For those born in the 80s, the first encounter with gold fingers can be traced back quite a while, although at that time it was not understood what it was all about. The Nintendo Famicom and Little Tyrant game console cartridges connected electrically through gold fingers.
Gold fingers (connecting fingers) are used in computer hardware, such as between memory modules and memory slots, graphics cards and graphics card slots, where all signals are transmitted through gold fingers. Gold fingers consist of numerous golden conductive contacts, and are named “gold fingers” due to their gold-plated surface and finger-like arrangement of the conductive contacts.
The characteristics of gold: it has excellent electrical conductivity, wear resistance, oxidation resistance, and reduces contact resistance. However, gold is extremely expensive, so it is only applied to local gold plating or chemical gold for gold fingers, such as bonding pads.
All signals between computer hardware, such as memory modules and memory slots, graphics cards and graphics card slots, are transmitted through gold fingers.
Gold fingers are composed of numerous golden conductive contacts, and are named “gold fingers” because of their gold-plated surface and the finger-like arrangement of the conductive contacts. Gold fingers are actually a layer of gold plated on a copper-clad board through an electroplating process. Due to the strong oxidation resistance of gold, it can protect internal circuits from corrosion, and its strong conductivity does not cause signal loss. Additionally, gold has a very high ductility, allowing for a larger contact area between the contacts under appropriate pressure, thus reducing contact resistance and improving signal transmission efficiency. Since the plating thickness is only a few tens of microns, it is easily worn, so unnecessary plugging and unplugging of components with gold fingers should be avoided to extend their lifespan.
The data flow and electronic flow of the memory processing unit are exchanged with the PC system through the contact of gold fingers with memory slots, acting as the input and output ports of the memory; therefore, the manufacturing process is crucial for memory connection.
Graded Gold Fingers Long and Short Gold Fingers
Long and short gold fingers have slightly less stress during insertion and removal.
Different connector shapes also have different stress.
Gold Finger Process Flow
Apply tape → Board grinding (micro-etching) → Washing → Activation → Washing → Nickel plating → Washing → Activation → Washing → Gold plating → Gold recovery → Washing → Air drying → Mounting
a. The purpose of applying tape
is to expose only the part of the board that needs to be gold plated, while covering the rest with tape to prevent plating. Applying tape is a delicate task, and it is not allowed for the tape to cover the gold fingers or be too far from them. Care must be taken to avoid cutting the board when removing excess tape to prevent manual scratches. During operation, ensure that the tape used does not have peeling issues to avoid residual adhesive on the copper surface. This step is the most labor-intensive, and the introduction of automatic tape application machines will bring another industrial revolution. The purpose of rolling tape is to ensure that the tape adheres firmly to the board surface using a roller machine, preventing cross-contamination of chemicals during nickel and gold plating due to the tape not being pressed firmly.
b. Board grinding (micro-etching)
Removes oil stains, oxidation skin, and green oil residues from the board surface, providing a bright, slightly rough copper surface to increase the adhesion between the copper layer and the nickel layer to be plated.
c. The role of activation
Removes slight oxidation from the copper surface and prevents re-oxidation, keeping the copper surface bright and clean, maintaining the activity of the copper or nickel layer, and enhancing the adhesion between the base metal and the metal to be plated.
d. Nickel plating
Acts as a barrier between the gold layer and the copper layer to prevent copper migration. To increase production speed and save gold usage, nearly all automatic nickel and gold plating equipment now uses conveyor belt upright types, with the main salt of the plating solution being nickel sulfamate (Nickel Sulfamate Ni (NH2SO3)2.4H2O), which has a very high nickel content and extremely low plating stress.
e. Gold plating
There is no fixed basic formula; gold salt (Potassium Gold Cyanide, abbreviated as PGC) is the main component.
