SMT manufacturing and soldering quality problems analysis and solutions

1. Analysis and summary of practical SMT processing and manufacturing and welding quality problems

The Chinese name of PCB is printed circuit board, also known as printed circuit board, printed circuit board, circuit board, PCB circuit board, etc., it is an important electronic component, is the support of electronic components, and provides electrical connections for electronic components. Since it is made by electronic printing, it is called a "printed" circuit board.

Characteristics of SMT process technology

The characteristics of the SMT process technology can be demonstrated by its comparison with the traditional through-hole technology (THT). From the perspective of assembly process technology, the fundamental difference between SMT and THT lies in "sticking" and "plugging". The differences between the two are also reflected in various aspects such as substrates, components, component forms, solder joint shapes, and assembly process methods.

THT through-hole plug-in soldering technology

THT uses components with leads, designs circuit connection wires and mounting holes on the printed board, inserts the component leads into the pre-drilled through holes on the PCB, and after temporary fixation, the other side of the substrate is welded by wave soldering and other soldering techniques, so as to form reliable solder joints and establish long-term mechanical and electrical connections, and the component body and solder joints are located on both sides of the substrate respectively. In this way, because the components have leads, when the circuit is dense to a certain extent, the problem of reducing the size cannot be solved. At the same time, it is difficult to eliminate faults caused by the proximity of the leads and interference caused by the length of the leads.

DIP package (Dual-in-line package) is a kind of part package in the THT plug-in process, also known as dual in-line package technology, which is a simple packaging method. It refers to the integrated circuit chip that is packaged in the form of double in-line, and the vast majority of small and medium-sized integrated circuits use this packaging form, and the number of pins generally does not exceed 100. The CPU chip in the DIP package has two rows of pins that need to be plugged into a chip socket with a DIP structure.

On a traditional THT printed circuit board, the components and solder joints are located on both sides of the board; On an SMT circuit board, the solder joints and components are on the same side of the board. Therefore, on SMT printed circuit boards, the through-holes are only used to connect the wires PTH on both sides of the board, the number of holes is much smaller, and the diameter of the holes is much smaller. This makes it possible to significantly increase the assembly density of the circuit board.

Previously, the boards were made with a DIP plug-in process.

The trend towards high-performance computing packaging is shifting from the original FCBGA and 2.5D package forms to 3D packages.

Electronic components are redesigned, the original pins are shortened or even eliminated, and the volume is also reduced, and then soldered directly to the printed circuit board (PCB) or other substrate surface by reflow or dip soldering. These small metal pieces or end caps are called Surface Mounted Devices (SMDs).

The difference between SMT and THT component mounting and soldering methods is shown in the figure. The evolution of the /THT plug-in process and the SMT chip process

2. Common defects and solutions in SMT dispensing process

Component displacement

The phenomenon is that the components are displaced after the SMD glue is cured, and the component pins are not on the pads when it is serious. The reason is that the amount of adhesive output of the patch adhesive is uneven, for example, one more and one less in the two dispensing glues of the chip component; The component is displaced or the initial adhesion of the patch adhesive is low during mounting; The PCB is placed for too long after dispensing, resulting in the glue being over-cured.

Solution: Check whether the glue nozzle is blocked and rule out the uneven glue discharge; Adjust the working state of the mounter; Replace the glue; The PCB should not be placed for too long (less than 4h) after dispensing.

Chips will fall off after wave soldering

The phenomenon is that the bonding strength of the components after curing is insufficient, which is lower than the specified value, and sometimes the pieces will fall off when touched by hand. The reason is that the curing process parameters are not in place, especially the temperature is not enough, the size of the component is too large, and the heat absorption is large; Curing lamp aging; insufficient glue volume; Contamination of components/PCBs.

Solution: Adjust the curing curve, especially to increase the curing temperature, usually the peak curing temperature of the hot curing glue is about 150 °C, and it is easy to cause film drop if the peak temperature is not reached. For the light curing glue, it should be observed whether the curing lamp is aging and whether the lamp is blackened; The amount of glue and whether the components/PCBs are contaminated are all issues to consider.

