SMT Process Introduction

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Mon, Apr 27, 2020

Surface Mount Technology is to attach surface mount components, soldering to printing is the application of solder paste on the circuit board pad, then place the surface mount components accurately on the solder paste coated pads, by heating the printed circuit board until the solder paste melts, after cooling, the connection between the components and the printed circuit is achieved.

Advantages of surface assembly technology

1)High assembly density, relatively speaking, using  SMT  can reduce the volume of electronic products by 60% and the weight by 75%.

2)High reliability, generally the rate of bad solder joints is less than 10 parts per million, which is an order of magnitude lower than the wave soldering technology of through-hole components.

Disadvantages of surface assembly technology

1)The nominal values on the components cannot be seen clearly, and the maintenance work is difficult.

2)It’s difficult to repair and replace the device and requires special tools.

3)The coefficient of thermal expansion(ETC) consistency between components and printed boards is poor. With the advent of special hand-in-hand disassembly and assembly equipment and new low-expansion-coefficient printed boards, they are no longer an obstacle to the further development of SMT.

Surface assembly process

The SMT process has two basic processes. One is the solder paste reflow soldering process, and the other is the patch-wave soldering process. In actual production, different process flows should be selected according to the type of components and production equipment used and the needs of the product. The basic process flow diagram is the following:

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SMT FLOW

1)Solder paste - reflow soldering process, the characteristics of the process are simple and fast, which is conducive to the reduction of product volume.

2)Patch - wave soldering process, the characteristics of this process is the use of double-panel space, the volume of electronic products can be further reduced, and through-hole components are still used. The price is low, but the equipment requirements are increased, the wave soldering process has more defects, and it is difficult to achieve high-density assembly.

3)Mixed installation, this process is characterized by full use of the double-sided space of the PCB board, which is one of the methods to minimize the installation area, and still keep the characteristics of the low price of through-hole components.

4) Solder paste- reflow soldering process is used on both sides. The characteristics of the process can fully utilize the PCB space and minimize the installation area. The process control is complex and strict. It’s often used in dense or ultra-small electrical products. Mobile phones are one of the typical products.

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We know that in the new materials, solder paste and glue are thixotropic fluids. They accounted for 60% of SMT total defects, training knowledge of these materials to ensure the quality of SMT. SMT involves a variety of loading process, such as printing, dispensing process, placement process, curing process, as long as any one of the process parameters drifts, it will lead to defective products, SMT process personnel must have a wealth of process knowledge, Monitor the process status at any time and predict the development trend.

Solder paste and printing technology

1.Solder paste

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It is a kind of paste which is a mixture of solder powder and pastes flux with soldering function. Usually, solder powder accounts for about 90%, and the rest is the chemical composition. It’s a complex material system. The manufacture of solder paste involves hydrodynamics, Comprehensive knowledge of metal smelting, organic chemistry, and physics.

1)For fluids such as solder paste, due to the long-chain structure and winding of macromolecules, or the presence of thixotropic agents, their flow behavior is much more complicated than that of low molecular fluids. The shear rate is no longer proportional, the viscosity of the liquid is no longer constant, and its flow behavior does not obey the rheological equation, so this kind of fluid is called non-fluid engineering.

2)When the external force of the solder paste increases, the viscosity of the solder paste decreases rapidly, but after a certain degree, it begins to stabilize. That is, when the solder paste is printed, it is subjected to the thrust of the scraper and its viscosity decreases. It reaches the minimum, so it can be successfully settled on the PCB pad through the window. With the stop of the external force, the viscosity of the solder paste comes back as soon as possible.

3)Solder paste is composed of solder ball power and paste flux. The particle size of the solder ball powder is generally controlled at 25UM-45UM. The too thick powder will lead to poor solder paste bonding performance, while in the paste flux, usually contains a certain amount of rosin or other resins, one of them is to increase the viscosity, and the other is to prevent the secondary oxidation of film-forming solder during the soldering process.

4)Viscosity and solder paste coating methods: one is metal template printing, the other is through the dispensing machine drip coating, the two types of methods require different viscosity of solder paste, the viscosity range is as follows:

5)Printing properties of solder paste:

In the SMT production, first requires the solder paste can be smoothly, constantly leaking through the solder pasteboard or distributor to PCB, if the solder paste printing performance is not good, will be closed the hole leakage plate, lead to can’t normal production, the reason is the lack of a solder paste printing aid or insufficient dosage, the shape of the alloy powders are not bad, particle size distribution is not in conformity with the requirements can also cause printing performance degradation.

