PCB surface treatment technology

The basic purpose of PCB surface treatment is to ensure good solderability or electrical properties. Due to the presence of copper in the air tends to be in the form of oxides, it is unlikely to maintain the original copper for a long time.

1.HASL
HASL, it is a process for coating molten tin (lead) on the surface of PCB and heating (compressed) by heating compressed air, to o form a coating layer that is resistant to copper oxidation and provides good solderability. Intermetallic compounds are formed at the junction of solder and copper. The knife can minimize the solder meniscus and prevent solder bridging.

2.OSP

OSP is a printed circuit board (PCB) with a process of RoHS instructions copper surface treatment. ,Simply speaking, OSP is on the surface of cleaning bare copper, chemically grown a layer of organic film. This film has an oxidation resistance, thermal shock resistance, moisture resistance, to protect the copper surface in the normal environment no longer continue to rust.

3.Immersion gold

The deposit of gold is a thick, electrical nickel alloy which is wrapped on the copper surface, which can protect PCB for a long time. In addition it also has other surface treatment process does not have the environmental endurance.

4.Immersion silver

The process of silver precipitation is between organic coating and electroless nickel plating. The process is simple and fast, and even if exposed to heat, humidity and pollution, silver can maintain good weldability, but it will lose luster.

5.Hard gold plating

In order to improve the wear resistance of products, increase the number of times and the hard gold plating.

PCB downstream market demand is flourishing, Highlight the advantages of local chemicals

With the global PCB capacity continues to shift to China and downstream product demand increased significantly, PCB industry will usher in rapid expansion opportunities.

MIIT statistics show, processing of various types of terminal electronic products in China, such as TV, monitor, notebook, mobile phone, their production has accounted for more than 50% of the global share. The rapid development of downstream industries will drive the rapid development of electronics related industries, including PCB.

At present, China's PCB output and output are ranked first in the world. Our PCB industry growth rate is significantly higher than the global growth rate.

PCB production process in each link is essential to use different chemicals, about 10% to 30% of the cost of PCB. Chemical quality will directly affect the performance of the PCB board. There are wide variety of chemicals, including Dry film photo resist, inks and PCB wet process chemicals. PCB wet process chemicals is expected to take the lead in achieving import substitution in the high-end field.

China's PCB chemicals companies started late, Technology accumulation and R & D capability is weak, there is still a certain gap with foreign enterprises.

At present, domestic chemical products are mainly concentrated in the low-end market,  low industry concentration, we play a leading role in the auxiliary materials with low value, but in electroplating process, PCB surface treatment, chemicals are still monopolized by foreign enterprises.

In recent years, with the domestic PCB chemical companies to increase R & D investment and product quality continues to improve, some high-end products have been accepted by PCB vendors, It is expected to gradually break the long-term monopoly of foreign enterprises PCB chemicals market situation.

Layout and type selection of printed circuit boards used in SMT processing

1 The main factors to be considered in mechanical design:
1)Selecting the best printed circuit board to make the best panel size;
2)Panel mounting hole, bracket, splint, clip, shielding box and radiator location;
3)suitable for heavy components of the fixed device;
4)Suitable aperture for component installation;
5)Assembly of the circuit board in the transport process to have compression and seismic resistance;
6)Circuit board assembly (vertical mounting / horizontal mounting);
7)Cooling mode;
Printed circuit board not only provides mechanical support,is also connected to the components installed on it.Therefore, for the designer of the printed circuit board,It is necessary to understand the entire panel designers physical size (frame size), the position of mounting holes, height restrictions and related details.

2 The choice of single or double sided board must meet the most effective cost utilization.
According to the experience, the double sided printed boards with plated through holes cost 5 -10 times as much as a single panel.Similarly, the cost of assembly components is an important aspect to consider.The cost of assembling a single printed circuit board component (manual) is about 25% -50%.While ,the assembly of a double-sided printed circuit board with PTH components, the cost is 15% -30%.
Double plate:Double sided board can be used or can not use PTH,because of the high price of PTH board.PTH will be used when the complexity and density of the circuit is required.In the layout design, the number of wires must be kept at least,to ensure easy access to the required materials.In the PTH board, plated through holes are used only for electrical connections but not for the installation of components.The number of holes should be kept to a minimum for economic and reliability considerations.To select a single panel or double-sided board, it is important to take into account the surface area of the component (C).The ratio of the surface area (C) of the component to the total area (S) of the printed circuit board is an appropriate constant proportion,which is useful for the installation of components.

