso offers a bright surface finish that can be adjusted according to customerspecifications.
Copper plating is typically utilized as a plating layer before the finallayer of metal is deposited. This surface finish is commonly used in circuitboards, automotive parts or the defense industry. The addition of copper to apart, before the final metal is deposited, can also improve the overallaesthetics of the finished part.
If a single metal does not provide the properties needed, it is alsopossible to co-deposit two or more metals for an electroplated alloy deposit.One example of this is a copper/tin/zinc alloy, also known as Tri-Metal orTri-M3, offered by the specialty plating company Electro-Spec, Inc.
Finding the Right Electroplating Company for Your Needs
When looking for a plating company, there are many criteria to considerbased on what your project requires and the plating company’s capabilities,including:
Size of the components
Volume of pieces (from prototype to volume production)
Plating metal to be used for the desired outcome
Budget of the project
Compliance to industry standards
Laboratory and testing capabilities/certifications
The method of electroplating is another consideration, as not allfacilities necessarily offer the same processes.
「Barrel」 plating can efficientlyplate large and small volumes of parts where adequate solution exchange andturnover is critical to meeting thickness requirements. Current density withinthe load of parts in the barrel is typically optimized through the part-to-partcontact during rotation.
However, there are some types of parts that are not ideal for mostconventional barrels. For example, parts that could become scratched, dented ordinged from part-to-part contact are much more susceptible to damage in mosttypes of barrels. Conversely, some flat parts are not ideal in a barrel due tothe parts sticking together during processing, which leads to missing platingor uneven plating thicknesses.
Depending upon some part geometries and tolerances, parts are also moreprone to 「nesting」 into each other during barrel plating.
Vibratory plating, used for small or fragile parts.(Photo courtesy of Electro-Spec, Inc.)
Vibratory plating is used for smaller parts that havedeep internal diameters, counter bores, fragile tips/ends or parts that couldbend through barrel plating. By incorporating a vibrating or pulsating basketthat transfers kinetic energy to the load, the parts move in a clockwisefashion across button contacts on the bottom of the basket. These contactstransfer current to the load of parts and provide very consistent amperageduring processing.
Largerparts that have excessive weight or parts that can become entangled or easilynest together cannot be plated in a vibratory basket as they will not move in auniform fashion. Conversely, smaller parts that do not have enough weightcannot be plated in a vibratory basket, either.
Rack plating issuitable for both delicate and larger parts. It works by holding parts in afixed position on a rack frame while suspended in the solution. This prevents damageto parts during processing and facilitates the processing of much larger partsthat could not be barrel plated.
The biggest issue with rack plating is that direct connection to the partson the rack causes plating distribution efficiency to be compromised due tohigh and low current density areas throughout the parts and the rack. Partsplaced on a rack also have poor solution movement, which is necessary tocontrol thicknesses, and they are more prone to rinsing and drying stains.
Selective plating processes isolatethe plating finish to a select area of the part. This process is done throughcontrolled depth plating, which involves fixing the part in a manner thatprovides continuous electrical contact and submerges the area to be plated atdefined depths through the plating solution.
Selective plating is ideal for specific applications where functionalplating is required for performance and/or cost savings with precious metal byreducing the surface area necessary to plate.
While this is an effective method for plating singulated parts andreducing costs, it does require the expense of tooling and labor for loadingthe parts. There are also some limitations on part size and geometry that canpreclude some parts from being able to be selectively plated.
Spouted Bed Electrode (SBE) plating isdesigned for small parts, flat parts, parts with counter bores, parts withtabs, parts that nest or have a difficult geometry that make either vibratoryplating or conventional barrel plating impossible or impractical.
The SBE process is accomplished in a chamber with ultrasonic action, and continuoussolution being pumped into and out of the chamber to facilitate part movementand supply fresh plating electrolyte during the plating process. The SBEensures very uniform plating coverage in high and low current density areas ofthe part as well as in counter bores.
The only limitation to SBE is the size of the parts, as the SBE chamberscan only accommodate specific sizes and weights that allow for movement insidethe chambers.