Component pins float/shift after curing

The phenomenon of this fault is that the component pins float or shift after curing, and the solder material will enter under the pad after wave soldering, and in severe cases, there will be short circuit and open circuit. The main causes are uneven patch adhesive, excessive amount of patch adhesive, or component deviation during mounting.

Solution: Adjust the dispensing process parameters; Control the amount of dispensing; Adjust the placement process parameters.

3. Analysis of solder paste printing quality

The common quality problems caused by poor printing of solder paste are as follows:

 (1) Insufficient solder paste (local lack or even overall deficiency) will lead to insufficient tin amount of solder joints of components after soldering, open circuits of components, deviation of components, and erection of components.

(2) Solder paste adhesion will lead to short circuit and component deviation after soldering.

(3) The overall deviation of solder paste printing will lead to poor welding of the whole board components, such as less tin, open circuit, deviation, vertical parts, etc.

(4) The tip of the solder paste is easy to cause short circuit after soldering.

The main factor that causes solder paste to be insufficient

When the printing press was working, the solder paste was not added in time. The quality of the solder paste is abnormal, and there are foreign substances such as hard lumps mixed in it. Solder paste that was previously unused has expired and is being reused. There is a quality issue with the board having an inconspicuous covering on the pads, such as solder mask (green oil) that is printed onto the pads. The fixed clamping of the circuit board inside the printing press is loose. The solder paste is leaked and the thickness of the screen is uneven. Solder paste is missing the screen or there are contaminants on the circuit board (such as PCB packaging, screen wipes, foreign objects floating in the ambient air, etc.). The solder paste scraper is damaged, and the stencil is damaged. The pressure, angle, speed and demoulding speed of the solder paste scraper are not appropriate. After the solder paste is printed, it is accidentally touched due to human factors.

The main factor that causes solder paste to stick

The board design is flawed and the pad spacing is too small. There is a problem with the stencil and the hole is not in the right position. The stencil is not wiped clean. Solder paste is poorly peeled off due to stencil issues. The performance of the solder paste is poor, and the viscosity and collapse are unqualified. The fixed clamping of the circuit board inside the printing press is loose. The pressure, angle, speed and demoulding speed of the solder paste scraper are not appropriate. After the solder paste is printed, it is squeezed and adhered due to human factors.

The main factor that causes the overall deviation of the solder paste print

The positioning fiducials on the board are not clear. The positioning datum point on the board is not aligned with the fiduciality point of the stencil. The fixed clamping of the circuit board inside the printing press is loose. The positioning ejector pin is not in place. Failure of the optical positioning system of the printing machine. The solder paste leaks the hole in the stencil and does not match the design file of the circuit board.

The main factor that leads to the drawing of the printed solder paste

There is a problem with performance parameters such as solder paste viscosity. There is a problem with the setting of the demoulding parameters when the circuit board is separated from the missing stencil. There are burrs on the hole wall of the hole of the missing mesh plate.

SMT patch quality analysis

The common quality problems of SMT patches include missing parts, side parts, turning parts, deviations, damaged parts, etc.

Fourth, the main factors that lead to the missing parts of the patch

The feeder of the component is not in place. The air path of the element nozzle is blocked, the nozzle is damaged, and the nozzle height is incorrect. The vacuum air circuit of the equipment is faulty and blocked. The circuit board is defectively instocked and deformed. There is no or too little solder paste on the pads of the board. There are quality problems in the components, and the thickness of the same variety is inconsistent. There is an error or omission in the program of the placement machine, or the selection of component thickness parameters is wrong during programming.

The main factors that lead to the flip and side parts of SMC resistors are mounted

The feeder of the component is abnormal. The nozzle height of the placement head is incorrect. The height of the placement head is incorrect. The size of the charging hole of the component taping is too large, and the component is flipped due to vibration. The direction of the bulk material when it is placed in the tape is reversed.

The main factor that causes component placement to be misaligned

When programming the placement machine, the X-Y coordinates of the components are incorrect. The reason for the patch nozzle is that the suction is unstable.

