Analysis of common solder joint defects and failure causes in "welding knowledge" PCBA processing

Date：2022-05-13 14:44:26 Views：1700

Solder joint reliability is usually a pain point in electronic system design. Various factors will affect the reliability of solder joints, and any one of them will greatly shorten the service life of solder joints. As the solder joints in the circuit board become smaller and smaller, and the mechanical, electrical and thermodynamic loads they carry become heavier and heavier, the requirements for stability are also increasing. However, the problem of PCBA solder joint failure will also be encountered in the actual processing process. It is necessary to analyze and find out the cause to avoid solder joint failure again. There are many common welding defects on circuit boards. The following describes the common welding defects, appearance characteristics, hazards and cause analysis in detail.

1. False soldering

Appearance features: there is an obvious black boundary between the solder and the lead of components or copper foil, and the solder is concave to the boundary.

Hazard: not working properly.

Cause analysis:

Component leads are not cleaned, tin plated or oxidized;

The printed board is not cleaned well, and the quality of the sprayed flux is not good.

2. Solder accumulation

Appearance features: the solder joint structure is loose, white and matte.

Hazard: insufficient mechanical strength, possible false welding.

Cause analysis:

Poor solder quality;

Insufficient welding temperature;

When the solder is not solidified, the component lead is loose.

3. Excessive solder

Appearance features: the solder surface is convex.

Hazard: solder is wasted and may contain defects.

Cause analysis: solder evacuation is too late.

4. Too little solder

Appearance features: the welding area is less than 80% of the pad, and the solder does not form a smooth transition surface.

Hazard: insufficient mechanical strength.

Cause analysis

Poor solder fluidity or premature solder withdrawal;

Insufficient flux;

Welding time is too short.

5. Rosin welding

Appearance features: Rosin residue is sandwiched in the weld.

Hazard: insufficient strength, poor continuity, and possible on and off.

Cause analysis:

The welding machine is too many or has failed;

Insufficient welding time and heating;

The surface oxide film is not removed.

6. Overheat

Features: rough and glossy metal surface, no welding spots.

Hazard: the pad is easy to peel off and the strength is reduced.

Cause analysis: the power of soldering iron is too large and the heating time is too long.

7. Cold welding

Appearance features: the surface is in the form of bean curd residue particles, and sometimes there may be cracks.

Hazard: low strength and poor conductivity.

Cause analysis: the solder shakes before solidification.

8. Poor infiltration

Appearance features: the interface between solder and weldment is too large and not smooth.

Hazard: low strength, no connection or on-off.

Cause analysis

The weldment is not cleaned;

Insufficient flux or poor quality;

The weldment is not fully heated.

9. Asymmetry

Appearance features: solder is not full of pads.

Hazard: insufficient strength.

Cause analysis

Poor solder fluidity;

Insufficient flux or poor quality;

Insufficient heating.

10. Looseness

Appearance features: wires or component leads can be moved.

Hazard: poor or non-conductive.

Cause analysis:

Gap caused by lead movement before solder solidification;

The lead wire is not well treated (poor infiltration or no infiltration).

11. Sharpening

Appearance features: tip appears.

Hazard: poor appearance, easy to cause bridging phenomenon.

Cause analysis:

Too little flux and too long heating time;

Improper withdrawal angle of soldering iron.

12. Bridging

Appearance features: adjacent wires are connected.

Hazard: electrical short circuit.

Cause analysis:

Too much solder;

Improper withdrawal angle of soldering iron.

13. Pinhole

Appearance features: holes can be seen by visual inspection or low power amplifier.

Hazard: the strength is insufficient and the solder joint is easy to corrode.

Cause analysis: the gap between the lead and the pad hole is too large.

14. Bubble

Appearance features: the root of the lead has a fire jet solder bulge, and there is a cavity inside.

Hazard: temporary conduction, but it is easy to cause poor conduction for a long time.

Cause analysis:

Large gap between lead and pad hole;

Poor lead infiltration;

The welding time of double-sided plate blocking through hole is long, and the air in the hole expands.

15. Copper foil warping

Appearance features: the copper foil is stripped from the printed board.

Hazard: the printed board is damaged.

Cause analysis: the welding time is too long and the temperature is too high.

16. Stripping

Appearance features: solder joints peel off from copper foil (not copper foil and printed board).

Hazard: open circuit.

Cause analysis: the metal coating on the pad is poor.

Main causes of PCBA processing solder joint failure:

1. Poor component pins: plating, pollution, oxidation, coplanar.

2. Poor PCB pad: coating, pollution, oxidation and warpage.

3. Solder quality defects: unqualified composition, impurities and oxidation.

4. Flux quality defects: low weldability, high corrosion and low sir.

5. Defects in process parameter control: design, control and equipment.

6. Defects of other auxiliary materials: adhesives and cleaning agents.

Methods for increasing the stability of PCBA solder joints:

The stability experiment of PCBA solder joint includes stability experiment and analysis. On the one hand, its purpose is to evaluate and identify the stability level of PCBA integrated circuit devices and provide parameters for the stability design of the whole machine. On the other hand, it is necessary to improve the stability of solder joints in PCBA processing. Therefore, it is necessary to analyze the failed products, find out the failure mode and analyze the failure cause. The purpose is to modify and improve the design process, structural parameters and welding process, and improve the finished product rate of PCBA processing. The failure mode of PCBA solder joint is the basis of predicting its cycle life and establishing its mathematical model.

I believe that by reading the above contents, we have a preliminary understanding of common solder joint defects and failure cause analysis. At the same time, we also hope that we can make a summary in the learning process, so as to continuously improve our professional level. Shenzhen Chuangxin Online Testing Technology Co., Ltd. is a well-known professional testing organization for electronic components in China. It has three standardized laboratories with an area of more than 1000 square meters. The scope of testing services covers: testing and verification of electronic components, identification of IC authenticity, product design, material selection, failure analysis, function testing, factory incoming material inspection, X-ray testing of components, tape braiding and other testing items.