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[Dry goods] SMT quality problems summary (by the SMT sector friends crazy pass)

From：定盈电子    Time：时间格式错误!

1, dispensing process common defects and solutions
1.1, drawing / tailing
1.1.1, drawing / trailing is a common defect in the dispensing, the common causes of glue mouth is too small, dispensing pressure is too high, glue from the PCB spacing is too large, patch glue expired or poor quality , Patch adhesive viscosity is too good to remove from the refrigerator after the failure to return to room temperature, dispensing volume is too large and so on.
1.1.2, the solution: change the larger diameter of the plastic mouth; reduce the dispensing pressure; adjust the "stop" height; glue, select the appropriate viscosity of the plastic; patch glue removed from the refrigerator should be restored to room temperature About 4h) and then put into production; adjust the amount of dispensing.
1.2, plastic mouth blockage
1.2.1, the phenomenon is the plastic nozzle is less or no glue out of the glue out of the cause of the pinhole is generally not completely clean; patch glue mixed with impurities, there are plugging phenomenon; immiscible glue phase mixing.
1.2.2 solution: for clean needles; for good quality patch adhesive; patch plastic grade should not be mistaken.
1.3, air hit
1.3.1, the phenomenon is only dispensing action, but no glue amount. The reason is that the patch glue into the bubble; plastic mouth blockage.
1.3.2, the solution: glue in the barrel should be carried out degassing treatment (especially their own installed plastic); replacement plastic mouth.
1.4, component shift
1.4.1, the phenomenon is the patch after curing the components of the device shift, serious component pins are not on the pad. The reason is due to the amount of adhesive glue glue is not uniform, such as chip components, two more glue Less; patch when the component shift or patch glue early low viscosity; dispensing after PCB placement time is too long glue semi-curing.
1.4.2, the solution: check the plastic mouth is blocked, remove the glue is not uniform phenomenon; adjust the placement machine working conditions; change glue; dispensing after PCB placement time should not be too long (shorter than 4h)
1.5, wave soldering will be lost after the film
1.5.1, the phenomenon is the strength of the adhesive component after curing is not enough, lower than the specified value, sometimes touch will appear out of hand. The reason is because the curing process parameters are not in place, especially the temperature is not enough, the component size is too large, Heat; light curing lamp aging; glue is not enough; components / PCB pollution.
1.5.2, the solution: adjust the curing curve, in particular, to improve the curing temperature, usually thermal curing gel peak curing temperature of 150 ℃ or so, can not reach the peak temperature easily lead to the film.For light solid glue, Whether the aging of the curing lamp, the lamp is black phenomenon; the number of glue and components / PCB whether there is pollution should be considered.
1.6, after the component pin floating / shift
1.6.1, the phenomenon of this failure is the chip components after curing floating or shift, wave soldering tin material will enter the pad, the serious will appear short circuit, open the cause of the main patch is not uniform, When the patch is too much or the component is offset.

1.6.2, the solution: adjust the dispensing process parameters; control the amount of glue; adjust the patch process parameters.

Second, solder paste printing and patch quality analysis
Analysis of solder paste printing quality
Poor solder paste caused by the quality of the common problems are the following:
①, lack of solder paste (lack of local or even the overall lack of) will lead to soldering components after the shortage of solder joints, components open circuit, components offset, components erect.
②, solder paste adhesion will lead to short circuit after welding, components offset.
③, solder paste printing the overall deviation will lead to poor welding of the entire board components, such as less tin, open, offset, vertical and so on.
④, solder paste can easily lead to short-circuit after welding.

