Wave soldering has been used for PCB mounting for more than 20 years, and has become a very mature electronic mounting technology, mainly used for soldering through-hole mounting components and surface components through hybrid assembly.
The name derives from the result of connecting metal components to a PCB using molten solder waves. This process uses a pot to hold a certain amount of molten solder. The component is inserted into the PCB or placed on the PCB. The board surface of the printed circuit board is in contact with the wave peak during soldering. The soldered part is first in contact with the first wave peak and then with the second wave peak. The first wave improves welding reliability and the second wave ensures welding quality. Automotive soldering is much faster than hand-welded parts and can produce higher quality products.
Defects and solutions for wave soldering
Because the wave soldering process involves machinery, the accuracy is more accurate than manual welding, and the error rate is lower. However, you need to carefully control the welding position and welding content. Otherwise, it is easy to encounter the following welding defects:
1. Insufficient Hole Fill
Insufficient of holes is a problem that occurs on printed circuit boards with pre-drilled holes that are used to mount components to the board. In essence, when there is not enough solder to fill the holes drilled into the component, underfilling of the holes occurs, which means that once cooled, the solder will not stick to the circuit board.
Solder joints are dry / incomplete / have holes, and the solder in the insertion holes and vias is not full.
A) PCB preheating and soldering temperature are too high, making the solder viscosity too low;
B) The hole diameter of the insertion hole is too large, and the solder flows out of the hole;
C) large lead pads for inserting components, solder is pulled onto the pads, so that the solder joints dry up
D) Poor quality of metallized holes or solder resist flowing into the holes;
E) The PCB climbing angle is too small, which is not conducive to solder exhaust.
A) The preheating temperature is 90-130 ℃, the upper limit is taken when there are many components, the tin wave temperature is 250 +/- 5 ℃, and the soldering time is 3 ～ 5S.
B) The hole diameter of the insertion hole is larger than the diameter of the lead by 0.15 ～ 0.4mm.
C) The size of the pads and the diameter of the pins should match to facilitate the formation of a meniscus;
D) reflected to the PCB processing plant to improve processing quality
E) The climbing angle of PCB is 3 ～ 7 ℃.
2. Excessive Solder
Component solder ends and leads are surrounded by excessive solder, and wetting angles are greater than 90 °.
1. The soldering temperature is too low or the conveyor speed is too fast, making the viscosity of the molten solder too large.
2. PCB preheating temperature is too low, the component and PCB absorb heat during soldering, which reduces the actual soldering temperature.
3. The activity of the flux is poor or the specific gravity is too small.
4. The solderability of the pad, the insertion hole or the pin is poor, and it cannot be fully wetted, and the generated bubbles are wrapped in the solder joint.
5. The proportion of tin in the solder is reduced, or the content of the impurity Cu in the solder is high, which increases the viscosity of the solder and deteriorates the fluidity.
6. Too much solder residue.
A) Tin wave temperature is 250 +/- 5 ° C, slow down the speed of the conveyor, and soldering time is 3 ~ 5S.
B) Set the preheating temperature according to the PCB size, board layer, number of components, and whether there are components mounted.
C) change the flux or adjust the appropriate ratio;
D) To improve the processing quality of PCB boards, components are used on a first-come-first-served basis, and should not be stored in a humid environment;
E) When the proportion of tin is <61.4%, some pure tin can be added in an appropriate amount, and the solder should be replaced when the impurities are too high;
F) Residues should be removed at the end of each day.
Tombstones, also known as Lifted components, which are components that are lifted from the PCB board during the soldering process. Several common causes of tombstones include:
* Incorrect lead length may cause the component to lift off the printed circuit board as it enters the solder bath.
* Wave soldering on a flexible PCB will bend while the remaining components remain flat, causing them lift on the circuit board.
* Use components that have different requirements for solder type and temperature.
To resolve incorrect lead lengths, review the leads you are using. If your lead is too long, hitting the solder bath may push it out of the through hole. To solve this problem, you can increase the immersion time in the wave, which reduces the heat demand on the wire and allows it to settle.
To correct PCB deflections or other deflection-related issues, double check what kind of PCB you are using and what its heat resistance is. Board deflection is common on large connectors and large IC packages or sockets.
Finally, after checking the thermal tolerances of the board, check the thermal tolerances of all components. Components with different temperature requirements or different lead solderability will also increase with wave contact, because some components will be soldered off, and excessive heat will push other components away. Ensuring that all components you use have the same requirements should help prevent such problems.
