1. Surface of PCB: OSP, HASL, Lead-free HASL, Immersion Tin, ENIG, Immersion Silver, Hard gold plating, Plating gold for whole board, gold finger, ENEPIG…
OSP: low cost, good solderability, harsh storage conditions, short time, environmental technology, good welding, smooth…
HASL: usually it’s multilayers HDI PCB Samples (4 – 46 layers), has been used by many large communications, computers, medical equipment and aerospace enterprises and research units.
Gold finger: it’s the connection between the memory slot and the memory chip, all signals are sent by gold finger.
Gold finger isconsists of a number of golden conductive contacts, which are called “gold finger” because of their gold-plated surface and their finger-like arrangement. Gold finger actually USES a special process to coat copper cladding with gold, which is highly resistant to oxidation and highly conductive. But the price of gold is expensive, the current tin plating is used to replace the more memory. From the last century 90 s, the tin material began to spread, the motherboard, memory, and video devices such as “gold finger” is almost always used tin material, only some high-performance server/workstation accessories will contact point to continue the practice of using gold plated, so the price has a little of expensive.
2. Why use the gold plating board?
With the integration of IC higher and higher, IC feet more and more dense. While the vertical tin spraying process is difficult to blow the fine welding pad flat, which brings difficulty to SMT mounting; In addition, the shelf life of tin spraying plate is very short. However, the gold plate solves these problems:
1.) For surface mount technology, especially for 0603 and 0402 ultra-small table mount, because the flatness of the welding pad is directly related to the quality of the solder paste printing process, on the back of the re-flow welding quality has a decisive impact, so, the whole plate gold plating in high density and ultra-small table mount technology is often seen.
2.) In development phase, the influence of factors such as components procurement is often not the board to the welding immediately, but often have to wait a few weeks or even months before use, shelf life of gold plated board is longer than the terne metal many times, so everyone is willing to adopt. Besides, gold-plated PCB in degrees of the cost of the sample stage compared with pewter plates
But with more and more dense wiring, line width, spacing has reached 3-4MIL
Therefore, it brings the problem of short circuit of gold wire: with the increasing frequency of the signal, the influence of signal transmission in multiple coatings due to skin effect becomes more and more obvious
(skin effect : High frequency alternating current, current will tend to concentrate on the surface of the wire flow. According to the calculation, skin depth is related to frequency.)
3. Why use the immersion gold PCB?
There’re some characteristics for the immersion gold PCB show as below:
1.) the crystal structure formed by immersion gold and gold plating is different, the color of immersion gold will be more good than gold plating and the customer is more satisfied. Then the stress of the submerged gold plate is easier to control, which is more conducive to the processing of the products. At the same time also because gold is softer than gold, so gold plate do not wear – resistant gold finger.
2.) Immersion Gold is easier to weld than gold plating, and will not cause poor welding and customer complaints.
3.) the nickel gold is only found on the welding pad on ENIG PCB, the signal transmission in the skin effect is in the copper layer, which will not affect the signal, also don’t lead short-circuit for the gold wire. The soldermask on the circuit is more firmly combined with the copper layers.
4.) The crystal structure of immersion gold is denser than gold plating, it is difficult to produce oxidation
5.) There will be no effect on the spacing when compensation is made
6.) The flatness and service life of the gold plate is as good as that of the gold plate.
4. Immersion Gold VS Gold plating
There are two kinds of gold plating technology: one is electrical gold plating, the other is Immersion Gold.
For the gold plating process, the effect of the tin is greatly reduced, and the effect of the gold is better; Unless the manufacturer requires the binding, or now most manufacturers will choose the gold sinking process!
Generally, the surface treatment of PCB can be divided into the following types: gold plating (electroplating, immersion gold), silver plating, OSP, HASL (with and without lead), which are mainly for FR4 or CEM-3 plates, paper base materials and rosin coating surface treatment; On the tin poor (eating tin poor) this if the removal of paste manufacturers and material processing reasons.
There’re some reasons for the PCB problem:
1.During PCB printing, whether there is oil permeating film surface on PAN, it can block the effect of tin; this can be verified by a solder float test
2.Whether the embellish position of PAN can meet the design requirements, that is, whether the welding pad can be designed to ensure the support of the parts.
3.The welding pad is not contaminated, which can be measured by ion contamination.
About the surface:
Gold plating, it can make PCB storage time longer, and by the outside environment temperature and humidity change is small (compared to other surface treatment), generally, can be stored for about a year; HASL or lead free HASL surface treatment second, OSP again, the two surface treatment in the environment temperature and humidity storage time to pay attention to a lot of under normal circumstances, silver surface treatment is a little different, the price is also high, preservation conditions are more demanding, the need to use no sulfur paper packaging processing! And keep it for about three months! On the tin effect, gold, OSP, tin spray is actually about the same, manufacturers are mainly to consider the cost performance!