There are often some bad phenomena during the soldering of electronic products, which brings a lot of troubles to the normal work. How do we solve it? Today I will share the causes and countermeasures of common solder defects.

First of all, understand the basic factors that result in poor solder.

① Equipment failure. It may leads to excessive or insufficient soldering time.

② Inappropriate material. There are impurities on the PCB pad, or there is a fracture. And the solder material is not suitable for the PCB pad or other object to be soldered.

③ Environmental factors. Abnormal temperature, conveyor speed, immersion depth and other problems caused by incorrect equipment parameter settings.

How to find the root cause of these problems?

① Check the soldering equipment. Use special electronic instruments to assist in checking equipment for parts aging or failure;

② Check whether the equipment parameters are set correctly: Check whether the parameters are correct;

③ Check whether there are impurities or fractures on the PCB pad and the soldering material is suitable for it.

Ideal solder joint
      Before checking the solder joint for defects, it is better to find an image of an ideal solder joint for comparison

ideal solder joints

For ideal solder joint, its surface has good wetting. That is to say, the molten solder should spread out on the surface of the metal to be soldered and form a complete, uniform and continuous solder covering layer. The correct amount of solder should be enough to cover the solder joint without too much or too little.

Types & Countermeasures of Common Solder Defects

1. Poor Solder Wetting

      Poor wetting is caused when the solder material is melted and the tin cannot cover the surface of object to be soldered, leaving the metal of the object exposed.

Causes & Solutions

(1) External pollution: Where there are any kind of contaminant, dirt or moisture present on your PCB or component, the wetting process begins to be hampered. It is always generally advised to clean the PCB you are working on prior to the soldering process.

(2) PCB pad oxidation. There will be oxidation when PCB pad is exposed to the air for too long or poor drying in the manufacturing process. To solve the problem, we can choose a strong active flux, or use a chemical solvent for etching, such as wiping the oxidation with a strong acid solution after appropriate dilution. After wiping, it must be tinned as soon as possible, otherwise it will cause more serious oxidation.

2. Solder Dewetting

      Solder dewetting is a surface phenomenon that occurs due to the reduction of surface energy between the solder and the substrate. It is a process where the solder material loses its wetting ability and adhesion to the underlying substrate. Then, these solders will form a "drop shape", which can compromise the mechanical stability and electrical connectivity of the solder joint.

Causes & Solutions

(1) Oxidation of the PCB surface: When the copper pad is exposed to air, it can form a layer of oxide that inhibits the wetting action of the solder.

Solution: Use an appropriate flux to help remove any oxides on the surface of the metals and promotes the formation of strong bonds between the solder and the metals.

(2) Surface contamination: Contaminants like oil, grease, and dust can lead to poor wetting and dewetting. When the surface is contaminated, the surface tension will be uneven, and some tin will fall off because their fluidity is greater than the cohesion, resulting in an uneven surface.

Solution: Use a suitable cleaning solvent and a lint-free cloth to clean the PCB and components before soldering.

3. Solder Balling

Solder balling is one of the most common defects that can occur on the surface of PCB component.

This is a process that often occurs with reflow or wave soldering. Small spheres of solder will appear around the pad and component.
        A few reasons why solder balls occur include:

          ·Excess humidity in the construction environment

          ·Dampness or moisture on the PCB

          ·Too much flux in the solder paste

Causes & Solutions

(1) Insufficient PCB preheating: increase preheating time so it is closer to the oven’s temperature. This will lessen the degree of change once heating begins inside, allowing the solder to melt evenly without overheating.

(2) High environment humidity: the use of air knife for operation can not only help preheat, but also prevent the fixture from bringing water back, which can effectively prevent the production of tin balls.

(3) PCB or tool moisture: Pay attention at all times to any wet components or tools in the process. Before production, check over each board for signs of dampness, and dry them off with anti-static cloths. Or blow dry the tool before use.

4. Cold Joints

      Where a solder joint has been formed at a lower than optimal soldering temperature, it can lead to a dull, lumpy or uneven joint.

      These solder joints have not formed properly as the solder has insufficiently melted. Therefore, the solder joint becomes at risk of both cracking or failing completely.

Causes & Solution

(1) Soldering iron tip or heat gun too cold: Make sure that the soldering iron tip or heat gun is hot enough. When using a soldering or rework station, set the temperature slightly above the solder’s melting point and give it enough time to reach it.

(2) Soldering tool was removed before achieving a solid bond: Give the soldering tool enough time to let the solder melt and flow over the workpieces properly.

(3) Uneven heat distribution: Always keep the tip clean and tinned before soldering.

(4) Contaminated workpieces or dirty tips: Clean the workpieces to remove any dirt or debris and prevent the solder from sticking correctly. Use flux when necessary to ensure a proper connection.

To remedy this, you can reheat the joint until the solder flows again.

5. Pseudo Soldering

      Pseudo soldering describes solder joints that exhibit poor wetting characteristic despite appearing shiny and properly formed after assembly. The term “pseudo soldering” comes from joints that mimic properly soldered connections visually, but lack reliable adhesion and bonding.

Causes & Solution

(1) Insufficient tin to enter PCB hole: there are impurities in the PCB hole or the ratio between the part hole and the part foot is incorrect. When this problem occurs, it is necessary to check whether the PCB is cleaned and whether there is a difference between the part and the PCB.

(2) Poor wetting through the hole wall: solder non-wetting on pads and component leads or solder mask flowing into the hole. If the PCB pad can not be changed, you should choose other solder with higher performance.

(3) Inactive flux: the flux is lost activity due to excessive heat, temperature or heating time should be appropriately reduced.

6. Excess Solder

Excess Solder is when the solder joint is covered with too much tin to determine whether it is a standard solder joint.

Causes & Solutions

(1) Incorrect tinning depth: tinning time or immersion time is too long, which need to be reduced.

(2) Flux mismatch: flux activity does not match the actual demand, and a more suitable flux should be replaced.

7. Solder Bridging

      Bridging refers to the solder connecting between two solder joints, and the solder joint to be connected to the other solder joint, which will cause either short circuiting or burning out of PCB.

Causes & Solutions

(1) Excessive soldering speed: The fast soldering speed makes it easy to tin the previous wet solder joint to the next joint. You should reduce the soldering speed appropriately.

(2) Contaminants on PCB pad or component feet: contaminants will guide the direction of the tin flow to cause bridging. It is advisable that clean the PCB board or components before soldering. 

(3) Too much solder being used: Too much tin will cause the gap between the two solder joints to become smaller, so it is easy to bridge, and the amount of tin should be reduced.

But what about the Bridges that have already occurred. When found, solder bridges will require a solder wick to remove the unwanted solder and drag it away.

      For more information, please keep in touch with RS. We are your reliable & stable assistant!