Introduction to PCB Soldering
Printed circuit board (PCB) soldering involves attaching electronic components to a PCB using molten metal solder. Proper PCB Design is critical for successful soldering and overall circuit board functionality. Several key factors must be considered when designing a PCB layout intended for soldering.
Common PCB Soldering Issues
Several common problems can arise during the PCB soldering process if the board is not properly designed:
Insufficient Pad Size
If the copper pads for component leads are too small, it can be difficult to get sufficient solder fill. This results in a weak solder joint. Pads should be sized appropriately for the component lead – not too large or too small.
Incorrect Hole Size
The holes drilled in the PCB for through-hole component leads must be the correct diameter. If holes are too small, component leads won’t fit. If holes are too large, solder can wick down into the hole, resulting in insufficient solder above for a good top-side fillet.
Insufficient Thermal Relief
For large copper pour areas like ground/power planes, solder pads should have a thermal relief pattern – thin spokes connecting the pad to the larger copper pour. This limits heat sinking during soldering so the pad can reach sufficient temperature. Without thermal reliefs, large pours can sink too much heat and result in poor solder joints.
Solder Mask Blocking Pads
The solder mask layer, which insulates the copper traces, must have openings for all solder pads. If solder mask overlaps the pads, solder will not adhere. Solder mask openings should be slightly larger than the copper pads.
Silkscreen Over Pads
The silkscreen text layer should not overlap any exposed copper pads. Silkscreen ink can interfere with solder wetting and cause soldering issues. Text should be offset from pads.
Designing PCBs for Optimal Soldering
By following some key PCB layout guidelines, you can ensure your boards are well suited for soldering:
Use Appropriate Pad Sizes
Pad size should be optimized based on component lead size. A good general guideline is that the pad diameter should be about twice the lead diameter or width. This allows for sufficient area for a strong solder fillet to form.
Some common pad sizes for standard components:
| Component Lead | Pad Diameter |
|---|---|
| 0.5 mm | 1.0 mm |
| 0.65 mm | 1.3 mm |
| 0.8 mm | 1.6 mm |
| 1.0 mm | 2.0 mm |
Optimize Hole Sizes
The hole diameter for a through-hole pad should be slightly larger than the component lead diameter, allowing the lead to easily insert. A typical minimum hole size is 0.2mm larger than the nominal lead diameter.
Some common hole sizes:
| Lead Dia. | Hole Dia. |
|---|---|
| 0.5 mm | 0.7 mm |
| 0.65 mm | 0.85 mm |
| 0.8 mm | 1.0 mm |
| 1.0 mm | 1.2 mm |
Provide Thermal Relief
Add thermal relief spoke connections to pads connected to large copper pours like ground and power planes. The thermal reliefs limit heat sinking and allow the pad to reach sufficient temperature for soldering.
A typical thermal relief uses four 0.25mm wide spokes connecting the pad to the larger copper area. Properly implemented thermal reliefs can greatly improve soldering results for dense PCB designs with heat sinking planes.
Ensure Proper Solder Mask Clearance
The solder mask layer must have openings larger than the copper pads to ensure no solder mask overlaps onto the pads. This solder mask clearance is typically 0.05mm – meaning the solder mask opening is 0.1mm larger in diameter than the copper pad.
Solder mask clearance rules in PCB design software help automate solder mask expansion so designers don’t have to manually size each opening. Insufficient solder mask clearance is a common error that can completely prevent soldering.
Keep Silkscreen Clear of Pads
The silkscreen text and graphics on a PCB should not overlap any exposed metal pads. Even if the silkscreen is not conductive, it can still interfere with soldering. A 0.2mm silkscreen offset from pads is typical – larger is better for easier soldering.
PCB design rule checks (DRC) help catch silkscreen over pad errors. Fixing these errors before manufacturing is important for soldering.
Inspecting and Testing Soldered PCBs
After PCB soldering, inspection and testing is important to verify manufacturing quality. Several inspection methods are useful for evaluating solder joints.
Visual Inspection
Careful visual examination of solder joints with the naked eye or minor magnification can catch many soldering issues. Look for:
- Insufficient solder fill or voids
- Solder bridging or splatter
- Cold or dull looking solder
- Misaligned components
- Physical damage
Visual inspection is an important first-pass check before in-depth testing. Catching visible errors early saves test time.
Automated Optical Inspection (AOI)
AOI machines use high resolution cameras to quickly scan a PCB and check for manufacturing defects, including soldering problems. AOI compares a PCB to the CAD data to verify correct part placement and solder fill. It can rapidly test 100% of joints and identify errors a human might miss.
AOI is widely used for medium to high volume PCB manufacturing. It does require programming with the PCB design data to be effective.
X-Ray Inspection
For some complex PCBs with fine pitch BGAs or hidden solder joints, x-ray inspection may be needed. X-ray imaging allows seeing solder joints under chips and between board layers that can’t be visually inspected.
X-ray is not typically used for all PCBs but is an important tool for select high-density designs in industries like aerospace, medical, etc. where assurance of solder joint integrity is critical.
Electrical Testing
Of course, the most definitive test of solder joint quality is an electrical test of the final assembled PCB. A bed-of-nails or flying probe tester makes contact with test points across the PCB and does continuity and isolation tests to verify electrical connectivity.
An in-circuit test (ICT) will power on the PCB and do Functional Testing as well. Passing an ICT is a good final validation that all solder joints are good and the board operates as intended.
Conclusion
Proper PCB design is critical for successful soldering during manufacturing. By optimizing pad sizes, drill holes, copper geometry, and soldermask and silkscreen clearances, you can ensure your PCB is an ideal soldering target. Careful post-soldering inspection and testing can then verify the quality of the final PCB Assembly.
With the right PCB soldering design practices and validation methods, you can have high confidence in the manufacturability, reliability and functionality of your circuit boards. Good soldering starts with good PCB design!
FAQ
What is a good pad size for soldering?
A: In general, solderable surface mount pads should have a diameter about twice the width of the component lead. E.g. a 0.5mm lead would have a 1.0mm pad. Larger 1:2 ratios are good for easy hand soldering.
How much bigger should a pad hole be than a lead?
A: Through-holes should have a diameter about 0.2mm larger than the component lead diameter to allow easy insertion. E.g. a 0.8mm lead would have a 1.0mm hole.
What is solder mask and why is clearance important?
A: Solder mask is the polymer coating that covers the PCB, except for the exposed metal pads. Solder mask prevents solder bridging between pads. The solder mask opening must be larger than the pad to ensure it doesn’t overlap onto the pad and interfere with soldering. 0.1mm solder mask clearance is typical.
What are thermal reliefs and why are they needed?
A: Thermal reliefs are thin spoke-like connections between a pad and a larger copper pour like a ground plane. The thin spokes limit heat transfer into the plane, allowing the pad to reach sufficient temperature for a good solder joint. Without thermal reliefs, the plane can sink heat away and cause a cold solder joint.
How can you test for soldering problems on a PCB?
A: A good test process includes:
1. Visual inspection for missing/bad solder joints
2. Automated optical inspection comparing a PCB to the CAD data
3. X-ray inspection for hidden solder joints
4. Electrical continuity and functional tests
A combination of visual checks and electrical validation is important for finding soldering issues.
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