4. Solder Ball
Spherical solder particles with a diameter greater than 0.13mm or within 0.13mm from the wire are called solder balls. During wave soldering, when a small amount of solder reattaches to the PCB, especially near the leads, a solder ball phenomenon occurs
1 PCB is wet during manufacturing or storage
2. High ambient humidity
3. Flux coating is uneven and there are omissions
4. Poor substrate construction
5. Preheating temperature is not suitable
6. Silver plated parts are dense
7. The peak shape is not suitable
A) Improve PCB storage conditions and reduce moisture
B) Flux is applied evenly
C) Change the PCB design plan and analyze the uniformity of heating
D) Select the appropriate waveform according to different boards
E) Bake the PCB circuit board
5. Pin Holes & Blow Holes
Pinholes or blowholes are the same thing and are caused by degassing of the printed circuit board during the soldering process. The formation of pins and pores during wave soldering is usually related to the thickness of the copper plating. Moisture on the PCB can escape through thin copper plating or voids in the plating. The plating in the through hole should be at least 25um to prevent the moisture on the board from turning into water vapor and generating gas through the copper wall during wave soldering.
The only way to solve this problem is to plate at least 25um of copper in the through holes to improve the quality of the circuit board.
6. Poor Wetting
Non-wetting or poor wetting means that the solder can no longer spread well on the pads, so that good solder joints cannot be obtained, which directly affects solder joint reliability
1 The plating on the surface of the PCB pad or pin is severely oxidized.The oxide layer separates the molten solder from the plating layer. The solder cannot be wetted and spread on the oxide layer.
2 Poor solderability
3 Solder or flux is contaminated
4 Insufficient welding temperature and short wave contact time:
5 Low preheating temperature or insufficient flux activity
6 Solder impurities exceed the standard;
A) Improve material solderability;
B) Use a plate that meets the requirements for coating quality. Generally, a plating thickness of at least 5μ is required to ensure that the material does not expire within 12 months.
C) Remove surface contaminants
D) Properly increase the preheating or welding temperature to ensure sufficient welding time;
E) Maintain the purity of solder
7. Solder Joint Bridge Or Short Circuit
1. PCB design is unreasonable, pad spacing is too narrow.
2. The pins of the plug-in components are irregular or skewed, and the pins have been approached or bumped before soldering;
3. PCB preheating temperature is too low, the component and PCB absorb heat during soldering, which reduces the actual soldering temperature;
4. The soldering temperature is too low or the conveyor speed is too fast, which reduces the viscosity of the molten solder;
5. Poor solder resist activity.
A) Design according to PCB design specifications. The long axis of the two end Chip components should be as perpendicular as possible to the PCB running direction during soldering. The long axes of SOT and SOP should be parallel to the PCB running direction. Widen the pad of the last pin of the SOP (design a tin theft pad).
B) The pins of the plug-in components should be formed according to the PCB pitch and assembly requirements. If a short plug-on soldering process is used, the component pins on the welding surface are exposed on the PCB surface 0.8 to 3 mm.
C) Set the preheating temperature according to the PCB size, board layer, number of components, and presence or absence of components.
D) Tin wave temperature is 250 +/- 5 ℃, soldering time is 3 ～ 5S. When the temperature is slightly lower, the conveyor speed should be slowed down.
F) Replace the flux.
8. Component Rupture
1) Destruction before assembly
2) The thermal mismatch between the plate and the component during the welding process caused the component to crack.
3) Over welding temperature
4) The cooling rate is too fast, causing stress concentration.
1) Use appropriate process curve;
2) Adjust the welding temperature appropriately
3) Adjust the cooling rate
9. Cold Solder
Generally refers to the uneven surface of the solder joint, which does not form a good solder strip. In severe cases, wrinkles or cracks are even generated around the pins, which affects the service life of the solder joint.
1) Oxidation of pads or pins;
2) The mechanical vibration of the conveyor belt or the guide rail causes the solder joints to be disturbed by external forces when they are cooling.
3) The welding temperature is too low or the soldering time is short;
A) Eliminate vibration sources during soldering
B) Check if the pins and pads are oxidized before soldering and deal with them in time
C) Prolong the soldering time appropriately
10 Flux Residues
Flux residue is mainly related to flux composition and harsh process conditions;
1) Improper flux selection
2) The content of rosin resin in the flux is too high or the quality is not good
3 Flux does not work effectively
4) Improper preheating temperature setting
5) Improper peak contact time
A) Correct selection of flux
B) Adopting strict preheating process
C) Extending the contact time with the wave can reduce the flux residue
D) Increase the flushing effect of the forward flow wave
1) Dirty PCB board surface: mainly due to high solid content of the flux, too much coating, too high or too low preheating temperature, and due to dirty conveyor claws, too much oxide and tin slag in the solder pot, etc.
2) PCB deformation: It usually occurs in large-sized PCBs. The large-sized PCBs have heavy weights or unbalanced components due to uneven component layout, This requires PCB components to be distributed as evenly as possible during the design of the process, and the process edges are designed in the middle of the large-sized PCB.
3) Loss of film (lost film): The quality of the adhesive is bad, or the curing temperature of the adhesive is incorrect. If the curing temperature is too high or too low, the bonding strength will be reduced.
4) Defects that cannot be seen: grain size of solder joints, internal stress of solder joints, internal cracks in solder joints, brittle joints, poor solder joint strength, etc. X-ray and solder joint fatigue tests are required. These defects are mainly related to soldering materials, adhesion of PCB pads, solderability of component soldering ends or pins, and temperature curves.