1, the main factor leading to insufficient solder paste
1.1, the printing press work, there is no timely added to add solder paste.
1.2, solder paste quality anomalies, which mixed with lumps and other foreign body.
1.3, has not been used before the solder paste has expired, was the second use.
1.4, the circuit board quality problems, the pad has inconspicuous cover, such as printed on the pad to the solder resist (green oil).
1.5, the circuit board in the printing machine fixed clamp loose.
1.6, solder paste leaky screen thickness is not uniform.
1.7, solder paste leakage screen board or circuit board with contaminants (such as PCB packaging, screen cleaning paper, floating air in the ambient air, etc.).
1.8, solder paste scraper damage, stencil damage.
1.9, solder paste scraper pressure, angle, speed and stripping speed and other equipment parameters set inappropriate.
1.10 solder paste printing is completed, because the human factors were accidentally knocked off.
2, leading to solder paste adhesion of the main factors
2.1, the design of the circuit board defects, pad spacing is too small.
2.2, stencil problem, the hole position is not correct.
2.3, the screen is not wiped clean.
2.4, stencil problems make solder paste off bad.
2.5, solder paste poor performance, viscosity, collapsed unqualified.
2.6, the circuit board in the printing press fixed clamp loose.
2.7, solder paste scraper pressure, angle, speed and stripping speed and other equipment parameters set inappropriate.
2.8, solder paste printing is completed, because the human factors were squeezed adhesion.
3, lead to solder paste printing the overall deviation of the main factors
3.1, the positioning point on the circuit board is not clear.
3.2, the positioning point on the circuit board and the reference point of the screen is not aligned.
3.3, the circuit board in the printing press fixed clamp loose. Positioning thimble is not in place.
3.4, the printer's optical positioning system failure.
3.5, solder paste leakage screen mesh openings and circuit board design documents do not meet.
4, leading to printing solder paste pulling the main factors
4.1, solder paste viscosity and other performance parameters.
4.2, circuit board and the gap between the release of the release of the layout parameters of a problem,
4.3, the gap between the stencil hole hole engraved hole.
Patch quality analysis
SMT patch common quality problems are missing pieces, side pieces, flip pieces, offset, loss pieces and so on.
1, the main factors leading to patch leakage
1.1, the feeder feeder (feeder) feeding is not in place.
1.2, the component nozzle of the air block, nozzle damage, nozzle height is not correct.
1.3, the equipment of the vacuum air circuit failure, blocking.
1.4, bad circuit board, resulting in deformation.
1.5, there is no solder paste or solder paste on the circuit board.
1.6, component quality problems, the same variety of thickness inconsistent.
1.7, placement machine call procedures are missing, or programming parameters of the thickness of the components of the wrong choice.

1.8, human factors accidentally knocked out.
2, leading to SMC resistor patch when the pieces, side pieces of the main factors
2.1, the feeder feeder (feeder) feeding abnormal.
2.2, the tip of the nozzle is not right.
2.3, the placement of the head grab the height of the wrong.
2.4, the size of the material packing hole is too large, the components due to vibration flip.
2.5 The direction in which the bulk material is placed in the braid is reversed.
3, leading to component chip offset the main factors
3.1, the placement machine programming, the components of the X-Y axis coordinates are not correct.
3.2, patch nozzle reasons, so that suction material instability.
4, leading to damage to the main components of the main factors
4.1, positioning thimble is too high, so that the location of the circuit board is too high, the components are squeezed in the placement.
4.2, the placement machine programming, the components of the Z-axis coordinates are not correct.
4.3, the mounting head of the nozzle spring was stuck.

