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Plane final coating

-

Author don Cullen

The characteristics of electroless nickel plating/gold leaching precipitation are introduced.

In recent years, the final coating process of PCB manufacturing has undergone major changes. These changes are the result of constantly demanding to overcome the limitations of HASL and more and more alternative methods of HASL.

The final coating is used to protect the surface of the circuit copper foil. Copper (Cu) is a good surface for welding parts, but it is easily oxidized. Copper oxide hinders the wetting of solder. Although copper is now covered with gold (Au) because gold will not be oxidized; Gold and copper will spread rapidly and penetrate each other. Any exposed copper will soon form solderless copper oxide. One method is to use a "barrier layer" of nickel (Ni), which prevents the transfer of gold and copper and provides a durable and conductive surface for the assembly of components.

Requirements of printed circuit board for non-electrolytic nickel coating

Non-electrolytic nickel coatings should have multiple functions:

Gold precipitation surface

The ultimate goal of the circuit is to form a connection with high physical strength and good electrical characteristics between PCB and components. If there is any oxide or pollution on the PCB surface, this welding connection will not happen under today's weak flux.

Gold naturally precipitates on nickel and will not be oxidized after long-term storage. But gold will not precipitate on oxidized nickel, so nickel must be kept pure between nickel bath and gold dissolution. Therefore, the first requirement for nickel is to keep it from being oxidized for a long enough time to allow gold to precipitate. A chemical immersion bath was developed to allow the phosphorus content in nickel precipitation to be 6~ 10%. The phosphorus content in non-electrolytic nickel coatings is considered as a careful balance between bath control, oxides and electrical and physical properties.

difficulty

Electroless nickel plating surfaces are used in many applications that require physical strength, such as automobile transmission bearings. The requirements of PCB are far less stringent than those of these applications, but a certain hardness is still important for wire bonding, touch points of touchpad, edge connectors and processing sustainability.

Wire bonding requires the hardness of nickel. If the lead wire deforms the deposit, friction loss may occur, which helps the lead wire to "fuse" to the substrate. SEM photos show that it does not penetrate the flat surface of nickel/gold or nickel/palladium (Pd)/ gold.

electrical specification

Because it is easy to manufacture, copper is chosen as the metal to form the circuit. The conductivity of copper is better than that of almost all metals (table 1) 1, 2. Gold also has good electrical conductivity, making it a perfect choice for the outermost metal, because electrons tend to flow on the surface of the conductive route (the "surface layer" benefit).

Copper 1.7? ωcm

Jin 2.4? ωcm

Nickel 7.4? ωcm

Non-electrolytic nickel coating 55~90? ωcm

Table 1. Resistivity of PCB metal

Although the electrical characteristics of most production boards are not affected by nickel layer, nickel will affect the electrical characteristics of high frequency signals. The signal loss of microwave PCB may exceed the designer's specifications. This phenomenon is proportional to the thickness of nickel-the circuit needs to pass through nickel to reach the solder joint. In many applications, the electrical signal can be obtained by specifying that the nickel precipitation is less than 2.5? M Return to the design specification range.

Transient resistance

Contact resistance is different from solderability because the nickel/gold surface remains solderless throughout the life of the final product. After long-term exposure to the environment, nickel/gold must maintain electrical conductivity in contact with the outside. The book 1970 of staghorn quantitatively expresses the contact requirements of nickel/gold surface. Various terminal environments are studied: 3 "65 C, the normal maximum temperature of electronic systems working at room temperature, such as computers; 125 c, the temperature at which the universal connector must work, which is usually specified for military applications; 200°C, which is becoming more and more important for flight equipment. "

For low temperature environment, nickel barrier layer is not needed. With the increase of temperature, the amount of nickel needed to prevent nickel/gold transfer increases (Table 2). three

The nickel barrier layer has good contact at 65°C, at125 C and at 200 C..