SpoutedBed Electrode plating. (Photo courtesy of George Hradil/Technic, Inc.)
經過幾十年的發展，金屬表面處理技術從經驗工藝變為了一種科學技術。
電鍍作為金屬表面處理的一種方法，廣泛應用於各種行業，包括航空航天、汽車、軍工、醫療、射頻微波、空間、電子、電池製造等。它是通過電離沉積方法將溶液中的金屬離子鍵合到金屬基底上的過程。電鍍前，對鍍件進行清洗，之後進行化學浴活化，在材料表面產生較大的附著力，在電沉積過程中形成較強的鍵合。
對電鍍浴過程涉及的組成成分變化進行監測。利用電源為鍍液中的鍍件和電子提供直流電流，電流將溶液中的離子吸附在金屬鍍件的表面。1摩爾電子轉移到鍍件上，相應的溶液中會有1摩爾的金屬離子附著在鍍件上。發生在鍍件表面的化學反應涉及離子的還原和氧化。
在電鍍前，工程師或設計師應該考慮什麼？
電鍍過程中鍍件的重疊。由於電鍍過程涉及鍍件表面的電化學反應，因此接觸鍍層的化學成分對於工件最終性能有十分重要的影響。鍍件的重疊會導致鍍件表面的粘附性不足或厚度不均。關鍵鍍件尺寸的公差與鍍層厚度有關，涉及鍍層參數時應使整體相互配合。例如：鍍件的工作環境。這將有助於確定鍍件所需的鍍層厚度，以使鍍件具有一定的耐腐蝕性或磨損周期。
電鍍過程中，鍍件的幾何形狀會影響電流密度。電鍍更容易在一些尖角處進行，如鋒利的角落，彎曲處或螺紋上。但是現有先進電鍍工藝可以防止此類問題的發生。鍍液引流（表面處理或化學浴電鍍）可以對鍍件的內表面實現均勻的鍍覆並且使鍍層與鍍件之間有很好的粘結強度。在確定鍍件所用的金屬類型時應考慮鍍件所需的性能（例如導電性，低摩擦，高強度和耐腐蝕，磨損等）。
電鍍的好處是什麼？
電鍍可以提高或改變金屬鍍件的性能。根據鍍件的應用，製造商可能想要具有更好性能的鍍件，比如耐磨性，防腐蝕，潤滑性好和低摩擦力，耐溫度和耐衝擊性，較高的導電和可焊性，較小的孔隙率，較高的硬度、強度或製造厚度小或尺寸小。電鍍工藝不僅會改變鍍件的機械特徵或功能性特徵，其成品的整體外觀也很重要。
電鍍的種類和方法
專業電鍍設備可以使用不同的基礎材料電鍍不同的基體：
常見基體材料：
·鈹銅 ·黃銅
·鋼 ·冷軋鋼
·銅 ·鎳
·磷青銅 ·不鏽鋼
·碲銅 ·鎳銀
常見鍍層材料：
·金 ·銀
·化學鍍鎳 ·銅
·電解鎳 ·合金共沉積
電鍍材料、電鍍方法和鍍件的基體材料都取決於鍍件的應用場合。
鍍金可以使鍍件具有優良的導電性，可以使之成為最好的電極、電流攜帶引腳和電路板組件的最佳材料之一。金可以保護材料免受過熱和腐蝕，且適用範圍廣泛。銀鍍件電阻較低，也經常被用於電子設備（銅的「閃光」）。
表面鍍鎳可以使鍍件具有優越的化學和耐腐蝕性以及更好的耐磨性，從而延長鍍件的使用周期。在電子產品中，鎳可以代替銀，在普通鋼表面鍍鎳可以替代更昂貴的不鏽鋼製品。鍍鎳工件有良好的表面光潔度，並且可以根據客戶要求進行調整。
鍍銅通常在鍍覆最後一層金屬沉積層之前。這種鍍件常用於電路板，汽車零件或國防工業。在最後的金屬沉積之前在鍍件上鍍銅，也可以提高鍍件的整體美觀性。如果單一元素不能滿足性能要求，也可以同時沉積兩種及以上的金屬以形成合金鍍層。例如：電鍍銅/錫/鋅合金，也稱為三金屬或Tri-M3電鍍法。