Currently, regardless of acidic, neutral, or even alkaline gold plating, the pure gold used comes from high-purity gold salts, which are pure white crystalline substances without crystallization water. Depending on the crystallization conditions, there are both large and small crystals, the former forming slowly and steadily in a high-concentration PGC aqueous solution, while the latter is obtained by rapid cooling and stirring, with the latter being more common in the market.
f. Acidic gold plating (PH=3.8~4.6)
Uses non-dissolving anodes, the most commonly used are titanium nets attached with platinum, or tantalum nets (Tantalam) with platinum layers, the latter being more expensive but having a longer lifespan.
g. Automatic forward groove style automatic gold plating
Places the anode on both sides of the groove, with the conveyor belt pushing the board into the central groove, the current is connected by brass brushes (on both sides of the conveyor belt above the groove) contacting the lines protruding from the board. As soon as the board enters the plating groove, the current is immediately connected. Each plating groove has a buffer chamber with rubber pads to reduce chemical loss and prevent mutual contamination of chemical solutions.
h. In acidic gold plating, the cathode’s current efficiency is not good; even with new solutions, it is only about 30~40%, and due to gradual aging and contamination, it reduces to about 15%, making stirring of the acidic gold plating solution very important.
i. During the gold plating process, the cathode produces more hydrogen gas due to reduced current efficiency, which decreases the hydrogen ion concentration in the plating solution, resulting in a gradual increase in PH value. This phenomenon occurs in acidic gold plating processes that use cobalt or nickel or both. When the PH value gradually increases, the cobalt or nickel content in the plating layer decreases, affecting the hardness and porosity of the plating layer, hence the PH value needs to be measured daily. Usually, there are large amounts of buffering conductive salts in the plating solution, so the PH value does not change significantly unless there are abnormal conditions.
Surface discoloration, yellow spots, yellow dots, and black spots on gold fingers are all forms of corrosion. Despite the differences in the surface forms of discoloration, yellow spots, yellow dots, and black spots, their essence is all a form of corrosion, differing only in severity.
Performance Parameters of Gold Fingers
Thickness of gold layer, thickness of nickel layer, and density of the plating layer
Common Issues with Gold Fingers
1. Poor adhesion
This gold finger had peeling before leaving the factory due to insufficient adhesion, partially detaching from the substrate. During the insertion into the connector, the gold finger that was initially lifted would be bent back due to the resistance from the spring contact. Afterward, when the gold finger is inserted into the connector, it experiences clamping force from the spring contact, causing two layers of gold fingers to be pressed together at the bending point. When the gold finger is pulled out of the connector, the friction from the spring contact pulls the gold finger towards the connector, and due to the weakness at the bending point, part of the gold finger breaks off during the removal process.
Laboratory simulations showed that lifting the gold finger part and inserting and removing it from the connector can replicate this phenomenon.
The main reasons for insufficient adhesion of gold fingers include poor pre-plating treatment, long power outages during plating solution, organic contamination in the plating solution, and low nickel tank temperature.
2. Poor gold color
Discoloration, yellow spots
Discoloration observed under magnification. From the magnified view, the darkened spots occur in the pits of the gold surface, which are actually pinholes in the gold layer. Through these pinholes, the underlying nickel is exposed to oxygen, causing discoloration. The aggregation of these darkened spots leads to widespread discoloration of the gold finger surface.
Yellow spots occur in small localized areas on the already discolored gold layer surface. Yellow spots are teardrop-shaped and are likely caused by splashes of saliva during assembly. Due to the corrosiveness of saliva, it penetrates through the pinholes in the gold layer to the underlying nickel, exacerbating the corrosion of the nickel layer and darkening its color. At this point, the nickel layer beneath the pinholes has turned black, and the black nickel oxide molecules have diffused to the surface of the gold layer, darkening it further.
Black spots
Large areas of black defects appearing on gold fingers, as shown in the figure. This is clearly the result of corrosive substances (such as residues from the plating solution, salt spray solutions, etc.) adhering to the surface of the gold layer.
The black disk phenomenon caused by the oxidation of the nickel plating layer occurs only in PCB ENIG Ni (P)/Au coatings; the black disk phenomenon is essentially the oxidation of nickel, where black nickel is nickel oxide (NixOy).