The main factor that causes damage to components when they are mounted

The positioning ejector pin is too high, so that the position of the board is too high, and the component is squeezed during placement. When the placement machine is programmed, the Z-axis coordinates of the components are incorrect. The nozzle spring of the placement head is stuck.

Factors that affect the quality of SMT reflow soldering

5. Influencing factors of solder paste

The quality of reflow soldering is affected by many factors, the most important of which are the temperature profile of the reflow oven and the composition parameters of the solder paste. Today's commonly used high-performance reflow ovens have been able to accurately control and adjust the temperature profile. In contrast, with the trend of high density and miniaturization, the printing of solder paste has become the key to the quality of reflow soldering.

The particle shape of the solder paste alloy powder is related to the soldering quality of the narrow pitch device, and the viscosity and composition of the solder paste must also be selected appropriately. In addition, the solder paste is generally stored refrigerated, and the lid can only be opened when it is returned to room temperature after use, and special attention should be paid to avoid mixing the solder paste with water vapor due to temperature differences, and stirring the solder paste with a blender when necessary.

The impact of welding equipment

Sometimes, excessive belt vibration in reflow soldering equipment is also one of the factors that affect the quality of welding.

Sixth, the influence of the reflow soldering process

After ruling out the quality anomalies of the solder paste printing process and the placement process, the reflow process itself can also lead to the following quality anomalies:

(1) Cold soldering is usually the reflow soldering temperature is low or the time in the reflow area is insufficient.

 (2) The temperature of the tin bead preheating zone climbs too fast (generally required, the slope of the temperature rise is less than 3 degrees per second).

 (3) Tin circuit board or components are damp, and excessive moisture content can easily cause tin explosion and produce tin.

 (4) Crack is generally the temperature drop in the cooling zone is too fast (generally the slope of the temperature drop of lead welding is less than 4 degrees per second).

SMT soldering quality defects ━━━ reflow soldering quality defects and solutions

The phenomenon of erection of monuments In reflow soldering, the phenomenon of erection of chip components often occurs, and the causes are: the fundamental reason for the occurrence of the phenomenon of erection is that the wetting force on both sides of the component is unbalanced, and thus the torque at both ends of the component is also unbalanced, which in turn leads to the occurrence of the phenomenon of erecting monuments.

The following conditions can lead to an imbalance in the wetting force on both sides of the component during reflow soldering:

(1) The design and layout of the pad are unreasonable. If the following defects exist in the pad design and layout, the wetting force on both sides of the component will be unbalanced. One of the pads on both sides of the component is connected to the ground wire or the area of the pad on one side is too large, and the heat capacity at both ends of the pad is uneven. The temperature difference between the various parts of the PCB surface is so large that the heat absorption on both sides of the component pad is uneven. Large devices around QFPs, BGAs, and heat sinks can have uneven temperatures at both ends of the pads. Solution: Change the pad design and layout. (2) There is a problem with solder paste and solder paste printing. The activity of the solder paste is not high or the solderability of the component is poor, and the surface tension is not the same after the solder paste is melted, which will cause an imbalance in the wetting force of the pad. The amount of solder paste printed on the two pads is uneven, and the melting time lags due to the increase in heat absorption of the solder paste, resulting in an imbalance in wetting power. Solution: Choose solder paste with high activity to improve the printing parameters of solder paste, especially the window size of the template.

(3) Chip displacement The uneven force in the Z-axis direction will lead to uneven depth of the component immersed in the solder paste, and the wetting force on both sides will be unbalanced due to the time difference during melting. If the component patch is displaced, it will directly lead to the erection of the monument. Solution: Adjust the process parameters of the placement machine.

(4) Incorrect furnace temperature curve If the furnace body of the reflow soldering furnace is too short and the temperature zone is less, the working curve of PCB heating will be incorrect, resulting in too large wet difference on the board surface, resulting in unbalanced wetting force. The solution: adjust the appropriate temperature profile for each different product.

(5) Oxygen concentration in nitrogen reflow soldering Nitrogen protection reflow soldering will increase the wettability of the solder, but more and more examples show that the phenomenon of monument erection increases when the oxygen content is too low. It is generally considered that the oxygen content is controlled to the minus 6th power of (100~500)×10.