Third, the impact of reflow quality factors
1, the impact of solder paste factors
The quality of the reflow is affected by many factors. The most important factor is the temperature curve of the reflow oven and the composition parameters of the solder paste, which is now widely used in the high performance reflow oven. In contrast, in the high density and miniaturization trend, the solder paste printing becomes the key to reflow quality.
The particle shape of the solder paste alloy powder is related to the welding quality of the narrow pitch device. The viscosity and composition of the solder paste must also be properly selected. In addition, the solder paste is generally stored in cold storage and can be opened to room temperature before taking office. To avoid the temperature difference between the solder paste into the water vapor, if necessary, stir the solder paste with a mixer.
2, the impact of welding equipment
Sometimes, the reflow soldering device conveyor belt vibration is also one of the factors that affect the quality of welding.
3, the impact of reflow process
After eliminating the solder paste printing process and the quality of the patch process after the abnormal flow reflow process itself will lead to the following quality anomalies:
①, cold welding is usually low reflow temperature or re-flow area of the lack of time.
②, tin beads preheating zone temperature climb too fast (generally required, the temperature rise of the slope of less than 3 degrees per second).
③, even tin circuit board or components damp, containing too much water easily lead to tin burst with tin.
④, the crack is generally the temperature drop in the cooling zone is too fast (generally have a lead drop temperature drop of less than 4 degrees per second).
Fourth, SMT welding quality defects ━ ━
Refuse quality defects and solutions
1, tombstones phenomenon reflow, the chip components often appear up the phenomenon, the reasons for the occurrence: the main reason for the occurrence of the phenomenon of monument is the uneven wetting force on both sides of the component, so the torque at both ends of the component is not balanced , Which led to the occurrence of tombstoning phenomenon.
The following conditions will lead to reflow soldering on both sides of the wetting force imbalance:
1.1, pad design and layout is unreasonable.If the pad design and layout of the following defects, will cause the wetting force on both sides of the component imbalance.
1.1.1, one of the two sides of the component pad and ground connection or side of the pad area is too large, the heat capacity at both ends of the pad is not uniform;
1.1.2, PCB surface temperature difference is too large so that both sides of the component pad heat absorption is not uniform;
1.1.3, large devices QFP, BGA, radiator around the small chip components at both ends of the pad will appear uneven temperature.
Workaround: Change the pad design and layout.
1.2, solder paste and solder paste printing problems. Solder paste activity is not high or poor solderability of the components, solder paste melting, the surface tension is not the same, will cause the pad wetting force imbalance. The amount of uneven, more side of the solder paste will increase due to heat, melting time lag, so that wetting force is not balanced.
Solution: Select the higher active solder paste to improve the solder paste printing parameters, especially the template window size.
1.3, the patch shift Z axis of the force is not uniform, will lead to the components immersed in the solder paste in the depth of uneven, due to the time difference between the two sides of the wetting force caused by imbalance.If the component patch shift will lead directly Tombstone.
Solution: adjust the placement process parameters.
1.4, the furnace temperature curve is not correct If the reflow furnace furnace is too short and too little temperature zone will cause the PCB heating curve is not correct, so that the board surface is too large humidity, resulting in wetting force imbalance.
Solution: Adjust the appropriate temperature profile according to each different product.
1.5, nitrogen reflow soldering oxygen concentration to take nitrogen protection reflow will increase the wetting of the solder force, but more and more examples show that the oxygen content is too low in the case of occurrence of the phenomenon of positive monument; Oxygen content control in the (100 ~ 500) × 10 negative 6 times the most appropriate.
2, tin beads
Tin beads are one of the common defects in reflow soldering, which not only affects the appearance but also causes bridging, and the tin beads can be divided into two types, one on the side of the chip component, often as an independent large sphere. A class appears in the IC pin around, was scattered beads. There are many reasons for the production of tin beads, are analyzed as follows:
2.1, the temperature curve is not correct Reflow curve can be divided into four sections, namely, preheating, heat preservation, re-flow and cooling. Preheat, the purpose of insulation is to make PCB surface temperature in 60 ~ 90s up to 150 ℃, and insulation about 90s, which can not only reduce the thermal impact of PCB and components, but also to ensure that the solder paste solvent can be part of the volatile, to avoid reflow soldering caused by too much solvent splash, resulting in solder paste out of the pad The formation of tin beads.
