PCB board surface treatment
Oxidation resistance, tin spraying, lead-free tin spraying, gold sinking, tin sinking, silver sinking, hard gold plating, whole plate gold plating, gold finger, nickel palladium OSP: lower cost, good solderability, harsh storage conditions, short time, environmental protection process, good welding, smooth.
Tin: Tinjet board is usually a multi-layer (4-46 layer) high-precision PCB sample, which has been used by many large communication, computer, medical equipment and aerospace enterprises and research units in China.
The gold finger is made up of a number of golden conductive contacts, which are gold-plated and arranged in a finger-like manner. Gold fingers are actually coated with a special layer of gold on top of copper-clad plates, because gold is highly resistant to oxidation and highly conductive. Because the price of gold is expensive, however, the current tin plating is used to replace the more memory, tin material from the last century 90 s began to spread, the motherboard, memory, and video devices such as “golden 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, the price expensive.
Why gilt plate (gold-plated PCB)
With the higher degree of integration of IC, IC feet are more and more dense. The vertical tin-spraying process is difficult to flatten the thin pad, which brings difficulties to SMT. In addition, the shelf life of tin-sprayed plates is very short. And the gold plate is a good solution to these problems:
★ For the surface mount technology, especially for 0603 and 0402 ultra small table paste, because the flatness of the pad is directly related to the quality of solder paste printing process, play a decisive influence on the reflow welding quality behind, so, the whole plate gold plating in high density and ultra small table paste technology is often seen.
★ In the trial production stage, under the influence of component procurement and other factors, the boards are often not welded immediately, but often wait for several weeks or even a month to use, and the shelf life of gold-plated plates is many times longer than that of lead-tin alloy, so everyone is willing to use them. Moreover, the cost of gold-plated PCB in the sample stage is almost the same as that of lead-tin alloy plate.
However, as wiring becomes more and more dense, the line width and spacing have reached 3-4mil.
Therefore, the problem of short circuit of gold wire is brought: with the increasing frequency of the signal, the effect of signal transmission in the multi-coating caused by skin effect on the signal quality is more obvious.
Skin effect refers to: 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.
Why do you use a countersunk plate
In order to solve the above problems of gold-plated plate, the PCB with gold-plated plate has the following characteristics:
★ Because of the crystal structure formed by sunken gold and gold plating is not the same, sunken gold will be more yellow than gold plating, the customer is more satisfied.
★ Because of the crystal structure formed by sunken gold and gold plating is not the same, sunken gold is easier to weld than gold plating, will not cause bad welding, causing customer complaints.
★ Because there is only nickel gold on the solder plate, the signal transmission in skin effect is in the copper layer and will not affect the signal.
★ Because the gold plate only has nickel gold on the pad, so it will not produce gold wire resulting in a little short.
★ Because the gold plate only has nickel gold on the pad, so the combination of resistance welding and copper layer on the line is more solid.
★ The spacing will not be affected when the project is compensated.
★ Because of the crystal structure formed by gold and gold plating is not the same, the stress of the gold plate is easier to control, for the products of the state, more conducive to the processing of the state. At the same time, because the gold is softer than gold, so the gold plate is not wear-resistant.
The smoothness and service life of the sunken gold plate are as good as that of the gold-plated plate.
Countersunk plate VS gold-plated PCB
In fact, the gold-plating process is divided into two kinds: one is electroplated, one is sunk gold.
For the gold-plating process, the effect of tin is greatly reduced, and the tin effect of gold is better; Unless the manufacturer is required to bind, now most of the manufacturers will choose to sink gold process! In general, the surface treatment of PCB is as follows: gold-plated (electroplated, gold-immersed), silver plated, OSP, tin-spraying (lead and lead-free), which is mainly for FR-4 or CEM-3 plate, paper base material and rosin coated surface treatment; On tin bad (tin eating bad) this if the exclusion of solder paste and other patch manufacturers production and material technology reasons.
Here only for PCB problems, there are several reasons:
★ In PCB printing, PAN position is oil permeable film surface, it can block the effect of tin; This can be verified by a tin-bleaching test.
★ PAN run position on whether to meet the design requirements, that is, pad design can be enough to ensure the support of parts.
★ Pad is not polluted, which can be obtained by ion pollution test results; The above three points are basically the key aspects considered by PCB manufacturers.
The advantages and disadvantages of several ways of surface treatment, are each have their strengths and weaknesses!
Gold-plating, it can make THE PCB storage time is longer, and by the external environment temperature and humidity change is small (relative to other surface treatment), generally can be saved for about a year; Tin spray surface treatment, OSP again, these two kinds of surface treatment in the environment temperature and humidity storage time to pay attention to many.
In general, the silver surface treatment is a little different, the price is also high, the preservation conditions are more demanding, need to use sulfur-free paper packaging processing! And keep for about three months! In terms of tin effect, sunken gold, OSP, tin injection is almost the same, manufacturers are mainly considering the cost performance!