Tin beads

Solder beads are one of the common defects in reflow soldering, which not only affects the appearance but also causes bridging. Solder beads can be divided into two categories, one appears on the side of the chip component, often as an independent large ball; The other type appears around the IC pins in the form of scattered beads. There are many reasons for the production of solder beads, which are analyzed as follows:

(1) The temperature curve is incorrect The reflow soldering curve can be divided into 4 sections, which are preheating, insulation, reflow and cooling. The purpose of preheating and heat preservation is to make the surface temperature of the PCB rise to 150 °C within 60~90s, and keep warm for about 90s, which can not only reduce the thermal shock of the PCB and components, but also ensure that the solvent of the solder paste can be partially volatilized, so as to avoid spatter caused by too much solvent during reflow soldering, causing the solder paste to flush out of the pad and form solder beads. Solution: Pay attention to the heating rate and take a moderate preheating so that there is a good platform for most of the solvent to volatilize.

(2) The quality of solder paste The metal content in solder paste is usually (90±0.5) c/o, and too low metal content will lead to too much flux composition, so too much flux will cause flying beads due to non-volatilization in the preheating stage. Increased levels of water vapor and oxygen in solder paste can also cause beading. Since solder paste is usually refrigerated, when taken out of the refrigerator, if the recovery time is not ensured, it will cause water vapor to enter; In addition, the cap of the solder paste bottle should be tightly closed after each use, if it is not closed tightly in time, it will also lead to the ingress of water vapor. After the solder paste printed on the template is finished, the remaining part should be disposed of separately, and if it is put back into the original bottle, it will cause the solder paste in the bottle to deteriorate, and solder beads will also be generated. Solution: Choose high-quality solder paste, pay attention to the storage and use requirements of solder paste.

(3) Printing and placement In the printing process of solder paste, due to the offset between the template and the pad, if the offset is too large, it will cause the solder paste to be immersed outside the pad, and solder beads are easy to appear after heating. In addition, the poor printing working environment will also lead to the generation of tin balls, the ideal printing environment temperature is 25±3 °C, and the relative humidity is 50C/O~65C/O. Solution: Carefully adjust the clamping of the template to prevent loosening. Improve the working environment of printing. The pressure in the Z-axis during the placement process is also an important cause of solder beads, but it is often not taken seriously. The Z-axis head of some placement machines is positioned according to the thickness of the component, such as improper adjustment of the Z-axis height, which will cause the phenomenon of squeezing the solder paste to the outside of the pad at the moment when the component is attached to the PCB, and this part of the solder paste will form solder beads during soldering. In this case, the size of the resulting solder beads is slightly larger. Solution: Readjust the Z-axis height of the mounter. The thickness of the formwork and the size of the opening. Excessive stencil thickness and opening size will lead to an increase in the amount of solder paste used, and will also cause solder paste to overflow outside the pad, especially if the stencil is made by chemical corrosion methods. Solution: Select the appropriate thickness of the template and the design of the opening size, the general template opening area is 90c/o of the pad size.

Wicking phenomenon

The wicking phenomenon, also known as the core-pulling phenomenon, is one of the common welding defects, which is mostly seen in vapor phase reflow soldering. Wicking causes the solder to move away from the pad and up the pin between the pin and the die body, often resulting in a severe virtual soldering phenomenon. The main reason is that the thermal conductivity of the component pins is large, so the temperature rises rapidly, so that the solder preferentially wets the pins, and the wetting force between the solder and the pins is much greater than the wetting force between the solder and the pads, and the upturned pins will aggravate the occurrence of wicking. Solution: For vapor phase reflow soldering, the SMA should be fully preheated before being put into the gas phase furnace. The solderability of the PCB pad should be carefully checked, and the PCB with poor solderability cannot be used for production. Sufficient attention is paid to the coplanarity of components, and devices that do not have good coplanarity cannot be used in production. In infrared reflow soldering, the organic flux in the PCB substrate and solder is a good absorbent medium for infrared, while the pin can partially reflect the infrared, so the solder is preferentially melted, and the wetting force between the solder and the pad will be greater than the wetting force between the solder and the pin, so the solder will not rise along the pin, so the probability of wicking is much smaller.