Solution: pay attention to the rate of heating and take a moderate preheat, so that it has a very good platform to make most of the solvent volatile.
2.2, the quality of solder paste
2.2.1, the solder paste in the metal content is usually (90 ± 0.5) ℅, the metal content is too low will lead to excessive flux components, so too much flux will be due to the preheating stage is not easy to cause flying beads.
2.2.2, the increase in water vapor and oxygen content in the solder paste will also cause flying beads. As the solder paste is usually refrigerated, when not removed from the refrigerator, if not to ensure recovery time, will lead to water vapor; The cover should be tightened after each use, and if it is not covered in time, it will also lead to the entry of water vapor.
On the template printed on the solder paste after the completion of the rest of the part should be handled separately, if returned to the original bottle, will cause the bottle solder paste deterioration, will produce tin beads.
Solution: choose high-quality solder paste, pay attention to the storage and use of solder paste requirements.
2.3, printing and patch
2.3.1, in the solder paste printing process, due to the template and the pad will shift, if the deviation is too large will lead to solder paste into the pad outside the solder, easy to appear after heating tin beads. Poor working environment will lead to the formation of tin beads, the ideal printing environment temperature of 25 ± 3 ℃, relative humidity of 50 ℅ ~ 65 ℅.
Workaround: Carefully adjust the clamping of the template to prevent loosening. Improve the printing work environment.
2.3.2, the process of Z-axis pressure is also an important cause of tin beads, but often does not cause people's attention. Part of the placement machine Z-axis is based on the thickness of the component to locate, such as Z-axis height Improper adjustment, will cause the components attached to the PCB on the moment the solder paste will be squeezed out of the phenomenon of solder pad, this part of the solder paste will be formed in the solder beads, in which case the size of the tin beads slightly larger.
Solution: Re-adjust the Z-axis height of the placement machine.
2.3.3, the thickness of the template and the opening size of the template thickness and opening size is too large, will lead to the increase in the amount of solder paste, solder paste can also lead to flow outside the pad, especially with chemical corrosion method manufacturing template.
Solution: Select the appropriate thickness of the template and the opening size of the design, the general template opening area for the pad size of 90 ℅.
3, wicking phenomenon
Picking phenomenon is also known as core pulling phenomenon, is one of the common welding defects, more common in the gas-phase reflow. Picking phenomenon so that the solder from the pad and along the pin up to the pin and the chip between the body, usually forming a serious The reason for the solder is that the wetting force between the solder and the pin is much greater than the wetting force between the solder and the pad, as long as the thermal conductivity of the component pin is large, In addition, the uptick of the pin will intensify the phenomenon of wicking.
Solution:
3.1, for the gas phase reflow SMA should be fully preheated and then into the gas phase furnace;
3.2, should be carefully checked PCB solderability, poor solderability of PCB can not be used for production;
3.3, full attention to the component of the coplanarity, poor coplanarity of the device can not be used for production.
In the infrared reflow, PCB substrate and solder in the organic flux is a good infrared absorption medium, while the pin is able to partially reflect the infrared, so the solder is preferred to melt, solder and pad wetting force on 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 occurrence of wicking phenomenon is much smaller.
4, the bridge is SMT production is one of the common defects, it will cause short circuit between the components, the bridge must be reworked. The reasons for causing a lot of bridges are:
4.1, the quality of solder paste.
4.1.1, solder paste in the high metal content, especially the printing time is too long, prone to metal content increased, leading to IC pin bridge;
4.1.2, solder paste low viscosity, preheated and flow to the outside of the pad;
4.1.3, solder paste tower drop poor, after the warm flow to the outside of the pad;
Solution: adjust the solder paste ratio or switch to good quality solder paste.
4.2, printing system
4.2.1, the printing machine repeat precision is poor, the alignment is not good (steel plate bit is not good, PCB alignment is not good), resulting in solder paste printing to the pad, especially fine pitch QFP pad;
4.2.2, template window size and thickness of the design is wrong and PCB pad design Sn-pb alloy coating is not uniform, resulting in solder paste too much.
Solution: adjust the printing press, improve the PCB pad coating;
4.3, posted pressure is too large, solder paste pressure after the full flow is the reason for the production of the other patch is not enough precision components will shift, IC pin deformation.
4.4, reflow furnace heating rate too fast, solder paste solvent and less volatile.
Solution: adjust the Z-axis height of the placement machine and reflow furnace heating rate.