The causes of the black nickel phenomenon are very complex. One theory explains that during chemical immersion Au/Ni, the dissolution of nickel and deposition of gold occur simultaneously in a displacement reaction. When the displacement reaction of the gold plating solution is too vigorous, it causes rapid oxidation of the nickel layer, turning it black.
The gold layer formed by the method of displacing gold on the nickel surface is thin and has many pinholes. The number of pinholes is related to the parameters of the ENIG Ni/Au process and its process control, as well as the thickness of the chemical gold layer. If the coating is too thin or the process parameters are not controlled properly, it may lead to poor quality of the gold layer covering the nickel, resulting in numerous pinholes that cannot prevent the growth of nickel oxide, leading to large areas of black nickel oxide.
Gold itself has extremely high corrosion resistance, but after the occurrence of the black disk phenomenon, the adhesion between the gold layer and the nickel oxide layer is lost, causing most of the gold layer to peel off the surface of the gold finger, exposing large areas of black nickel oxide.
This defect is sporadic, and its occurrence is unpredictable, posing significant risks.
3. Rough gold surface
The surface is not smooth, with bumps
Solutions to Issues
(1) Improve the PCB ENIG Ni (P)/Au process conditions and fine control of the process to increase the thickness of the gold layer and reduce pinholes in the gold layer.
(2) It is recommended to use nickel/gold plating for gold fingers.
Some content excerpted from “Analysis of Gold Finger Discoloration Process Cases” by Green Board Observation
【1】Interest-driven passion
【2】Should hardware engineers design PCBs themselves?
【3】How long should PCB traces be?
【4】How wide should PCB traces be?
【5】The inner layers of PCBs
【6】Via holes
【7】Can PCB traces have sharp angles and right angles?
【8】Should dead copper be retained? (PCB islands)
【9】Can vias be placed on pads?
【10】What materials does FR4 refer to in PCBs?
【11】Why is the solder mask layer usually green?
【12】Steel mesh
【13】Pre-layout
【14】Key points of PCB layout and routing
【15】Cross-segment routing
【16】Signal reflection
【17】Dirty signals
【18】Surface treatment processes such as immersion gold, gold plating, and tin spraying
【19】Trace width
【20】Positioning of capacitors
【21】Crosstalk
【22】PCB flying needle testing
【23】Overview and simulation of FPC
【24】Why do PCBs deform and bend? How to solve it?
【25】Understanding “Characteristic Impedance” in one article
【26】PCB stacking design
【27】High-speed circuit PCB reflow path
【28】Power processing and plane segmentation in PCB design
【29】Zigzag PCB routing – Tabbed routing
【30】What is the loss angle of PCB dielectric? “∠”?
【31】The impact of PCB copper foil roughness on high-speed signals
【32】Why shouldn’t crystal oscillators be placed at the edge of the PCB?
【33】What are high-speed signals?
【34】What is a transmission line?
【35】Pre-emphasis, de-emphasis, and equalization
【36】How to utilize PCB heat dissipation
【37】”Stub” in PCB design
【38】Controversy: Should there be a GND protection line between traces?
【39】PCB copper pour
【40】Rules to follow when designing PCBs
【41】”Fake eight layers” in PCB stacking design
【42】In addition to strip lines and microstrip lines, there are also “coplanar waveguides”
【43】Parameters related to PCB pad design
【44】Why should grounding holes be drilled at the edges of PCBs?
【45】PCBs that dissipate heat more easily: aluminum substrates
【46】Why should reference planes be hollowed out?
【47】PCB design of crystal oscillators
【48】Designing PCB for DC/DC power supply with EMC considerations
【49】PCB corners, not necessarily rounded lines are the best
【50】Why should vias be blocked? “Conductive via plugging process”
【51】Key points for power PCB layout and routing
【54】Rigid-flex boards (soft-hard combined boards)
<55】Mixed-signal PCB design
<56】Capacitor placement in PCB design
<57】The impact of via stub in PCB design
<58】Decoupling capacitors in PCB design: placement and routing
<59】PCB design checklist: structure
<60】PCB design checklist: power
<61】PCB design checklist: routing
<62】PCB design checklist: high-speed digital signals
【63】Process edges
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