When it comes to printed circuit board assembly and SMT equipment, the question isn't if you will have quality issues from time to time, but when. The key to keeping the "when" to a minimum is to start with good equipment, have a reliable source of advice on hand, and know where to begin when these issues pop up.
At Capital Equipment Exchange, we've put together information to help you spot and troubleshoot some of the most common causes of SMT problems or PCB defects.
Common Causes Of SMT Defects
- Solder Balls & Solder Beads
- Bridging & Short Circuits
- Open Or Insufficient Bonds
- Tombstoning - Standing Components
- Hot Slump Or Cold Slump
- Dewetting - Bad Attachments
- Orange Skinning
Solder Balls & Solder Beads
Solder balls are pretty easy to spot. You have solder balls if you see lots of tiny balls just outside the flux residue areas. If these balls are appearing alongside your PCB components, you have beading.
Preventing Solder Balls & Solder Beads
Start with your stencils. Solder paste can get smeared underneath your stencil. Stencil cleaner may be getting left behind. You may have a stencil thickness or squeegee pressure problem. Stencil alignment also plays a role in keeping your boards ball and bead free.
Bridging & Short Circuits
Sometimes you'll start to have solder paste running between component contacts. If left unchecked, you have a quick recipe for short circuits in progress. A batch of short-circuiting PCBs should be checked for bridging problems.
How To Avoid Bridging In PCBs
Like troubleshooting for balls and beads, you'll want to ensure your squeegee pressure, stencil thickness, and alignment aren't off. If everything looks good there, pay attention to your paste viscosity, screen print speeds and temperatures, and reflow ramp-up times.
Open Circuits & Insufficient Bonds
Sometimes no solder, or not enough solder, will be placed. This can lead to an open circuit or insufficient solder on the joint between a component lead and pad.
How To Avoid Open PCB Circuits
These solutions are going to start sounding familiar. It makes sense if you think about how many issues result from improper solder application.
- Reduce Squeegee Pressure & Speed
- Use A Harder Type Of Squeegee
- Clean Your Printing Stencil
- Check Solder Paste Viscosity
Tombstoning - Standing Components
If you are starting to notice specific components standing upright on one end, you have yourself a case of PCB tombstoning. Usually, you'll have passive components, like resistors or capacitors lifting from the board, creating an open circuit, and ruining your PCBs.
Smaller components, higher temperatures, and lead-free solders can contribute to tombstoning problems.
Preventing PCB Tombstoning
When you start troubleshooting a component tombstoning problem, you'll want to start with the pad. Is your component pad the right size?
- If you're working with smaller components, consider subbing OSP, immersion tin, or immersion gold for a more traditional HASL finish.
- Ensure all traces connecting your pads are equal in width.
- Check the inner layers of your PCBs for even copper placement.
- Check your solder mask thicknesses.
Hot Slump Or Cold Slump
You set up your equipment to place solder paste in a specific area and expect it to stay there. Unfortunately, your paste will tend to spread or slump. This can lead to bridging and short circuits. So, whether you're dealing with hot slump or cold slump, you'll want to nip this issue in the bud ASAP.
While hot slump is most likely happening as your paste gets warm during preheating, cold slump can happen for various reasons: temperature, paste viscosity, drying times, and the height of the deposit.
How To Reduce Cold Slump
- Switch to a higher viscosity solder paste.
- Make sure no solvents are left after screen cleaning.
- Reduce squeegee and dispensing syringe pressures.
- Check screen printer temperature.
How To Reduce Hot Slump
- Shorten your preheater ramp-up rate.
Dewetting - Bad Attachments
With dewetted solder joints, you will notice good component lead and pad contact but no bonding between the solder and pad. Your first clue that dewetting is occurring might be intermittent connections. Dewetting is usually caused by PCB contamination.
- Minimize Contamination - Hands off! Keep handling to a minimum.
- Clean Your Boards - Besides fingerprints, manufacturing residues and excess flux can cause contamination.
- Repair Where Necessary - Don't be afraid to rework where necessary if you start to notice dewetted joints.
Solder Joint Orange Skinning
If orange skinning is your problem, you will start to notice the texture of your solder joints looking dull, rough, and well, like an orange peel. PCB orange skinning issues are usually temperature-related.
How To Avoid Orange Skinning On Your PCBs
- Lower Your Peak Zone Temperatures
- Shorten Times Between Activation & Reflow
- Lower Preaheating Temperatures
Replace Or Repair Your SMT Machines
If, as you troubleshoot, you realize your PCB machines are the problem, buying new isn't your only option. Save time and money with used SMT equipment from Capital Equipment Exchange. We not only stick around after your purchase for any issues that arise, but we can help you find spare parts to extend the life of your existing machines as well. Contact us today to get the latest on our inventory or find out more about our streamlined purchasing process.