PCB Circuit board welding procedure specification

1 Preparation before welding
1.1Material: pay attention to the polarity of welding components,Whether there is the oxidation or not oil in the components foot.
1.2 Tool:Quantity should be consistent with the bom list.It’s 2 particles at most for each time to be taken.We should put the material back to the original palce.
1.3 pcb board:Check board circuit, whether there is short circuit, open circuit, etc on the board..
1.4 The list:Please make sure that the list is correct.
1.5 Workbench:Must make sure that it is neat and clean.If there is anti-static requirements, then we should pay attention to the use of anti-static workers appliances.At the same time the operator should wear anti-static wrist str
2 Implement welding
2.1 Safe use and scientific use of iron,Keep the tip clean.Iron head working temperature: lead welding temperature at 350 ~ C, lead-free welding general at a temperature of 380 DEG C, turn off the power when not in use.
2.2No pressure should be applied during welding.General solder joints takes about 2 ~ 3 seconds 
to complete it. Notice:should not be taken before the tin solder was complete freezing. In order 
to avoid insufficient solder we should correctly use flux. 
2.3Correct posture of welding operation.Normally,distance from Iron to nose should be not less 
than 20cm, Usually 30cm is suitable .
2.4Discrimination of polarity of welding components.The welding components should be neat, 
middle, paste board.The welding sequence of the device shall be based on the principle that the 
first welded element does not affect the welding of the following elements.First, we weld small 
capacitance and other devices,and then weld the larger elements,lastly we weld plug.
2.5we should use the tools properly, and put them away after using them.
2.6Treatment of waste after welding.we should Clean up the waste and throw it in the trash.
2.7Try to avoid repeated welding
2.8Sealing-off the components.
1)Proper use of tools,depends on the welding element set (iron, hot air guns, tweezers, etc.).
2)The correct use of tin gun
3)Temperature and time should be reasonable.
3 Treatment after welding
3.1Pay attention to the following points after welding:
1)Check leak welding, welding fault (polarity welding anti), short circuit phenomenon, weld etc.
2)Check solder joints for proper solder,Whether there is good gloss and even on the surface .There should be no burrs, gaps and cracks on the surface.Solder surface should be clean.
3.2To properly use the wash plate water:Clean the residue on the PCB board, such as tin dross, tin scrap, component feet, etc. The user should do a good job protection measures.Because Plate washing water is volatile and flammable,the leftover should be installed in place.Don't waste water.
3.3Power on detection .
1)Measure the input terminal of the power supply with Multimeter resistance block,to see if there is a short circuit.If yes ,we should be excluded before power up.
2)According to the schematic diagram of the circuit module circuit inspection,If you have any questions, you can ask the Engineer
3)After the completion of the power must be assembled according to the list of IC, check the error
3.4PCBA should be checked with a good electrostatic bag packaging, can not be placed at random.

Circuit board welding process

Technology principle

BGA welding using the principle of re-flow soldering.Let’s introduce tin ball in the back-flow mechanism in the process of welding.

Circuit board welding preheating:First of all, First of all, used to achieve the required viscosity and printing properties of the solvent evaporated,Temperature rise must slow (about 5 ° C per second),to limit the boiling and splashing,to prevent the formation of small beads.Some components of internal stress is more sensitive.If the external temperature rising too fast,it can lead to fracture.

Active flux (cream),Chemical cleaning action began.Water-soluble flux (cream) and disposable type flux (cream) will occur the same cleaning action.Temperature is a little different.Will some pollution from the combination of metal oxides and metal removed and solder particles.Good metallurgy tin solder joint requirements on the surface of the "clean".
When the temperature continues to rise.Solder particles separate melt in the first place.Liquefaction and surface tin absorption and start the process of "rush".So in all possible covering on the surface, and began to form tin solder joints.

Circuit board welding back-flow:The most important at this stage.When all individual solder particles melt, combine together to form a liquid tin,the surface tension then began to form surface welding feet.If the clearance between the PCB welding plate and pin over 4 mil,Is most likely due to divide the pins and welding plate surface tension, namely tin caused by open circuit.

Circuit board welding cooling:If cooling fast, tin some strength slightly larger.But not too fast otherwise it will cause the temperature stress inside the element.

Circuit board welding temperature area:BGA solder ball, divided into lead-free and has two kinds of lead.Have lead solder ball in 183 ℃ ~ 220 ℃, melting point of lead-free solder ball melting point at 235 ℃ ~ 245 ℃.
No matter how lead welding or lead-free soldering.Tin ball melting stage are in the re-circulation region.Only the temperature is different.Before re-flow curve can be viewed as a slow process of heating and heat preservation.