Bridging

Bridging - is one of the common defects in SMT production, it will cause a short circuit between components, and must be repaired when encountering bridging, there are many reasons for bridging:

(1) The quality of solder paste The metal content in solder paste is high, especially if the printing time is too long, the metal content is easy to increase, resulting in IC pin bridging. The solder paste has a low viscosity and spreads out of the pad after preheating. The solder paste tower has poor drop, and it diffuses to the outside of the pad after preheating. Solution: Adjust the solder paste ratio or switch to a good quality solder paste.

 (2) Printing system The printing machine has poor repeatability and uneven alignment (poor alignment of steel plate and poor alignment of PCB), resulting in solder paste printing outside the pad, especially the fine-pitch QFP pad. The size and thickness of the stencil window are not designed correctly, and the PCB pad design is not evenly plated with Sn-pb alloy, resulting in too much solder paste. Solution: Adjust the printing machine to improve the PCB pad coating.

(3) The placement pressure is too large, and the full flow of solder paste after pressure is a common reason in production. In addition, insufficient placement accuracy can cause component dislocation and IC pin deformation.

(4) The reflow oven heats up too fast, and the solvent in the solder paste does not have time to volatilize. Solution: Adjust the height of the Z-axis of the placement machine and the heating speed of the reflow oven.

Wave soldering quality defects and solutions

(1) Drawing refers to the appearance of excess needle-like solder at the end of the solder joint, which is a defect unique to the wave soldering process. Causes: inappropriate PCB transfer speed, low preheating temperature, low tin pot temperature, small PCB transfer inclination, poor wave peaks, flux failure, and poor solderability of component leads. Solution: Adjust the transfer speed to the appropriate degree, adjust the preheating temperature and tin pot temperature, adjust the PCB transfer angle, optimize the nozzle, adjust the wave peak shape, replace the new flux and solve the lead solderability problem. Recommended setting values for each section of the reflux temperature curve:

 (1) Room temperature - the start point of preheating

(2) Preheating area: Excessive preheating will lead to deepening oxidation and deterioration of flux.

(3) Preheating end point - reflow soldering area

(4) Reflow Soldering Zone - Cooling Zone The peak temperature of SMT reflow soldering usually depends on the melting point temperature of the solder and the temperature that the assembled part can withstand. First of all, it is necessary to consider whether the components and PCB board can withstand this temperature, because the composition of leaded solder paste and lead-free solder paste is different, the lead composition is: tin 63%, lead 37%, the melting point is generally 183, the peak is 205 - 230, lead-free is: tin 96.5%, silver 3%, copper 0.5%, so the melting point is higher, generally 217, the peak is 245 - 250, the general peak temperature should be about 25 - 30 °C higher than the normal melting point temperature of the solder paste, in order to successfully complete the welding operation, if lower than this temperature, This may cause defects such as cold soldering and poor wetting.

(2) Causes of virtual soldering: poor solderability of component leads, low preheating temperature, solder problems, low flux activity, large pad holes, oxidation of the printed board, pollution of the board surface, too fast transmission speed, and low temperature of the tin pot. Solution: Solve the solderability of the lead, adjust the preheating temperature, test the tin and impurity content of the solder, adjust the flux density, reduce the pad hole during design, remove the PCB oxide, clean the board surface, adjust the transfer speed, and adjust the temperature of the tin pot.

(3) Causes of tin thinness: poor solderability of component leads, large pads (except for the need for large pads), large pad holes, large welding angles, too fast transmission speeds, high temperature of the tin pot, uneven coating of flux, and insufficient tin content of solder. Solution: Solve the solderability of the lead, reduce the pad and pad hole during design, reduce the soldering angle, adjust the transfer speed, adjust the temperature of the solder pot, check the pre-applied flux device, and test the solder content.

