Introduction
Soldering is a critical process used to attach electronic components to printed circuit boards (PCBs) and form reliable electrical and mechanical joints. The two most common soldering methods are reflow soldering surface-mount devices (SMDs) and wave soldering through-hole dual in-line package (DIP) components. This article examines the key differences between soldering SMD and DIP components in terms of processes, equipment, challenges, and applications.
SMD Components Overview
Surface mount devices (SMDs) are electronic components designed with terminations flush on their housing to attach directly to pads on the PCB surface:
- Small plastic, ceramic or metal component package
- Flat metal terminations on underside
- Hundreds of standard shapes and sizes
- Examples: resistors, capacitors, ICs, QFPs, BGAs
SMDs allow high component density and miniaturization.
DIP Components Overview
Dual in-line packages (DIPs) house integrated circuits with long metal pins that insert into holes drilled in a PCB:
- Plastic or ceramic housing with two rows of pins
- Pins allow through-hole mounting
- Breadboarding friendly but larger size
- Includes ICs, sockets, connectors, switches, etc.
DIPs allow simple prototyping and board assembly but take up more space.
SMD Soldering Methods
Reflow soldering is the primary technique to solder surface mount components:
- Solder particles suspended in flux medium
- Dispensed or printed onto pads
- Machine precisely places components on pads
Reflow
- Heat melts solder paste to attach components
- Typically uses infrared, vapor phase, or convection
Inspection
- Automated optical inspection after reflow
This achieves accurate, high speed, repeatable SMD soldering.
DIP Soldering Methods
DIP components are soldered by:
Through-Hole Board
- PCB with plated through-holes for DIP pins
Wave Soldering
- Bottom side passes over molten solder wave
- Solder wicks into pin holes to attach
Manual Soldering
- Individual joints hand soldered with iron
- Low volume prototyping assemble
Inspection
- Visual inspection of joints afterwards
Wave soldering achieves high volume production.
Comparing Reflow Versus Wave Soldering
| Metric | Reflow Soldering | Wave Soldering |
|---|---|---|
| Board Side | Top side | Bottom side |
| Components | SMDs | DIPs |
| Process | Pick-and-place <br>followed by reflow | Wave solder after<br>through-hole assembly |
| Automation | Highly automated | Moderately automated |
| Speed | Very fast | Relatively fast |
| Volume | Medium to very high | Medium to high |
| Rework | More challenging | Simpler |
Table 1: Comparison of SMD reflow versus DIP wave soldering attributes
SMD Soldering Challenges
Some issues encountered soldering SMDs:
- Tombstoning – Chip standing vertically if one pad not wetted
- Splashing – Solder balling up on pads
- Voids – Trapped bubbles in solidified joint
- Bridging – Solder connecting adjacent pads
- Solder Beads – Excessive solder buildup around joints
- Warping – Board warpage after reflow
Preheating, thermal balancing, paste deposition control, and inspection help avoid these defects.
DIP Soldering Challenges
Some common DIP soldering issues:
- Cold Solder Joints – Weak joint from insufficient heat
- Disturbed Joints – Solder bumps or movement while cooling
- Icicles – Spikes of solder from dragged pads
- Bridging – Solder bridging gap between pins
- Flux Residues – Failed cleanup leaving residue
- Pin Misalignment – Bent or skewed pins
Adjustments to flux chemistry, temperature, conveyor speed, and cleaning address these.
X-Ray Inspection
X-ray imaging provides internal inspection of solder joint quality for both SMD and DIP:
- Verifies proper wetting and fillet shape
- Finds hidden defects like voids or cracks
- Checks for poor pin through-hole fill
- Ensures no bridging under component
This revealing view validates assembly integrity.
Shear and Pull Testing
Mechanical shear and pull testing determines solder joint strength:
Shear Testing
- Measures force required to horizontally shear the joint
- Checks Coplanarity and pad adhesion
Pull Testing
- Pulls component vertically from board
- Assesses adequacy of through-hole pin soldering
Statistics identify process issues.
Thermal Cycling
Repeated thermal cycling evaluates joint integrity:
- Subjects board to temperature extremes
- Cycles between high and low extremes like -40°C to 100°C
- Monitors electrical continuity during cycling
- Checks for cracked joints due to expansion mismatch
This accelerates fatigue testing.
Typical Defect Limits
IPC-A-610 sets soldering defect limits:
| Defect | Target | Acceptable |
|---|---|---|
| Solder Voids | 0% | <25% |
| Solder Balls | 0 | <3 per board |
| Bridging | 0 | <2 bridged |
| Cold Solder | 0 | <3 cold |
| Disturbed Joint | 0 | <10% disturbed |
Table 2: Example solder joint defect limits per IPC-A-610
SMD Rework and Repair
Common SMD rework methods:
- Soldering Iron – Directly reflow joint
- Hot Gas/Air – Nozzle targeted heat
- Infrared – IR lamp spot heating
- Solder Paste – Reprint paste and reflow
Rework systems automate heating profiles.
Figure 6: SMD rework using hot air soldering station
Skilled technicians required not to damage boards or components.
DIP Rework and Repair
Typical DIP rework techniques:
- Solder Sucker – Vacuum desoldering tool
- Solder Wick – Braided copper wicks up solder
- Hot Air – Simultaneous airflow heating
- Manual Iron – Direct soldering iron heat
Simply reheating and reinserting DIP pins often suffices to repair joints.
Lead-Free Soldering
Lead-free solder alloys like SAC305 or Sn/Ag present challenges:
- Higher melting point requires higher heat
- Increased likelihood of thermal damage
- Tombstoning more common with SMDs
- More difficult wetting on pads
- Opens possibility of silver electromigration
But lead-free is mandated for environmental safety.
Applications
SMD Soldering
- Consumer electronics
- Computers and servers
- Telecommunications
- Automotive electronics
- Aerospace and military
DIP Soldering
- Legacy equipment
- Prototyping
- Small scale production
- Academic labs
SMD dominates most new applications requiring miniaturization.
Conclusion
In summary, soldering is critical for attaching and interconnecting electronic components to circuit boards. Both SMD reflow and DIP wave soldering satisfy this for different applications. With SMD soldering enabling small, high density PCBs and DIP soldering suitable for prototyping and small scale production, manufacturers can utilize the best approach based on their requirements. Careful process control and inspection is key to attaining high solder joint quality and minimizing defects. As electronics assembly continues diversifying, competent soldering and rework skills remain essential foundations.
FAQs
Q: What are the main advantages of using SMD components versus DIPs?
A: SMDs allow great miniaturization and component density due to their small size. They suit automated assembly for high volume production.
Q: Are soldered SMD connections less mechanically robust than through-hole DIP?
A: Properly made solder joints for both SMD pads and DIP pins can be very reliable. But DIP does withstand more vibration and thermal cycling stress.
Q: What causes tombstoning defects when soldering SMDs?
A: Uneven heating of pads lifts one side of the chip. Using appropriate pad size, paste volume, preheating, and thermal mass balances minimize this.
Q: Can DIPs be soldered using reflow like SMDs instead of wave soldering?
A: It is possible to glue DIPs first and reflow solder but requires careful heat control. Wave soldering is a faster and more reliable process.
Q: What is the primary advantage of using hot air tools for SMD rework versus soldering irons?
A: Hot air provides uniform area heating that avoids localized overheating damage. It suits reworking small fine-pitch ICs.
