Reflow quality defects and solutions

1, tombstone reflow phenomenon, the chip components often appear erected phenomenon causes: the root cause of tombstoning phenomenon is unbalanced wetting force elements on both sides, so the torque is uneven across an element , resulting in the occurrence of tombstoning.
The following conditions will result in wetting force of on both sides of component imbalance during reflow:
1.1, the pad design and layout is unreasonable. If pad design and layout has the following defects will cause wetting force elements on both sides of the imbalance.
1.1.1. One of both sides of the pad and the ground component connected to one side or the pad area is too large, uneven pad heat capacity at both ends;
1.1.2 throughout the PCB surface temperature is too large so that elements on both sides of the pad absorbs heat unevenly;
1.1.3, large devices QFP, BGA, small chip components at both ends of the pad around the radiator temperature will appear uneven.
Solution: Change the pad design and layout.

1.2, there is a problem with the solder paste printing. Solder paste of low activity or element can be poor solderability, after the solder paste melts, surface tension is not the same, the pad will cause the wetting force imbalance. Two pads of the printed solder paste uneven amount, while many will be increased due to solder heat absorption, melting time lag, so unbalanced wetting force.
Solution: use higher activity solder paste, solder paste printing parameters to improve, especially the template window size.

1.3 patch displacement discontinuity Z-axis direction will lead to solder components immersed in depth uneven melting due to the time difference caused by unbalanced wetting force on both sides. If the element is a direct result of the shift patch tombstone.
Solution: Adjust the placement machine process parameters.

1.4, temperature curve is not correct if the reflow furnace body temperature zone is too short and too little will cause the PCB heating curve is incorrect, so that the wet surface of the plate is too large, resulting in unbalanced wetting force.
Solution: According to each of the different products regulate the proper temperature curve.
1.5, nitrogen reflow oxygen concentration to take nitrogen reflow solder wetting force will increase, but more and more of illustration, in the event of low oxygen content tombstoning phenomenon but increased; usually considered oxygen content control (100 to 500) negative about 6 × 10 th most appropriate.
2, tin beads
Tin beads are reflow soldering defects common one, it not only affects the appearance of the bridge and would lead solder balls can be divided into two categories, one side appeared in chip components, often as a separate large spherical; Another Appearing around the IC pins were scattered small bead of solder balls have a lot of reasons, are as follows:
2.1, the temperature profile is not correct reflow curve can be divided into four sections, respectively, is preheated, heat preservation, reflow and cooling. Preheating, thermal insulation is intended to make the PCB surface temperature to rise to within 60 ~ 90s 150 ℃, and incubated for about 90s, which can not only reduce the thermal shock PCB and components, but mainly to ensure that the solder paste can be part of the volatile solvent, the solvent to avoid causing too much splash reflow, resulting paste out of the pad and the formation of tin beads.
Solution: Note that the heating rate, and take moderate warm, making it a good platform for the majority of the solvent to evaporate.

2.2, the quality of solder paste
2.2.1, solder metal content is usually at (90 ± 0.5) ℅, metal content is too low will lead to excessive flux component, so the excess flux due to warm-up phase and cause less volatile fly beads.
2.2.2, solder paste and the oxygen content of water vapor can cause increased flying beads of solder paste is usually due to cold, when removed from the refrigerator, make sure that if there is no recovery time, will lead to water vapor to enter; Also paste bottles every time the lid tightly after use, if not timely Gaiyan also cause water vapor to enter.
Printed solder paste on the template upon completion. The remaining part should be addressed separately, if more back into the original bottle, the bottle will cause deterioration of solder paste, will produce tin beads.
Solution: Select the quality of solder paste, attention solder paste custody and use requirements.