(4) Causes of missing soldering: poor solderability of the lead, unstable solder wave, flux failure or uneven spraying, poor local solderability of PCB, conveyor chain jitter, immiscible pre-coated flux and flux, and unreasonable process flow. Solution: Solve the solderability of the lead, check the wave device, replace the flux, check the pre-applied flux device, solve the PCB solderability (cleaning or return), check and adjust the transmission device, use the flux uniformly, and adjust the process flow.

 (5) After welding, the solder mask film of the printed board is blistered. SMA will appear light green bubbles around individual solder joints after soldering, and in severe cases, bubbles the size of fingernails will appear, which not only affects the appearance quality, but also affects the performance in severe cases, this defect is also often seen in the reflow soldering process, but it is more common during wave soldering. Causes: The root cause of solder mask blistering is that there is gas or water vapor between the solder mask mold and the PCB substrate, these trace gases or water vapors will be entrained in it in different processes, and when encountering high soldering temperatures, gas expansion leads to delamination between the solder mask film and the PCB substrate, because the pad temperature is relatively high during soldering, so the bubbles first appear around the pad.

 Any of the following reasons can cause water vapor entrainment on the PCB: (

1) PCB often needs to be cleaned and dried in the processing process before the next process, for example, after corrosion, it should be dried and then pasted with solder mask, if the drying temperature is not enough at this time, it will entrainment water vapor into the next process, and bubbles will appear when encountering high temperature during soldering.

(2) The storage environment before PCB processing is poor, the humidity is too high, and the soldering is not dried in time.

(3) In the wave soldering process, aqueous flux is often used now, if the PCB preheating temperature is not enough, the water vapor in the flux will enter the PCB substrate along the hole wall of the through hole, and the water vapor will enter first around the pad, and bubbles will be generated after encountering the high temperature of soldering.

(4) Strictly control each production link, the purchased PCB should be inspected and put into storage, usually the PCB should not appear blistering within 10 seconds at a temperature of 260 °C.

(5) The PCB should be stored in a ventilated and dry environment for no more than 6 months.

(6) The PCB should be pre-baked in an oven at a temperature of (120±5) °C for 4 hours before soldering.

(7) The preheating temperature should be strictly controlled in wave soldering, and it should reach 100 - 140 °C before entering wave soldering, and if water-containing flux is used, the preheating temperature should reach 110 - 145 °C to ensure that the water vapor can be volatilized. Blistering on the PCB substrate after SMA soldering The main reason for the appearance of nail-sized bubbles after SMA soldering is also the entrainment of water vapor inside the PCB substrate, especially for multilayer board processing. Because the multi-layer board is made of multi-layer epoxy resin prepreg preg and then hot pressing, if the storage period of the epoxy resin prepreg is too short, the resin content is insufficient, the pre-drying to remove the water vapor is not clean, and it is easy to entrain water vapor after hot pressing molding, and it will also leave bubbles due to the insufficient glue content of the semi-solid sheet itself and the insufficient adhesion between the layers. In addition, after the PCB is purchased, due to the long storage period and humid storage environment, the patch is not pre-baked in time before production, and the damp PCB patch is also prone to blistering. Solution: After the PCB is purchased, it should be put into storage after acceptance, and the PCB should be pre-baked at a temperature of (120±5) °C for 4 hours before PCB mounting. IC pin soldering after open circuit or virtual soldering Causes: Poor coplanarity, especially for FQFP devices, due to improper storage caused by pin deformation, if the placement machine does not have the function to check the coplanarity, sometimes it is not easy to be found. Poor solderability of pins, long storage time of ICs, yellowing of pins, and poor solderability are the main causes of false soldering. The solder paste is of poor quality, low metal content, and poor solderability, and the metal content of the solder paste commonly used for FQFP device soldering should not be less than 90%. If the preheating temperature is too high, it is easy to cause oxidation of the IC pins, which makes the solderability worse. The small window size of the printing template results in an insufficient amount of solder paste. Pay attention to the storage of the device, and do not pick up the components or open the package. During production, the solderability of components should be checked, and special attention should be paid to the fact that the IC should not be stored for too long (within one year from the date of manufacture), and high temperature and humidity should be avoided during storage. Double-check the template window size so that it is neither too large nor too small, and take care to match the PCB pad size.