6)Bonding force of solder paste: it is necessary for solder paste to retain sufficient viscosity after being placed for a period of time(8H) after printing.

7)Collapse degree of solder paste:

Is a term to describe whether the solder paste is still in good shape after printing on the PCB and passing a certain paste temperature. This phenomenon will often lead to “bridging, tin bead” after reflow.

Solder paste

Solder paste printing technology:

The printing of solder paste is the first process in SMT. The printing of solder paste involves three basic contents---- solder paste, stencil, and printing machine, a reasonable combination of the three, to achieve the quality of solder paste Quantitative distribution is very important. Solder paste has been said before, and now mainly describes the module and printing machine.

1) Manufacturing method of metal stencils:

A. Chemical corrosion method, due to the presence of side corrosion, the window wall finish is not enough, and the effect on stainless steel materials is poor, so the leakage effect is also poor.

B. Laser cutting method, uses a microcomputer to control the CO2 or YAG laser generator, cutting the window directly on the metal template like a light painting.

C. Electroforming method: the template made by the electroforming method is obviously expensive, suitable for use in fine pitch device welding products.

2) Printing machine:

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Description Automatic printing machine usually has an optical alignment system. By identifying the alignment marks on the PCB and the template, the automatic alignment of the template window and the PCB pad is achieved. The accuracy of the printing machine reaches 0.01MM, but there are many types of printing machines. Process parameters, such as blade speed, blade pressure, demolding speed, and the gap between template and PCB still need to be set manually.

3) Factors affecting printing effect:

A. The smoothness and the diameter-to-depth ratio of the template window.

B. The effect of solder paste thixotropy.

4) Solder paste printing process:

A. Solder paste preparation: Solder paste should be placed in the refrigerator to refrigerate (between 0-10 degrees). When in use, remove the room temperature (4 hours) from the refrigerator and then open the lid. Conditional factories are stirred by the machine and can be used after about 0.5-1H. It should be noted whether the viscosity and particle size of the solder paste meet the requirements of the current product. (Viscometer to measure)

B. Install and calibrate the template, semi-automatic can help align with CCD.

C. Printing solder paste: the initial dosage is not easy to be excessive, pay attention to environmental quality: no wind, clean, temperature (23 = 3) degree, relative humidity is less than 70%.

D. Completion/cleaning template.

5) Adjustment and influence of printing machine process parameters:

A. Scraper speed: generally between 12-40mm / s

B. Squeegee pressure: generally 0.5KG / 25MM

C. squeegee width: the best for PCB length plus 50MM

D. Printing gap: generally controlled at 0-0.07MM

E. Separation speed:

F. Scraper shape and materials.

6) defects of solder paste printing, causes, and countermeasures:

A. Welding paste pattern dislocation:

Causes: improper alignment of the steel plate and offset of welding pad; The printing press is not accurate enough.

Hazard: easy to cause a bridge connection.

Countermeasures: adjust the steel plate position, adjust the printing machine.

B. Solder paste drawing tip with dent:

Causes: scraper pressure is too large, rubber scraper hardness is not enough, the window is large.

Harm: solder quantity is not enough, easy to appear virtual welding, welding spot strength is not enough.

Countermeasures: adjust the printing pressure, change the metal scraper to improve the template window design.

C. too much solder paste:

Causes: the size of the template window is too large, the clearance of the PCB template is too large.

Harm: easy to cause bridge.

Countermeasures: check template window size, adjust printing parameters, especially PCB template clearance.

D. uneven graphics with breakpoints.

Causes: the template window wall luminosity is not good, the printing plate many times, not timely wipe off the solder paste, solder paste thixotropy is not good.

Harm: easy to cause insufficient solder, such as false welding defects.

Action: wipe the template.

E. Figure contamination:

Causes: many times of template printing, not timely wipe clean, solder paste quality is poor, the steel plate left when shaking.

Harm: easy bridge.

Countermeasures: wipe the template, change the solder paste, adjust the machine

In short, the parameters of the solder paste should be paid attention to when the solder paste printing will change at any time, such as particle size/shape, thixotropy, and welding performance, in addition, the parameters of the printing press can also cause changes, such as printing pressure/speed and environmental temperature, solder paste printing quality has a great influence on the welding quality, therefore should be carefully treated each of the parameters in the process of printing, and often observe and record the related coefficient.