0.0 ? m 100% 40% 0%

0.5 ? m 100% 90% 5%

2.0 ? m 100% 100% 10%

4.0 ? m 100%

Table 2. Contact resistance of nickel/gold (result of 1000 hours)

Nickel used in antle's research is electroplated. As Bodland confirmed, it is expected to improve from non-Lieutenant General electrolytic nickel. But these results are correct for 0.5? M of gold, where the plane usually precipitates 0.2? M. It can be inferred that the plane is sufficient for the contact elements working at125 C, but the elements with higher temperature will need special tests.

Antle suggested: "The thicker the nickel, the better the barrier layer, which is correct in all cases, but the actual situation of PCB manufacturing encourages engineers to precipitate only the required amount of nickel. Flat nickel/gold has now been used in mobile phones and pagers using touch pads. The specifications of such components shall be at least 2? M nickel.

connector

Non-electrolytic nickel/immersion gold is used to produce circuit boards with spring fit, press fit, low-voltage sliding fit and other solderless connectors.

Plug-in connectors require longer physical durability. In these cases, the strength of non-electrolytic nickel coating is enough for PCB application, but gold immersion is not enough. Very thin pure gold (60~90 knops) will be worn off from nickel by repeated friction. When gold is removed, the exposed nickel is rapidly oxidized, resulting in an increase in contact resistance.

Electroless nickel plating/gold immersion may not be the best choice for plug-in connectors, because they will undergo multiple insertions throughout the product life cycle. Nickel/palladium/gold surface is recommended for multi-purpose connectors. five

barrier layer

Non-electrolytic nickel has three blocking effects on the plate: 1) to prevent copper from diffusing into gold; 2) prevent the diffusion of gold to nickel; 3) Sources of nickel formed by 3)Ni3Sn4 intermetallic compound.

Diffusion of copper to nickel

The transfer of copper through nickel will lead to the decomposition of surface gold by copper. Copper will be oxidized quickly, resulting in poor solderability during assembly, which will occur in the absence of nickel plating. Nickel is needed to prevent migration and diffusion during storage and transportation of empty plates and during assembly when other areas of the plates have been welded. Therefore, the temperature of the barrier layer is required to be lower than 250℃ for less than one minute.

Turn and Owen6 studied the effects of different barrier layers on copper and gold. They found that "... the comparison of copper permeability values at 400°C and 550°C shows that hexavalent chromium and nickel with phosphorus content of 8~ 10% are the most effective barrier layers studied". (Table 3)

Thickness of nickel 400 c for 24 hours 400 c for 53 hours 550 c12 hours.

0.25 ? m 1？ m 12？ m 18？ m

0.50 ? m 1？ m 6？ m 15？ m

1.00 ? m 1？ m 1？ m 8？ m

2.00 ? Non-proliferation, non-proliferation, non-proliferation

Table 3. Copper penetrates into gold through nickel.

According to Arrhenius equation, the diffusion at lower temperature is exponentially slow. Interestingly, in this experiment, the efficiency of non-electrolytic nickel is 2~ 10 times higher than that of electroplating nickel. Instead, Owen pointed out, "... Ann (8%) 2? m(80？ Inches) barrier reduces the diffusion of copper to a negligible level of six.

It can be seen from this extreme temperature test that there are at least 2? The thickness m of nickel is a safe specification.

Diffusion of nickel into gold

The second requirement of non-electrolytic nickel is that nickel should not migrate through "particles" or "pores" impregnated with gold. If nickel comes into contact with air, it will oxidize. Nickel oxide is not solderable and difficult to remove with flux.

There are several articles about using nickel and gold as ceramic chip carriers. These materials are subjected to the extreme temperatures of assembly for a long time. A common test for these surfaces is at a temperature of 500°C for 15 minutes. seven

In order to evaluate the ability of electroless nickel/gold immersion surface to prevent nickel oxidation, the solderability of temperature aging surface was studied. Different heat/humidity and time conditions were tested. These studies show that nickel is fully protected by gold leaching and still has good weldability after long-term aging (figure 1).

In some cases, such as gold thermal ultrasonic wire bonding, the diffusion of nickel into gold may be the limiting factor of assembly. In this application, the nickel/gold surface is not as good as the nickel/palladium/gold surface. Iacovangelo studied the diffusion characteristics of palladium as a barrier between nickel and gold, and found that 0.5? M palladium can prevent migration even at extreme temperatures. This study also proves that there is no copper through 2.5? Nickel/palladium in rice.