5, wave soldering quality defects and solutions
5.1, pull the tip refers to the end of the solder in the excess needle-like solder, which is the wave soldering process unique defects.
Reason: PCB transmission speed, preheating temperature is low, tin pot temperature is low, PCB transmission angle is small, poor peaks, flux failure, component lead solderability is poor.
Solution: adjust the transfer speed to the appropriate date, adjust the preheat temperature and tin pot temperature, adjust the PCB transmission angle, preferably the nozzle, adjust the peak shape, exchange new flux and solve the problem of lead solderability.
5.2, Weld causes: components lead poor solderability, preheating temperature is low, solder problems, low flux activity, the solder hole is too large, lead plate oxidation, board surface pollution, transmission speed too fast, tin pot Low temperature.
Solution: Solve the solderability of the lead, adjust the preheat temperature, test the tin and impurity content of the solder, adjust the flux density, reduce the solder hole in the design, remove the PCB oxide, clean the board, adjust the transfer speed, adjust the tin pot temperature The
5.3, tin thin causes: component lead solderability is poor, the pad is too large (except for large pad), the pad hole is too large, the welding angle is too large, the transmission speed is too fast, tin pot temperature is high, Uneven coating, solder tin content is insufficient.
Solution: Solve the solderability of the lead, the design to reduce the pad and pad hole, reduce the welding angle, adjust the transmission speed, adjust the tin pot temperature, check the pre-coated flux device, test solder content.
5.4, leakage welding causes: lead solderability is poor, solder peak instability, flux failure or uneven spraying, PCB local poor solderability, conveyor chain jitter, pre-coated flux and flux is not compatible, the process is unreasonable.
Solution: Solve the solderability of the lead, check the crest device, replace the flux, check the pre-flux device, solve the PCB solderability (cleaning or return), check the adjustment of the transmission, the uniform use of flux, adjust the process.
5.5, after welding printed circuit board solder film foam
SMA in the welding will be in the individual solder around the emergence of light green vesicles, severe nail caps will appear when the size of the bubble, not only affect the appearance of quality, serious also affect the performance, this defect is reflow process In the frequent problems, but when the wave soldering.
Cause: The root cause of the solder mask foaming is that there is gas or water vapor between the solder mask and the PCB substrate. These trace gases or water vapor are entrained in the process. When the welding temperature is high , Gas expansion and lead to solder mask and PCB substrate layering, welding, the pad temperature is relatively high, so the bubble first appeared in the pad around.
One of the following causes causes the PCB to entrain moisture:
5.5.1, PCB in the processing process often need to clean, dry and then do the next process, such as rotten after drying should then paste solder film, if the drying temperature is not enough, will entrain the water vapor into the next process, In the welding when the high temperature and the emergence of bubbles.
5.5.2, PCB processing before the storage environment is not good, humidity is too high, there is no timely drying when welding.
5.5.3, in the wave soldering process, now often use the water flux, if the PCB preheating temperature is not enough, the flux of water will flow along the hole through the hole into the PCB substrate, the first around the pad Into the water vapor, after welding high temperature will produce bubbles.
Solution:
5.5.4, strictly control the production process, the purchase of the PCB should be checked after storage, usually PCB temperature at 260 ℃ within 10s should not appear bubbling phenomenon.
5.5.5, PCB should be stored in a ventilated and dry environment, the storage period of not more than 6 months;
5.5.6, PCB should be placed in the oven before welding (120 ± 5) ℃ pre-bake for 4 hours.
5.5.7, wave soldering in the preheating temperature should be strictly controlled into the wave soldering should reach 100 ~ 140 ℃, if the use of water flux, the preheating temperature should reach 110 ~ 145 ℃, to ensure that the water vapor can be evaporated.
6, SMA welding on the PCB substrate after the bubble
SMA welding after the emergence of nail size of the bubble, the main reason is the PCB substrate entrainment of water vapor, especially the multi-layer board processing. Because the multi-layer by the multi-layer epoxy resin prepreg pre-hot and then from, If the epoxy resin prepreg storage period is too short, the resin content is not enough, pre-drying to remove the water vapor is not clean, then the hot forming is easy to entrain with water vapor.Also because the semi-fixed film itself is not enough glue, between layers In addition, the PCB purchase, due to storage period is too long, the storage environment is damp, patch production before the pre-bake, damp PCB patch is also prone to blistering phenomenon.
Solution: PCB purchase should be accepted after the storage; PCB patch before the (120 ± 5) ℃ temperature pre-bake 4 hours.
7, IC pin after welding open or Weld
cause:
7.1, poor coplanarity, especially FQFP devices, due to improper storage caused by pin deformation, if the placement machine does not check the coplanarity function, and sometimes difficult to be found.
7.2, pin solderability is not good, IC storage for a long time, the pin is yellow, poor solderability is caused by the main reason for Weld.
7.3, solder paste poor quality, low metal content, poor weldability, usually used for welding FQFP device solder paste, metal content should be no less than 90%.
7.4, preheating temperature is too high, easy to cause IC pin oxidation, so that poor solderability.
7.5, printing template window size is small, so that solder paste is not enough.
Solution:
7.6, pay attention to the custody of the device, do not just pick up the components or open the packaging.
7.7, the production should check the solderability of components, with particular attention to IC storage period should not be too long (since the date of manufacture within one year), custody should not be high temperature, high humidity.
7.8, carefully check the template window size, should not be too large should not be too small, and pay attention and PCB pad size matching.

                                                                       Nanjing DingYing electronic Technology Co., Ltd.