Mounter

SMT technology and SMT machine

In SMT production, the SMT technology usually refers to the accurate placement of chip components on the PCB in a certain way. This process is called "pick and place" in English, obviously, it refers to the two actions of drawing/picking and placing.

In the past 30 years, the placement machine has developed from the early low speed (1-1.5 seconds/piece) and low precision (mechanical alignment) to high speed (0.08 seconds/piece) and high precision (optical alignment, placement accuracy ± 60um / 4q) High-precision automatic placement machine is high-tech electromechanical equipment composed of computer, optics, precision machinery, ball screw, linear guide, linear motor, harmonic drive, vacuum system, and various sensors.

1)Structure and characteristics of the placement machine:

The surface can be divided into the frame, PCB transmission mechanism and support table X, Y, and Z/Q servo, positioning system, optical recognition system, placement head, feeder, sensor, and computer operating software.

2)Functions of each part:

Bottom platform: supporting all components, it should have sufficient rigidity.

Feeder: It accommodates components in various packaging forms, and sends the components to the picking site. After the PCB positioning work, the PCB is supported and correctly positioned in the X and Y directions.

Placement head: Take the components, and after positioning correction, paste the components to the set position.

Positioning system: affects the placement accuracy and placement rate. Currently, DC servo motors are generally used to drive toothed belt ball screw drives.

Computer control system: the brain of the placement machine, the command center for all placement operations.

3)Type of attaching machine:

According to different purposes, the placement machine has different classification methods:

A. Classification by mounting method:

Sequential, online, simultaneous, simultaneous/online

B. Classification by mounting speed (mounting rate):

Low speed, medium speed, high speed

C. Classification by the price:

  Low-end(under USD100,000), mid-range (USD100,000-200,000), high-end (USD200,000)

D. The main factors affecting the performance of the placement machine:

Switchboard mechanical structure X-Y transmission mechanism Other factors

In short, the accuracy of the placement machine mainly talks about the offset of the attached parts and the pad. For example, the length of the CHIP part cannot exceed 1/3 of the pad, and the width cannot exceed 1/3 of the pad and so on.

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Reflow soldering temperature curve

Reflow

Reflow, it’s original intention is to form a solder joint by remelting the preplaced solder surface, and no additional solder is added during the soldering process. Reflow soldering technology has become the mainstream process of SMT.

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1.Compared with traditional wave soldering, reflow soldering has the following advantages:

1) The solder paste can be quantitatively distributed, with high accuracy, less solder heat, fewer impurities, and relatively less usage.

2) It’s suitable for welding all kinds of high-precision and high-demand components, such as 0603 resistors and capacitors and chip packaging devices such as QFP, BGA, and CSP.

3) Less welding defects, the rate of bad solder joints is less than 10*10(-6).

2.The basic structure of the reflow furnace.

A typical infrared-forced heat circulation reflow furnace is a heating device that combines hot air convection and remote heating together. It concentrates the advantages of both infrared reflow furnace and forced hot air convection, so it can effectively overcome the shadow effect of far infrared reflow furnace.

Usually consists of five zones, the first and fourth heater temperature zone configuration planar far outside, from first to fourth zones each configuration for hot blast heater, the temperature range of the fourth district for the welding temperature, the heat insulation effect of the second and third temperature area, mainly in order to make the SMA heated more evenly, to ensure that the SMA in sufficiently good condition to the welding temperature zone.

1) Heating body, the heating body is almost all aluminum plate or stainless steel heater, some manufactures also coated its surface with infrared coating, in order to increase the ability of infrared hair XIE.

2) Transfer system: the transfer system of the reflow furnace has three kinds, one is heat-resistant tetrafluoroethylene glass fiber cloth, two is stainless steel mesh, three is a chain guide rail.

3) Forced convection system: tangential fan convection system should be preferred when conditions are right.

4) Temperature control system.

3.Adjust welding temperature curve

The ideal temperature curve usually consists of four temperature zones, the preheating zone, the heat preservation zone/active zone, the reflow zone, and the cooling zone.

A. Preheating zone: Some solvents in the solder paste are evaporated in time, and components, especially the chip resistance and capacitance components, are heated slowly to adapt to the high temperature in the future.

B. Heating preservation zone: at this point, the volatiles are further removed, the activator starts to activate, and effectively remove the oxide on the welding surface. The surface temperature of SMA can be kept uniform under the influence of hot air convection, and the temperature on the plate is close to the minimum.

