Advantages, disadvantages and application scene of different PCB surface treatment technics

PCB technology has undergone tremendous changesalong with the continuous development of electronic science and technology, and the manufacturing technicsneeds to be improved. At the same time, the technicsrequirements of PCB boards have gradually improvedin each industry. For example, in the circuit boards of mobile phones and computers, bothgoldandcopperare used, which makes the board's advantages and disadvantages gradually become easier to distinguish.

Let us learn aboutthe surface technicsof the PCB board, compare the advantages and disadvantages of different PCB board surface treatment technicsand applicationscene.

Simply from the appearance, the externallayer of the pcb board has three main colors: gold, silver, and light red. Classified by price: gold is the most expensive, silver is second, light red is the cheapest,it is easy to judge whether the hardware manufacturers have the actionof cutting material and technics from the color . However, the internal circuitof the board is mainly pure copper, which is bare copper.

First, pcb bare copper board

The advantages and disadvantages are obvious:

Advantages: low cost, smooth surface and good weldability (Under no oxidationcondition).

Disadvantages: It is easy to be affected by acid and humidity. It can't be placedfor a long time. It needs to be usedupwithin 2 hours after unpacking, because copper is easily oxidized when exposed to air; it can't be used in doublesided board becausethe second side is oxidized after the first reflow. If there is a test point, solder paste must be applied to prevent oxidation, otherwise the subsequent contact with the probe will not be good.

Pure copper is easily oxidized if exposed to air, and the externallayer must have the above protective layer. And someonethink that the golden color part is copper, whichis absolutelywrong idea, because it is the protective layer on the copper. Therefore, it is necessary to plate a large area of gold on the circuit board, which is the immersion gold technicsthat you have learned before.

Second, gold plating board

Goldcoloris the real gold. Even if only a very thin layer is plated, it already accounts for nearly 10% of the cost of the board. In Shenzhen, there are many merchants who specialize in buying used circuit boards. It is a good income to wash out gold through certain means.

Gold is used as the coatinglayer, firstlyfor the convenientwelding and secondly for anti-corrosion. Even the golden fingers that have been used for several years are still flashing. If copper, aluminum, and iron were used, they are now rusted into a pile of waste.

The goldplatinglayer is widely used in board places of component pads, gold fingers, connector shrapnel, etc. If you find that the circuit board is actually silver, then needless to say, directly call the consumer rights hotline, it is certainly the manufacturers cut down material and technics, do not use the materials wellbut use other metal to fool customers. The motherboards ofmobile phone circuit boards are mostly goldplatingboards, and the immersion gold boards, computer motherboards, audio and small digital boards are generally not goldplating board.

It’s not difficult to conclude the advantage and disadvantage of immersion gold.

Advantages: It is not easy to oxidize, can be stored for a long time, and the surface is flat. It is suitable for soldering fine gap pins and components with small solder joints.Whichis thepreference for a button PCB board (such as a mobile phone board, that can be repeatedmultiple timesover reflow solderingand it is less likely to reduce solderability. Can be used as substrate for COB (Chip On Board) wire bonding.

Disadvantages: high cost, poor soldering strength, because the usageof electroless nickel process, easy to have black padproblems. The nickel layer oxidizes over time and long-term reliability is a problem.

Now we know that goldcoloris gold.Silvercoloris silver? Of course not, it is tin.

Third, spray tin circuit board

The silver board is calledspray tin board. Spraying a layer of tin on the externallayer of the copper circuitcan also aid in welding. But it can’t provide long-lasting contact reliability like gold. It has nearly no effect on the components that have been welded, but for pads that are exposed to the air for a long time, reliability is not enough, such as ground pads, pin sockets, etc. Long-term usingis prone to oxidative corrosion, resulting in poor contact. Basically circuit board used for small digital products, without exception, arespray tinboard due to cheapcost.

Its advantages and disadvantages are summarized asbelow:

Advantages: low price and good welding performance.

Disadvantages: It is not suitable for soldering fine gap pins and small volumecomponents because the poor fatness of spray tin board. Solder bead is easy to be produced in PCB processing, and it is easy to cause short circuit to fine pitch components. When used in the double-sided SMT technics, because the second side has been through oncehigh-temperature reflow soldering, it is very prone to re-melting the tin to produce solder beador ball-shapetin spots similar to water droplets that are dripped by gravity, resulting in serious unfairness surfaceand thus affecting the welding.

Previously we said the cheapest light red circuit board, namely the miner's lamp thermoelectric separation copper substrate

Fourth, OSP technic board

Organic solder mask. Because it is organic, not metal, it is cheaper than the tinspraying technics.

Advantages: with allthe advantages of bare copper boardwelding, the expired board can also be re-surface treatment.

Disadvantages: susceptible to acid and humidity. When used in secondary reflow soldering, it needs to be completed within a certain period of time. Usually, the effect of the second reflow soldering will be poor. If the storage time exceeds three months, it must be re-surface treatment.it needs to be used up within 24 hoursonce unpacking. The OSP is an insulating layer, so the solder paste must be applied to the test point to remove the original OSP layer in order to contact the pin point for electrical testing.

The only effect of this organic film is to ensure that the inner copper foil is not oxidized beforewelding. This film evaporates as soon as it is heated. Solder canweldcopper wires and components together.

But it is notresistant to corrosion. An OSP board can't be soldered if it is exposed to the air for ten days.

There are OSP technicsin manycomputer motherboards. Because the board area is too large, it’s too expensive to usegold plating.