PCBA Tech - ​6 kinds of wave soldering quality defects and solutions

1. Poor wetting, missing welding, and virtual welding

cause:

Oxidation or pollution of the solder ends, pins, and pads of the printed circuit board substrate, and the PCB is damp;

Chip component tip metal electrode adhesion is poor or single-layer electrode is used, which will cause capping at the welding temperature;

The PCB design is unreasonable, and the shadow effect during wave soldering causes solder leakage;

PCB warping makes the contact between the PCB warping position and wave soldering poor;

The two sides of the conveyor belt are not parallel (especially when the PCB transmission frame is used), so that the contact between the PCB and the wave crest is not parallel;

The wave crest is not smooth, and the height of the two sides of the wave crest is not parallel, especially the tin wave nozzle of the electromagnetic pump wave soldering machine, if it is blocked by oxide, the wave crest will appear jagged, which is easy to cause missed soldering and false soldering;

Poor flux activity, resulting in poor wetting;

PCB preheating temperature is too high, so that the flux is carbonized, loses activity, and causes poor wetting

Solution:

The components are used first come first, do not store in a humid environment, do not exceed the specified use date, clean and dehumidify the damp PCB;

Wave soldering should choose surface mount components with three-layer terminal structure. The body of the component and the soldering end can withstand the temperature impact of 260 degree Celsius wave soldering more than twice;

When SMD/SMC adopts wave soldering, the component layout and arrangement direction should follow the principle of smaller components in front and avoid mutual shielding as much as possible. In addition, the remaining pad length after the components can be lengthened appropriately;

PCB warpage is less than 0.8%~1.0%;

Adjust the horizontal level of wave soldering machine and transmission belt or PCB transmission rack

Clean the wave nozzle;

Replace the flux;

Set the proper preheat temperature.

2. Sharpen the tip

cause:

The PCB preheating temperature is too low, which makes the temperature of the PCB and the components low, and the components and the PCB absorb heat during soldering;

The soldering temperature is too low or the conveyor speed is too fast, which makes the viscosity of the molten solder too large;

The wave crest height of the electromagnetic pump wave soldering machine is too high or the pin is too long, so that the bottom of the pin cannot touch the wave crest, because the electromagnetic pump wave soldering machine is a hollow wave, and the thickness of the hollow wave is 4~5mm

Poor flux activity;

The ratio of the lead diameter of the welding component to the insertion hole is not correct, the insertion hole is too large, and the large pad absorbs a large amount of heat.

Solution:

Set the preheating temperature according to the PCB, board layer, number of components, whether there are components mounted or not, and the preheating temperature is 90~130 degree Celsius;

The tin wave temperature is (250±5) degree Celsius, and the welding time is 3~5s. When the temperature is slightly lower, the conveyor belt speed should be slowed down;

The height of the wave crest is generally controlled at 23 points of the thickness of the PCB. The pin forming of the plug-in component requires that the pin be exposed to the PCB soldering surface by 0.8~3mm;

Replace the flux;

The aperture of the insertion hole is 0.15~0.4mm larger than the diameter of the lead (the lower limit is taken for the thin lead, and the upper limit is taken for the thick lead).

3. The solder mask of the printed board blisters after welding

SMA will appear light green around individual solder joints after welding. For example, when the ice height is severe, a fingernail-sized bubble will appear, which not only affects the appearance quality, but also affects the performance in severe cases. This kind of defect is also a common problem in the reflow soldering process, but it is common in wave soldering.

cause:

The fundamental reason for the blistering of the solder mask is the presence of gas or water vapor between the solder mask and the PCB substrate. These traces of gas or water vapor will be entrained in it during different processes. When the soldering high temperature is encountered, the gas expands. This leads to the delamination of the solder mask and the PCB substrate. During soldering, the soldering temperature is relatively high, so bubbles first appear around the pad.

One of the following reasons will cause the PCB to entrain moisture.

PCB often needs to be cleaned and dried before doing the next process. For example, the solder mask should be dried after being etched. If the drying temperature is not enough at this time, it will entrain moisture into the next process. Bubbles appear at high temperature;

The storage environment before PCB processing is not good, the humidity is too high, and it is not dried in time when soldering;

In the wave soldering process, water-containing flux is often used now. If the PCB preheating temperature is not enough, the water vapor in the flux will enter the inside of the PCB substrate along the hole wall of the through hole. Air bubbles will be generated after welding high temperature.

Solution:

Strictly control all production links. The purchased PCB should be put into storage after inspection. Usually, the PCB should not be blistered within 10s at 260°C;

PCB should be stored in a ventilated and dry environment, and the storage period should not exceed 6 months;

The PCB should be pre-baked in an oven at (120+5) degree Celsius for 4h before soldering;

The preheating temperature in wave soldering should be strictly controlled, and it should reach 100~140 degree Celsius before entering wave soldering. If water-containing flux is used, the preheating temperature should reach 110~145 degree Celsius to ensure that the water vapor can be completely volatilized.

4. Pinholes and stomata

Pinholes and pores all represent bubbles in the solder joints, but they have not yet expanded to the surface. Most of them occur at the bottom of the substrate. When the bubbles at the bottom are completely diffused and condensed before exploding, pinholes or pores are formed. The difference between pinholes and pores is that the diameter of the pinholes is smaller.

cause:

Organic pollutants are stained on the pins of the substrate or parts. Such polluting materials come from automatic plug-in machines, pin forming machines, and poor storage.

The substrate contains water vapor generated by electroplating solutions and similar materials. If the substrate uses cheaper materials, it may inhale such water vapor, and enough heat is generated during soldering, which vaporizes the solution and causes pores;

Too much substrate storage or improper packaging, absorbing moisture in the surrounding environment;

The flux tank contains water;

Compressed air for foaming and hot air knives contains too much moisture.

The preheating temperature is too low to evaporate water vapor or solvent. Once the substrate enters the tin furnace, it will be in contact with the high temperature instantly and burst;

If the tin temperature is too high, it will burst immediately when it encounters moisture or solvents.

Solution:

Use common solvents to remove organic pollutants on the pins; due to the difficulty of removing silicone oil and similar products containing silicon, if you find that the problem is caused by silicone oil, you must consider changing the source of lubricating oil or release agent;

Bake the substrate in an oven before assembly to remove the moisture contained in the substrate;

Bake the substrate in the oven before assembly;

Replace the flux regularly;

The compressed air needs to be equipped with a water filter and exhausted regularly;

Increase the preheating temperature;

Lower the temperature of the tin furnace.

5. Rough welding

cause:

Improper time-temperature relationship;

Incorrect solder composition;

Mechanical vibration before solder cooling;

The tin is contaminated.

Solution:

Adjust the conveyor belt speed and correct the welding preheating temperature to establish an appropriate time-temperature relationship;

Check the solder composition to determine the type of solder and the appropriate soldering temperature for a certain alloy;

Check the conveyor belt to ensure that the substrate does not collide or shake during welding and solidification;

Check the types of impurities that cause contamination, and use appropriate methods to reduce or eliminate the contaminated solder in the tin bath (dilute or replace the solder)

6. Welding into blocks and welding objects protruding

cause:

The conveyor belt speed is too fast;

The welding temperature is too low;

The secondary welding waveform is low;

Improper waveform or improper angle between the waveform and the board, and improper output waveform;

Surface contamination and poor solderability.

Solution:

Slow down the speed of the conveyor belt; increase the temperature of the tin furnace at the latest point;

Re-adjust the secondary welding waveform;

Re-adjust the waveform and the angle of the conveyor belt;

Clean the PCB surface to improve its solderability.