2.3 Printing and placement
2.3.1, the solder paste printing process, due to the template and the pad will be shifted to happen, if the offset is too large will cause the solder pads soaked flow, the solder beads prone to heating. Also print poor working conditions will lead to the generation of solder balls, ideal printing ambient temperature 25 ± 3 ℃, relative humidity 50℅ ~ 65℅.
Solution: Carefully adjust the clamping of the template to prevent loosening improve the printing environment.
2.3.2, the pressure in the Z-axis placement process is caused by a solder beads important reasons, but often do not attract attention. Part Mounter Z axis head is based on the thickness of the elements to locate, such as the Z-axis height improper adjustment will cause element attached to the moment on the PCB solder pads squeeze the phenomenon outside the solder balls which will form part of the paste during soldering. in this case the tin produced slightly larger beads.
Solution: Readjust the Z-axis height of placement machine.
2.3.3, thickness and size of the opening of the template. The thickness of the template with the opening size is too large, it will lead to increasing the amount of solder paste, solder paste can also cause overflow to the outside of the pad, especially the use of the chemical etching technique manufactured formwork.
The solution: use the appropriate thickness and size of the opening of the template design, general template opening area for pad size 90℅.

3, wicking
Wicking phenomenon known as the pulling phenomenon is one of the common welding defects, common in the vapor phase reflow. Wicking solder pads from along the pin between the pin and chip up to the body, usually in the form of serious Weld phenomenon causes long as it is due to the large thermal conductivity component pin, so heating up quickly, so that the solder pin priority humid, wet strength is much greater than between the solder and lead solder wetting forces and between the pads, Furthermore upturned pin will aggravate wicking phenomenon.
Solutions:
3.1, for the vapor phase reflow soldering SMA should first fully preheated oven and then into the gas phase;
3.2, you should carefully check the PCB pad solderability, weldability bad PCB can be used to produce;
3.3, full attention coplanarity element of coplanarity of the device can not be bad for the production.
In the infrared reflow, PCB substrate and organic solder flux is a good infrared absorbing medium, and the pin was able to partially reflecting infrared, so the contrast priority melted solder wetting and solder pads on the force will be greater than the wetting force between the solder and pin, so the solder will not rise along the pins, so the probability of the occurrence of wicking much smaller.

4, bridging ━━ is one of the common defects in SMT production, it will cause a short circuit between the elements encountered bridging must rework caused by many of the major reasons for bridging are:

4.1, the quality of solder paste.
4.1.1, the high metal content in the paste, especially the printing time is too long, prone to metal content increased, leading IC pin bridging;
4.1.2, paste viscosity is low, after the warm-up to the pad outside overflow;
4.1.3, paste tower off poor, after preheating to the pad outside overflow;
Solution: Adjust the ratio or the use of good quality solder paste.

4.2, the printing system
4.2.1, printing repeatability is poor, bit uneven (steel on the bit bad, PCB counterpoint bad),. resulting solder paste printing to the pad outside, especially in fine-pitch QFP pads;
4.2.2 Template window size and thickness as well as PCB design right pad design Sn-pb alloy coating uneven, resulting paste side.
Solution: Adjust the printing press, improve PCB pad coating;
4.3, stickers to put too much pressure, pressure full flow after the paste is the production of common causes. Another component placement accuracy is not enough will appear shift, IC pin deformation.
4.4, reflow oven temperature too fast, too late to the volatile solvent pastes.
Solution: Z-axis height adjustment Mounter and reflow furnace heating rate.