Circuit board welding preheating zone:Is used to set the temperature of the PCB from ambient temperature to the activity of required temperature.Circuit boards and components of the heat capacity is different.Their actual temperature increase rate is different.Pcb and the temperature of the components shall be no more than 2 ~ 5 ℃ per second speed continuous rise.If too fast, can produce thermal shock.Circuit boards and components may be damaged.Such as ceramic capacitor of tiny cracks.And the slow temperature rise, solder paste will be excessive heat.Inadequate solvent evaporation, affects the welding quality.Furnace preheating zone of 15 to 25% of the entire heating zone length.

Circuit board welding heat preservation area:In this area generally account for heating area 30 ~ 50%,The active area of each element on the main purpose is to make the PCB temperature to stabilize.To minimize the temperature difference.In this area to give enough time to make the heat capacity of the temperature of the components to catch up with smaller components.To the end of the activity area, solder, solder ball and element oxide on the pin be removed, the whole circuit board temperature equilibrium.Attention should be paid to the PCB all components at the end of this area should have the same temperature.Or into the back-flow zone will be because of uneven temperature of each part to produce all kinds of bad welding phenomenon.General common active temperature range is 120 ~ 150 ℃.If active zone temperature setting is too high.Flux (cream) don't have enough time to activity.Increasing temperature curve slope is an upward slope.Although some solder paste manufacturers allow some increase in temperature during activation.But the ideal temperature curve should be a steady temperature.

Circuit board welding back-flow zone:The role of the ward is will increase the temperature of the PCB from the active temperature to the recommended peak temperature.Activity is always lower than the alloy melting point temperature, temperature.And the peak temperature is always on the melting point.Is the scope of a typical peak temperature solder paste alloy melting point temperature of 40 ℃ or so.Re-circulation region scope of working time is 20 to 50 s.The zone temperature setting is too high will make its temperature rising slope over 2 ~ 5 ℃ per second.Or make re-flow peak temperature is higher than recommended.Or work time is too long can lead to excessive crimp PCB, take off a layer or damage, and damage the integrity of the component.Re-flow peak temperature is lower than that of the recommended, work time is too short possible defects such as cold welding.

Circuit board welding cooling zone:This area of the solder paste tin alloy powder is melted, and fully connected surface wetting.Should be cooled quickly as possible.This will help the alloy crystal formation to get the bright spot.And has good appearance and low contact angles.Slow cooling leads to more decomposition of impurities into the tin of circuit board, resulting in a rough welds.In extreme cases, it may cause poor wetting and weaken the solder joint binding force.Cooling section of the cooling rate is commonly 3 ~ 10 ℃ / S.

FPC flexible circuit board main features

FPC flexible circuit board flexura and reliability
1.The FPC has four kinds: single side, double side, multi-layer flexible plate and just flexible plate.

