A backbone of electronics manufacturing, soldering is the process to connect two types of metal by melting solder. Using a hot iron, a metal, usually tin or lead, is heated to its melting point and attached to leads or pads to connect components and printed circuit boards. This article addresses common issues in the soldering process to be watched for.
Soldering Issues
Solder Starved
A solder staved joint is a joint that doesn’t have enough solder to make a proper electrical connection. Even if there is a connection, over time, a weak joint and cause stress cracks and fail over time.
Solder Skips
Solder skips happen when a solder joint is not wetted with solder. Visually, there is a clear dividing line between the solder and the lead of the component or copper foil, resulting in an open circuit. The open circuit can prevent your electronic from functioning properly.
The cause of a solder skip is generally the result of a mistake in design or during the PCB printing. An uneven pad size or the wave could have been set to the wrong height can cause solder skipping.
Too Much Solder
Having too much solder over a connection is common issue is soldering. Too much solder can cause electrical shorting but can also hide mistakes in the soldering process. When a pin and pad are completely covered by a blob of solder, establishing a proper connection can be difficult to ensure. The pin or the pad can be without the proper wetting to create the electrical connection. Below are some examples of excess solder and the problems they can create.
Solder Bridge
A solder bridge is two points on a PCB are inadvertently connected by excess solder. This problem can be found when dealing with small components, with the bridge being at times undetectable visually. When a bridge is formed, an electrical short circuit can damage the component or entire board.
Generally, a bridge is formed for the following reasons:
-The excessive application of solder between joints
-Using two large of solder tips
-The solder tip was withdrawn at a bad angle
To prevent this from happening, one can use the correct lead length for through hole placement. Additionally, practicing proper soldering techniques will also help prevent the excess solder.
To remove extra solder, heat the solder up so that it liquifies. The excess solder should then be able to be drawn from the problematic area.
Solder Balls
Solder balls usually form during wave or reflow soldering. The solder ball appears as a small sphere consisting of solder attached to the laminate, resist, or conductor surface. A solder ball forms mostly for two reasons:
-Moisture near the through holes becomes steam due to the high temperatures within the oven. As the solder is heated, if there is solder blocking the hole wall, the solder will become trapped and bubble out, forming a ball of solder when cooled.
-The other issue is solder balls can form on the backside of the board. This would be the side that contacts the wave crest. This is generally caused by improper settings for the wave machine. As the board is heated and moisture is evaporated, the solder will splash out from the tin bath, creating solder balls on the printed circuit board.
Insufficient Wetting
Insufficient wetting is when there isn’t enough solder on a pin, pad, or board so that the components and board will not attach to one another. This can be caused by a few things, generally, the pin, pad, or both not having enough solder melt to where it needs to be. Three examples of insufficient wetting are:
–Pad: When the leads are wetted but the pad is not. This can cause the leads to separate from the pads as there is not enough solder to make the connection. This can be caused by a dirty PCB or by heat not being applied to both the pad and pins.
–Pin: This happens when the pin and pad are not wetted enough. This is caused by the pins and pad not being given enough heat for the solder to flow. Heating the pins and pad evenly will prevent this from being an issue.
–Surface Mount: When the pin is heated exclusively from the pad, the solder won’t flow onto the solder pad. To prevent this, heat the pad first to ensure the solder will flow.
Stray Solder Spatters/Webbing
This happens when extra solder is left over on the board. The solder remnants are held to the board with sticky flux residue. An issue can arise if the solder works itself free, causing short circuiting on the board. Cleaning them off with tweezers or a knife can prevent the spatters from causing electrical issues for your product.
Process
Cold Joint
When an insufficient amount of heat is transferred, preventing the solder from melting. This can be caused by a few things:
The soldering iron or joint was given the proper time to heat
-The iron temperature is set too low to melt the specific type of solder
-A design flaw in the pads or traces
Cold joints can be difficult to find during production, usually noticed once the product is in use. This can cause serious issues with the products as the cold joint has a weak conductive strength.
Overheated Joint
Overheating the joints can cause to much moisture to be cooked out of the solder. The joints will appear white with a rough surface and no metallic luster. This can be caused by:
-The soldering iron being set too high
-Solder failing to flow due to the PCB already having a layer of oxide
Overheating can cause damage to the PCB. Pads can be lifted, preventing the board from operating.
Untrimmed Leads
Untrimmed leads are leads that extend past an appropriate length, potentially causing damage as they overreach into another joint. The joints connecting or bridging can cause damage to the board. A lead that is too long is also in jeopardy of being bent and creating an unwanted contact. It is best to trim the lead right above the solder joint as to have a uniform height for the solder joints.
Pin Holes and Blow Holes
A pin hole and blow hole are what happens when moisture within the boards is heated into a gas during the soldering process. As the gas is released through the soft solder, it continues releasing until the solder has cooled, leaving a hole in the joint. The hole can be small or big, hence the names pin for small and blow for large holes.
The circuit will conduct temporarily but the hole can cause conductivity to lessen over time. One thing that can be done to prevent this is to preheat the boards to remove moisture, as well as using a minimum of 25um in copper plating thickness for the through holes.
Placement/Components
Disturbed Joint
A disturbed joint is a joint that has been shifted as the solder solidifies. The solder can appear frosted, crystalline, or rough in appearance. A disturbed joint can look a lot like a cold joint. The way to fix a disturbed joint is to reheat the component and let it cool, free of movement.
Tombstoning
Tombstoning is when the component, usually a capacitor or resistor, is lifted off the board on one side. This happens when one side of the pad isn’t wet, causing the component to lean. This can happen because traces have inconsistent thickness, or a lack of thermal relief factored into the design. The components can also be pushed by the wave machine if the height setting is too low.
Lifted Pad
A lifted pad is when the pad becomes detached from the surface of the PCB. Generally, the lifted pads are common on boards with thin copper layers or where there is no through-plating on the holes. It can also happen when the joint is overworked, a symptom of desoldering or repairing a component, causing the adhesive bond between the joint and copper to weaken.
If in need of help with circuit board printing or other electrical assemblies, reach out to us at Intellitec’s electronics manufacturing website to see how we can help bring your product from design to reality.