5, wave soldering quality defects and solutions
5.1 icicling means any excess solder in the solder the ends of the needle, which is the wave soldering process-specific defects.
Cause: PCB improper transmission speed, low preheat temperature, solder pot temperature is low, small PCB transfer dip, poor peak flux failure, component leads poor weldability.
Solution: Adjust the transmission speed to the appropriate date, adjust the preheating temperature and solder pot temperature, adjust the angle of PCB transfer, preferably nozzles, adjust peak shape, change and flux of new leads solderable solve problems.
5.2 Weld causes: component leads poor weldability, preheat temperature is low, the solder problems, flux activity is low, the pad hole too big, lead oxide system board, the board there is pollution, the transmission speed is too fast, the tin pot low temperature.
Solution: Solving leads solderable, preheat temperature adjustment, testing and impurities tin solder, flux density adjustment, designed to reduce the pad hole, remove PCB oxide, cleaning the plate surface, adjust the transmission speed, solder pot temperature adjustment .
5.3, because the thin tin produced: component leads poor weldability, pads too (except require large pad), pad the hole is too big, welding angle is too large, too fast transmission speed, solder pot temperature is high, the flux coating apply unevenly insufficient solder containing tin.
Solution: Solving leads solderable, designed to reduce the pad and the pad hole, reducing welding angle, adjust the transmission speed, adjust the solder pot temperature, pre-coated solder inspection equipment, laboratory solder content.
5.4, weld causes: Lead poor solderability, solder wave instability, failure or spray uneven flux, PCB partial poor weldability, conveyor chains jitter, pre-coated solder and flux are not compatible, the process is unreasonable.
Solution: Solving leads solderable, check crest device replacement flux, solder precoating inspection device, solve PCB solderability (cleaning or returned), check the adjustment gear, uniform use of flux adjustment process.
5.5 After soldering PCB solder mask blistering
SMA will appear in the welded joints of individual vesicles around the green light, there will be severe blister the size of a fingernail, not only affects the appearance of quality, performance impact will be severe, this defect is the reflow process often appear in the problem, but for more time to wave.
Reason: the root cause is the presence of solder mask blistering gases or vapors between the die and the PCB solder substrate, these traces of gases or vapors to be entrained in a different process in which, when the soldering temperature encountered , gas expansion resulting layered PCB substrate with solder resist film, soldering, pad temperature is relatively high, so bubbles first appeared in the pad around.
One of the reasons of the following will result in PCB entrained water vapor:
5.5.1, PCB in the process often requires washing, drying and then do the next process, as will rot engraved paste solder mask should be dry before, if the drying temperature is not enough this time, the water vapor will be entrained into the next process, when the temperature and the air bubbles in the welding.
5.5.2, the former PCB processing and storage environment is not good, high humidity, welding time and no time drying.
5.5.3, the wave soldering process, is now often the use of aqueous flux, if the PCB preheat temperature is not enough, the hole in the wall of the flux of water vapor will be along the through-hole PCB substrate into the interior, which is around the pad first into the vapor, will produce bubbles after encountering soldering temperatures.
The solution:
5.5.4 To strictly control the production processes, the PCB should be examined after the purchase of storage, usually at a temperature of 260 ℃ PCB should blistering within 10s.
5.5.5, PCB should be stored in a dry environment, the storage period not exceeding six months;
5.5.6, PCB prior to soldering should be placed in the oven at (120 ± 5) ℃ temperature pre-bake under four hours.
5.5.7, wave soldering preheating temperature should be strictly controlled, top-wave should reach 100 ~ 140 ℃, if the use of aqueous flux, the preheating temperature should reach 110 ~ 145 ℃, to ensure complete water vapor can be volatile.

6, blistering on the PCB board after soldering SMA
SMA fingernail-sized blister after welding, the main reason is the internal PCB substrate entrained water vapor, especially plywood processing. Because plywood epoxy prepregs multilayer preform again after pressing together, If the epoxy prepreg storage period is too short, the resin content is not enough, the pre-dried to remove moisture removal is not clean, then after thermoforming easily entrained moisture. also due to a semi-solid gel content piece itself is not enough, between the layers the combined force is not enough and leave bubbles. in addition, the PCB purchase, because the storage period is too long, humid storage environment, there is no time before pre-baking chip production, PCB after damp patch also prone to blistering.
Solution: PCB after the purchase can only be put in storage after acceptance; former PCB patch should be (120 ± 5) ℃ temperature pre-bake under four hours.

7, IC pin welded or soldered open
Reason:
7.1 coplanarity is poor, especially FQFP devices, due to improper storage pin deformation, if the placement machine without checking coplanarity function, and sometimes difficult to detect.
7.2, the pin is not good weldability, IC stored for a long time, pin yellow, poor solderability is the main cause of Weld.
7.3, paste of poor quality, low metal content, weldability is poor, usually used for soldering paste FQFP devices, metal content should not be less than 90%.
7.4, preheat temperature is too high, can lead to IC pin oxidation, weldability deteriorates.
7.5, a small stencil window size, so that the amount of solder paste is not enough.
The solution:
7.6, take good care of the device, do not just pick up elements or unpacking.
7.7, the production of components should be checked weldability, special attention should not be too long IC storage period (within one year since the date of manufacture), should not be kept high temperature and humidity.]
7.8, double-check the template window size, not too big nor too small, and pay attention to the PCB pad size to match.