C. Reflow area: The solder melts, the activator is further broken down at the same time, effectively removes all kinds of oxide, with high temperature, reduces the surface tension, solder climbed to low, components of the pin a certain height. The backflow area, the surface tension of the solder paste melt after an automatic calibration pin migration caused by the patch process components, but also due to the incorrect welding plate design a variety of welding defects, such as vertical BEI bridging.

D. Cooling zone: The solder joints cool down quickly, the solder solidifies, and the solder joints cool rapidly to refine the solder lattice, improve the bonding strength, and brighten the solder joints.

The furnace temperature curve is very important for the welding effect of components, so the equipment requirements for measuring the temperature of the finance are also very strict.

Welding quality assessment and inspection

1.Excellent solder joint appearance

1)Good wetting.

2)The solder is continuous on the surface of the solder joint, and the closer to the edge of the solder joint, the thinner the solder layer. The contact angle should generally be less than 30 degrees. For the solder joint with a smaller pad edge, a concave meniscus should be seen. No solder barrier and other contaminants are allowed.

3)The solder layer at the solder joint should be moderate to avoid too much or too little.

4)The position of the solder joint must be accurate, the end/pin of the component should be in the center of the pad, and the width and length should not exceed.

5)The solder joint plane should be continuous and smooth, and the solder joint formed by reflow soldering should have a bright appearance.

In principle, the above requirements can be applied to all solder joints, no matter what method it uses to solder the board, and no matter where it is on the PCB, it should make people feel that they are even, smooth and full.

2.  Common quality defects and solutions in SMT production

1)Tombstone phenomenon

During reflow soldering, chip components often stand up, which is called a tombstone. The fundamental reason for this is that the wetting force on both sides of the component is unbalanced, and the force-distance between the two ends of the board surface component is also unbalanced, resulting in the tombstone The occurrence of phenomenon is mainly caused by the following reasons:

A. Pad design and layout are unreasonable: the area on one side of the pad is too large or the temperature difference across the PCB surface is too large so that the component pad heat absorption is not uniform, the solution is to change the pad design and layout.

B. Solder paste and solder paste printing: The solution is to select a solder paste with higher activity and change the solder paste printing parameters, especially the window size of the module.

C. SMT: The uneven force in the Z-axis direction will cause the thickness of the component to be immersed in the solder paste. The displacement will directly cause the tombstone. The solution is to adjust the parameters of the placement machine.

D. Furnace temperature curve The PCB working curve is incorrect because the temperature difference on the board surface is too large. The solution is to adjust the temperature curve according to each product.

E. Oxygen concentration in N2 reflow soldering: the use of N2 will increase the wetting power of the solder, but the phenomenon of tombstones will increase when the oxygen content is too low. It is generally considered that the oxygen content is controlled at about 100 * 10 (-6) the best.

2)Tin beads are generated during reflow soldering

A. The temperature curve is not correct; usually pay attention to the heating rate, and take a moderate preheat, and have a good platform to make most of the solvent volatilize, thereby inhibiting the formation of tin beads.

B. The quality of solder paste, too low metal content will lead to too much flux composition and not easy to cause flying beads, the increase of water vapor/oxygen content in solder paste will also cause, if the rest of the template is returned to the original bottle, it will cause The deterioration of China Tin will also produce tin beads.

C. Printing and SMT: The alignment of the template and the pad is offset, and the printing environment is not good. During the placement process, the Z-axis head is positioned according to the thickness of the component, so it will cause the component to be attached to the PCB and the solder paste will be folded in an instant 3. The phenomenon of squeezing out of the pad. The solution is to carefully adjust the clamping of the template, there should be no looseness, and the height of the Z-axis can be adjusted.

D. Thickness and opening size of the template: the opening is too large, especially the template made by a chemical etching method, the solution is to choose the appropriate thickness template and size design.

3)Blistering of solder mask on printed board after welding:

The fundamental reason is that there is gas/water vapor between the solder mask and the PCB substrate. They will go through different processes. When a high temperature is encountered, the gas expands, resulting in delamination of the solder mask and PCB substrate, and the pad temperature Higher, so bubbles appear first around the pad.