Nickel-tin intermetallic compound

During surface mounting or wave soldering operations, atoms on the PCB surface will be mixed with solder atoms, which depends on the diffusion characteristics of metals and the ability to form "intermetallic compounds" (Table 4). seven

Metal temperature c diffusivity (? Inches per second. )

Jin 450

486 1 17.9

167.5

Copper 450

525 4. 1

7.0

Palladium 450

525 1.4

6.2

Nickel 700 1.7

Table 4. Diffusion rate of PCB material in welding

In nickel/gold and tin/lead systems, gold will immediately dissolve into loose tin. The solder forms strong adhesion to the underlying nickel by forming Ni3Sn4 intermetallic compound. Enough nickel should be deposited to ensure that the solder does not reach under the copper. Bader's measurement shows that it does not need to exceed 0.5? M nickel to maintain this barrier layer, and even have to go through more than six temperature patrols. In fact, the maximum observed intermetallic compound layer thickness is less than 0.5? m(20？ Inches).

porousness

Non-electrolytic nickel/Gold has only recently become a common final PCB surface coating, so industrial procedures may not be suitable for this surface. There is now a nitric acid vapor process (IPC-TM-650 2.3.24.2)9 for testing the porosity of electrolytic nickel/gold used as plug-in connectors. Non-electrolytic nickel/gold impregnation failed the test. A European porosity standard using potassium ferricyanide has been developed to determine the relative porosity of planar surfaces, and the results are given as the number of holes per square millimeter (number of holes /mm2). A good plane surface should have less than 10 holes per square millimeter when magnified by 100 times.

conclusion

For reasons of cost, cycle time and material compatibility, PCB manufacturing industry is interested in reducing the amount of nickel deposited on circuit boards. The minimum nickel specification should help prevent copper from diffusing to the gold surface, maintain good solder joint strength and reduce contact resistance. The specification of maximum nickel content should allow flexibility in sheet manufacturing, because there is no serious failure mode related to thick nickel precipitation.

For most circuit board designs today, 2.0? m(80？ Inches) is the minimum required nickel thickness. In practice, a series of nickel thicknesses are used in the production batch number of PCB (Figure 2). The change of nickel thickness will be the result of the change of chemical characteristics of immersion bath and the change of residence time of automatic hoist. To guarantee 2.0? The minimum value of m, the specification from the end user should require 3.5? M, minimum 2.0? M, maximum 8.0? m .

This specific range of nickel thickness has been proved to be suitable for producing millions of circuit boards. This range meets the requirements of electronic products for solderability, shelf life and contact. Because different products have different assembly requirements, it may be necessary to optimize the surface coating for each special application.

refer to

Mallory, G.( 1990). Electroless plating AESF Publishing Company.

West Sappho Laneque (1986). Properties of electrodeposited metals and alloys. AESF Publishing Company.

Antell, M.( 1970, June). Gold plated contacts: the influence of heating on reliability. Electroplating.

Baudrin, D. (198 1,1February). Electroless nickel plating is used to reduce the demand for gold. Electroplating and surface treatment.

Kudrak, E. et al. (199 1, March). Wear reliability of gold-plated palladium and hard gold in high-speed digital connector system. Electroplating and surface treatment.

Turn, J.C. and Owen, E.L. (1974, 1 1 month). Metal diffusion barrier of copper-electrodeposited gold system. Electroplating.

Sibad (1969,1February). Dissolution of gold, silver, palladium, copper and nickel in molten tin-lead solder. Welding.

New autocatalytic gold bath and diffusion barrier coating.

Karen, D.C. (Tel: 1997). TR- 104。 Wire bonding of electroless deposited metal circuit board facing. Proceedings of the IPC Expo.

Don Cullen is the electronic technology manager of MacDermid Inc in waterbury, Connecticut; (203) 575-5700. Please contact him for the full version of this article.

(22 May 2006, 5438+0)