基於需求找到合適的電鍍公司
當尋找一個電鍍公司時，許多的選擇標準需要基於項目需求和電鍍公司的資質等多方面考慮，從而選擇最合適的公司，其中的標準包括：組件的尺寸；工件的體積（從原型生產到批量）；鍍層金屬；項目預算；對行業標準的符合性；實驗室測試性能與認證。
另一個需要考慮的問題是電鍍方法，因為不是所有的方法都可以提供相同的電鍍效果。
「滾鍍」可以高效地電鍍大型和小型鍍件，一定的電鍍液的交換率與流動率是滿足鍍層厚度要求的關鍵。鍍件上加載的電流密度，通常是通過旋轉時零件之間的聯繫來進行優化的。
但是對於幾種鍍件來說，傳統的滾鍍方法並不理想。比如說，由於零件之間接觸導致鍍件刮擦、壓傷、碰撞，這種類型的鍍件則極易在滾鍍時受損。相反，一些扁平鍍件也不宜用滾鍍，因為在電鍍過程中鍍件會粘在一起，這樣會導致部分表面未得到電鍍或者鍍層不均勻。
由於鍍件特殊的幾何尺寸以及公差，在電鍍過程中鍍件更容易重疊在一起。
振動電鍍，用於小巧易碎鍍件。振動電鍍用於尺寸更小的鍍件，這些鍍件有著寬的內部直徑、埋頭孔、易碎的小件/末端或在滾鍍中容易彎曲的部分。通過引入一個振動或脈衝的籃子將動能傳遞給加載物，鍍件以順時針方向躍過籃子底部的按鈕接觸器。這些接觸器在電鍍過程中為加載的鍍件輸送電流以及與之相一致的電流強度。
超重的鍍件或者易相互纏繞、堆疊的鍍件不能置于振動室來電鍍，因為這樣的鍍件不能均勻地運動。相反，重量較小的鍍件也不能置于振動籃子裡。
掛鍍既適用於精細鍍件又適用於大型鍍件。掛鍍是將鍍件固定在掛具上，再懸掛進電鍍液裡。這樣可以防止鍍件在電鍍處理時受損，且可以對更大型的不能滾鍍的鍍件進行電鍍。掛鍍最大的問題在於，鍍件與掛架直接的連接，由於鍍件與掛架上的高低電流密度區域導致鍍層分布效率大打折扣。鍍件掛在掛架上，也使得溶液運動過少，而足夠的溶液運動是控制厚度的必要因素，並且鍍件還易於清洗和乾燥汙漬。
選擇性電鍍，具有特定的應用場合，如功能性鍍層或者減少貴金屬電鍍面積來節約成本的需要。
選擇性電鍍是對鍍件的部分區域進行表面電鍍處理。它可以控制鍍層深度，為鍍件提供連續電氣連接，並將鍍件的待電鍍深度浸入電鍍溶液中。
儘管這對於電鍍單一鍍件和降低成本來說是一種高效的方法，但它仍然需要工藝裝備和人工費用。此外，鍍件幾何尺寸的適用性也有限，因此可在電鍍前即排除掉不需要電鍍的區域。
獨立鍍件的選擇性電鍍
噴泉床電鍍（SBE）專為小型鍍件、扁平鍍件、帶埋頭孔的鍍件、突出物的鍍件、容易堆疊的鍍件以及幾何形狀複雜無法應用振動電鍍或傳統滾鍍的鍍件設計。
噴泉床電鍍（SBE）是在一個加裝超聲作用的密室裡實現的，在電鍍過程中向密室裡連續地提供新鮮的電鍍液。SBE能夠保證鍍件的電流密度不同區域都能夠得到均勻的鍍層。噴泉床電鍍（SBE）唯一的不足是它對鍍件的尺寸有限制，因為噴泉床電鍍（SBE）的腔體只能夠容納特定尺寸和重量的鍍件使得鍍件能在腔體裡運動。
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