Solution:

All links should be strictly controlled. The purchased PCB should be inspected and stored. The PCB should be stored in a dry and ventilated environment with a storage period of no more than 6 months. The PCB should be placed in an oven to preheat 105 degrees 4-6H before welding. The preheating temperature in wave soldering should be strictly controlled. Before entering wave soldering, it should reach 100 degrees-120 degrees. When using aqueous flux, the preheating temperature will reach 110 degrees-125 degrees to ensure that the water vapor can be evaporated.

4)Wicking:

It means that the solder is separated from the pad along with the pin and the chip body, and a serious virtual soldering phenomenon will be formed, mainly for the vapor phase reflow soldering. The solution is to first preheat the half SMA and then put it in solderability in the furnace and inspection of the pads, as well as the coplanarity of the components are also relevant.

5)The cracking of chip components is common in multilayer chip capacitors (MLCC):

The main reason is that the preventive measures caused by thermal stress and mechanical force are: carefully adjust the welding process curve, especially the temperature of the preheating zone should not be too low. The placement and placement height of the Z-axis of the placement machine should be carefully adjusted during placement. The shape of the blade of the panel and the warpage of the PCB, especially after welding, should be corrected accordingly.

6)Solder joints are not bright / there are many residues:

After the SMA is released, failure to force air cooling will also cause the phenomenon of dullness and more residues. In addition, if the metal content in the solder paste is low, the medium is not easy to volatilize, and the dark color will also highlight the phenomenon of excessive residue.

7)PCB distortion:

For many reasons, we have the above solution: in the case of price and space permitting, choose TG to paste PCB or increase the thickness of the PCB to obtain the best length-to-width ratio, and rationally design the double-sided steel foil area of the PCB, Covered with a copper layer in the place where there is no circuit, and appear in the form of a network to increase the rigidity of the PCB, pre-bake the PCB before placement, the condition is 105 degrees / 4H, adjust the fixture or clamping distance to ensure that the PCB is Thermal expansion space; the welding process temperature should be adjusted as low as possible. When slight distortion has occurred, it can be placed in a fixed fixture and heated up in place to release stress.

8)Bridging

There are four reasons for bridging:

A. Solder paste quality problem: the metal content in the solder paste is high, especially after the printing time is too long, the metal content is prone to increase, the viscosity of the solder paste is low, and it flows out of the pad after preheating: the pad slump is poor, After preheating, it will overflow to the outside of the disk, which will cause IC pin bridging. The solution is to adjust the solder paste.

B. Printing system: poor repeatability of the printing machine, uneven alignment, solder paste printed outside the silver bar, this situation is more common in fine-pitch QFP production, poor steel plate alignment, and PCB alignment, and steel plate window size / The thickness is not correct and the PCB pad design SN / PB alloy plating is not uniform, resulting in a large amount of solder paste, which will cause bridging. The solution is to adjust the printing machine and change the PCB pad coating.

C. Placement: Z-axis height should be adjusted.

D. Preheating: the heating speed is too fast, the solvent in the solder paste is too late to volatilize.

9)Other common welding defects:

A. Poor wettability, which is manifested in poor soldering of PCB pads or poor soldering of component pins. The causes are: component pins / oxidized/contaminated; excessive reflow temperature, solder paste Poor quality, these will lead to poor wettability, and in serious cases, virtual welding will occur.

B. Tin quantity is small, which is manifested in dissatisfaction with solder joints, and the small meniscus of the IC pin root. The causes: small printing template window, wick phenomenon (temperature curve difference), low solder paste metal content,

The C pin is damaged, which is manifested by the poor coplanarity or bending of the device pins, which directly affects the quality of the soldering. The cause: it is damaged during transportation/pick and place.

E. Insufficient amount of solder paste, a phenomenon that often occurs in production. Cause: The first PCB printing/printing after the machine stops, the printing process parameters change, and the steel plate window is blocked,

For this SMT quality inspection, the 3D microscope is equipped with a 50-400 magnification paste-resolution CCD (2.11 million pixels) large depth of field (3MM) lens, which uses halogen light to pass the image of the measured object through the high-definition display (360 degrees) display. It can clearly check the solder joints on the PCB. It also has a dedicated BGA-Z lens, which can be deeply seen at the bottom of the BGA to see the actual status of the BGA solder ball solder joints.

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X-RAY inspection technology application

PCBA   repair

In SMT production, especially in the development of new products, there are often various problems such as device displacement, bridging, and virtual soldering after soldering. It is necessary to repair QFP BGA type devices. BGA has a special rework station.