How to Prevent Poor Solder Wetting

Poor solder wetting is often a behind-the-scenes culprit of poor joints and unreliable soldering procedures. However, just because poor soldering is a common problem doesn’t mean it’s inevitable. With some tried-and-true strategies in your toolbox, you can prevent poor solder wetting from occurring and ensure your joints are built to last. This Gold Phoenix PCB Knowledge Center article explains what solder wetting is, why good solder wetting matters, and how to prevent poor solder wetting in PCB soldering.

What Is Solder Wetting?

Soldering involves joining two metals together through a solder alloy. This technique is one of the oldest recorded methods for permanently joining metals. Solder wetting is the part of the process in which metal in the solder bonds with metal on the printed circuit board (PCB) or component. During the wetting process, the solder becomes fluid molten and can adhere properly to the component for an optimal solder joint.

Solder wetting — and subsequent metal bonding — requires a unique environment to properly take place. Wetting requires a copper surface that’s free of contaminants and has reached the proper temperature. Below, we’ll explore exactly why good solder wetting is crucial, and how to prime your operation for success.

Why Good Solder Wetting Is Important

Good solder wetting is vital for proper metal joining. Without it, metals won’t adhere properly and likely won’t pass industry standards for acceptable use, making them essentially defective. Proper solder wetting will lead to well-made solder joints that can stand the test of time.

You can often tell good solder wetting apart by its appearance — it will create a shiny, smooth-looking solder that has clearly reached maximum flow. Poor solder wetting, on the other hand, is often visible. The solder may look grainy, dull and porous, reflecting its poor adhesion to components. This solder will often be unusable in a commercial setting, resulting in wasted time, capital and productivity.

Causes of Poor Solder Wetting

Poor solder wetting can be the result of several different issues. Common culprits include:

• Oxidation on solder surfaces. If a solder tip is left hot without a covering, it can react with oxygen and lead to improper wetting.

• High or low soldering temperatures. If the temperature is too low, your solder won’t reach the proper fluidity level and will not contact adequately with the components. On the other hand, too-high temperatures can cause the flux to burn off rapidly as vapor, impeding proper solder wetting.

• Over-holding the solder tip. Holding a solder tip to the components for an extended period can lead to burned flux and damaged components.

• Insufficient wetting. Dirty circuit boards or failure to apply heat to both the pad and pin can lead to insufficient wetting and lack of bonding.

How to Prevent Poor Solder Wetting

Preventing poor solder wetting requires following a well-rounded checklist to ensure you’re creating the right temperature and environment.

Clean Your Surfaces

Clean your surfaces and components with a general solvent wipe as a quick and effective way to remove grease or any dirt that could impede solder wetting.

Tin Tips With Solder

Adding solder to a hot tip helps to prevent oxidation. Always tin your tips before turning off the iron to keep it ready and primed for proper wetting. You’ll also save yourself the hassle and investment of continually buying replacement tips.

Use High-Activity Solder Paste

High-activity solder pastes generally provide better wetting, particularly if you’re dealing with a complicated surface finish. High-activity solder pastes will be especially helpful to prevent poor wetting during reflow.

Preheat Your Soldering Iron

A cold joint occurs when the solder doesn’t melt completely, causing unreliable adhesion. Prevent this from occurring by preheating your soldering iron properly with the power it needs to achieve an optimal temperature.

If you’re noticing the signs of poor wetting during soldering, don’t worry — you can typically repair joints. Stop soldering, let the joint cool and remove any burnt flux. Let your iron heat up to the proper temperature, reheat your joint and begin again.