①Single flexible plate is the lowest cost, when the PCB for electrical demand is not high。Single flexible circuit board has a chemical etching out the layer of conductive graphics，In flexible conductive graphics layer on the surface of the insulating substrate for rolled copper foil.Insulating base material can be polyimide, polyethylene terephthalate, aromatic polyamide fiber resin and PVC.
　　②Double flexible plate is on the two sides each have a layer of insulating base film etching made of conductive graphics.Metallized hole to both sides of the insulation materials connection form a conductive path, in order to meet the deflection design and use of the function.While covering film can protect the single and double wires and components placed.
③Multi-layer flexible plate is a level 3 or more layers of single sided or double sided laminated flexible circuit together.，Through the drill away L form metal, electroplating, the formation of a conductive path between different layers.Multi-layer circuit in higher reliability, better thermal conductivity and convenient assembly has huge function differences in performance.When designing the layout, assembly shall be taken into consideration, layer number and size of flexible influence each other.
　　④Traditional just flexible plate is made from rigid and flexible substrate has a choice of formation pressure together.Structure closely to metallization away L form electrical connection..If a PCB is, opposite element, just flexible plate is a good choice.But if all the components are on one side, selects the double flexible plate, and on the back of the laminated layer on FR4 reinforced material, will be more economic. 
⑤Hybrid structure is a kind of sandwich plate, flexible circuit conductive layer made of different metals.A eight layer plate using FR - 4 as inner medium, use polyimide as the outer layer of the medium,Out from three different direction of the motherboard, lead each wire made of different metals.Constantan alloy, copper and gold as independent lead, respectively.This kind of hybrid structure is mostly used in electrical signal conversion and is quite harsh relationship and performance of heat transformation of low temperature circumstances, is the only feasible solution.　　
　　2．FPC flexible circuit board of the economy
　　If the circuit design is relatively simple, total volume is not large, and space is appropriate, even within the traditional way is a lot cheaper.If the line is complex, with many signals or have special electrical or mechanical performance requirements, flexible circuits is a better design options.When the size and performance of the application beyond the ability of rigid, flexible assembly is the most economical way.On a thin film can be made with 5 12 mil mil hole welding plate and 3 mil line and spacing of flexible circuits.Therefore, in the SMT chip directly on thin film is more reliable.Because do not contain ions may be drill WuYuan flame retardants.These may have a protective film and curing under high temperature, high glass transition temperature.Flexible material compared with the rigid material cost saving reason is exempt from the connector. 
　　The high cost of raw materials is flexible circuits of the main causes of high price.Cost of raw material price difference is bigger, the lowest polyester raw materials used in flexible circuit cost is 1.5 times that of the raw materials used in rigid circuit;High-performance polyimide flexible circuits, up to 4 times or more.At the same time, the flexible material could make it not easy to automate process in the manufacturing process, leading to the decline in output;Liable to occur in the final assembly process defects, these defects including off flexible attachment, line breaks.When the design is not suitable for application, such cases are more likely to happen.Under the high stress caused by bending or molding, often need to choose the reinforced material or reinforcing materials.Despite the high cost of raw materials, manufacture of trouble, but folding, flexible and multi-layer makeup function, will reduce the overall component size, use less material, reduce the total assembly cost. 
　
　　3．The cost of FPC flexible circuit board
　　Despite the cost of the above factors, the flexible assembly price is falling, become and close to the traditional rigid circuit.The main reason is the introduction of updated materials, improved the production process and the change of the structure.Now so that the thermal stability of the product is higher, there are few materials do not match.Some of the updated material for copper layer is thinner and can make more precise line, make the component more light, more suited to put into a small space.In the past, the rolling process of copper foil adhesion in coated with adhesives on the medium, now, can not directly use adhesives in medium generated on the copper foil.These techniques can get several microns thick copper layer, get a 3 m. 1 even more narrow width precision line.After removed some adhesives flexible circuit has the flame retardant performance.It can accelerate the uL certification process and can further reduce the cost.The surface of the flexible circuit board solder mask and other coating made of flexible assembly further reduce the cost. 
　　In the next few years, smaller, more complex and assembly cost higher FPC flexible circuit boards will require more novel methods of assembly, and mixed flexible circuits may be increased.For flexible circuits industry challenge is to use the technical advantages, keep with the computer, telecommunication, consumer demand and market of active sync.In addition, the flexible circuits will play an important role in utilizing action.

PCB assembly production line working environment requirements

For many people,the requirement for the PCB assembly production line is not very clear,and we will show you about this
First, temperature
The SMT print machine required a temperature 23 degree and tolerance is 3 degree,so the working environment temperature should between this range.Generally,the working temperature between 17-28 degree is ok,if it can’t reach this,it can over 15-35 degree.
Second, humidity
Humidity has a great influence on product quality in the production environment. If too high ,some element is easy to absorb moisture, it’s not good to moisture sensitive components, and if solder paste exposed to humid air ,it is easy to absorb moisture, cause welding defects. If the humidity is too low and the air is dry, it is easy to produce static electricity, which is unfavorable to the electrostatic sensitive element (ESD). Therefore, the environmental humidity must be controlled. Generally, the relative humidity between 45% to 70 % RH is good.And there are also some rules is 30% to 55%RH,and some not very district rule is 40%-80% RH.

Third,Air cleanliness

If there are too much dust in the workshop,it will have very serious influence for the small components,like 0201 and 01005 and the small pitch(0.3mm)SMT components,and it will speed up the attrition of the equipment,and it can even cause the equipment fault,increase some extra work for equipment repaid and maintenance.In the SMT assembly environment,except the dust,there maybe some chemical gas,if there is harmful thing in the gas,it will bring some harm for the workers;if the gas have corrosivity,it will influence the reliability of the product a lot.So the work environment need to be clean,no dust,no corrosivity,and no smell gas.Then it can make use the PCB assembly products quality,the equipment can work well,and the worker’s health.

And the workshop cleanliness should reach 100 thousand grade(BGJ73－84).If under the air conditioner,there need be a certain of wind,and try the best to control the CO2 below 1000PPM,and to make sure the worker’s health.

To make sure the factory cleanliness reach 100 thousand grade(BGJ73－84),the factory should pay for a lot of money for that.And the normal factory can’t get that.In order to get this kind of environment,when the workers come to the workshop,they have to extract to vacuum,and there are negative pressure in the whole workshop.In today's highly competitive market environment,the companies’ profit is very small,if you can’t reach this requirement,you have to have some rules with the air cleanliness,such as the carton which can brought a lot of dust can’t get to the SMT workshop.