Soldering is a critical process used to mechanically and electrically join components in electronic assemblies. Insufficient solder can lead to poor quality solder joints that are unreliable both electrically and mechanically. There are several potential causes that can result in insufficient solder during the soldering process:
Poor Solderability of Parts
The solderability refers to how well the parts to be soldered can wet and adhere to solder. Following factors affect solderability:
Oxidation or Contamination
- Metal surfaces like component leads and PCB pads get oxidized over time
- This makes solder wetting difficult resulting in poor joints
- Oil, grease or other residues also reduce solderability
- Certain metals like stainless steel and aluminum do not solder well
- Lead-free solders have worse solderability than leaded solders
- Mismatch between surfaces and solder alloy reduces wetting
- Components stored for long periods have degraded solderability
- Moisture absorption also reduces solderability of parts
Lack of Solder Coating
- Components without pre-applied solder coating have poorer solderability
- Coatings like hot solder dip, ENIG, immersion tin improve wetting
- Overheating parts during soldering destroys solderability
- Burnt or charred surfaces will not accept solder properly
Proper handling, storage and pretinning along with protective coatings is needed to ensure good solderability of parts.
Low Soldering Temperature
Sufficient temperature is essential for proper melting and wetting of solder. Insufficient temperature causes:
- Solder joint is formed before flux can properly activate and remove surface oxides
- Solder does not become fully molten and evenly flow over the joint surfaces
- Partial wetting occurs resulting in weak joints with cold solder spots
Common reasons for low soldering temperature:
- Soldering iron tip is not properly tinned or maintained
- Low iron power rating or large tip being used
- Poor contact between iron tip and joint being soldered
- Excessive heat sinking in large ground planes and leads
- Fast motion of iron tip does not allow time for heating
Using adequate tip size, power rating, contact time/motion and re-tinning maintenance helps avoid low temperatures.
Problems with Solder Paste Deposition
For reflow soldering, insufficient solder paste deposit will lead to poor joints. Some contributing factors are:
- Excessive gap between component and PCB pad
- Misalignment of paste deposits with pads
- Too little paste dispensed due to volume setting issues
- Clogging or leaks in solder paste stencil
- Outgassing and drying of the paste deposit before reflow
- Slumping of paste due to high ambient temperatures
Good process controls, stencil cleaning, monitoring of paste deposits and proper storage helps avoid these issues.
Defects in PCB and Component
PCB and component defects that absorb solder and restrict flow result in insufficient solder:
- Voids in ground or thermal planes acting as heat sinks
- Poor pad design with insufficient wetting area
- Contamination like oils and residues on pads
- Cross-talk barriers blocking flow between leads
- Tight lead spacing preventing access to solder
- Warped leads or gaps between lead and PCB pad
Inspecting PCBs and components and checking pad dimensions ensures such issues are avoided.
Flux removes surface oxides enabling solder flow and wetting. Following flux related reasons reduce soldering effectiveness:
- Too little flux applied to joint
- Flux drying out before completing soldering
- Weak or water-soluble flux that is too mild
- Low activity of aged flux reducing cleaning capability
- Baked on or burnt flux residues interfering with wetting
Adequate amount of appropriate rosin-based flux should be applied to maintain solderability.
Problems with Solder Wire
Issues with solder wire composition and condition also affect soldering:
- Impurities and voids in solder wire reducing fluidity -Insufficient wire diameter to thermal mass of joint
- Oxidation or contamination of solder wire surface
- Mismatch between alloy melting point and process temperature
- Low tin-lead percentage of alloy increasing melting point
Proper solder wire handling and selection compatible with process requirements avoids these problems.
Other Process Issues
- Excessive heat sinking due to large ground planes
- Jigging misalignment resulting in loss of contact between tipped iron and joint
- Soldering for too short a duration to allow adequate heating
- Vibration or movement disturbing solder bead formation
- Poor fume extraction exposing joints to corrosive flux residues
Control and monitoring of process parameters is needed to counteract these effects.
Troubleshooting Insufficient Solder
- Visually inspect joint closely under magnification to identify poor wetting, cold spots etc.
- Use solderability testing chemicals like rosin that react when applied to oxidized/contaminated areas
- Thermally profile temperatures at joint during soldering to check if adequate temperature is reached
- Review process parameters like heat application duration, wire gauge, tip size etc.
- Evaluate PCB design – thermal planes, pad dimensions, spacing etc.
- Test flux activity and assess paste condition
- Check for issues with solder bath contamination or dross buildup if wave soldering
Preventing Insufficient Solder
- Use proper storage and handling of components to maintain solderability
- Apply solderability preservatives like benzotriazole on surfaces
- Ensure PCB and component cleanliness before soldering
- Select the right solder alloy matched to process temperature
- Use adequate flux and apply uniformly to joints
- Clean and tin soldering iron tips regularly
- Optimize soldering temperature, duration and motion
- Inspect stencil condition and paste deposits
- Ensure adequate fillet wicking over joint
- Monitor the soldering process continuously and make adjustments as needed
With proper analysis of root causes and preventive steps, issues due to insufficient solder can be eliminated resulting in reliable, high quality solder joints.
Q1. How can I identify if insufficient solder is causing poor quality joints?
Look closely under magnification for joints with dull finish, grainy structure, dark spots, non-wetting and dewetting of surfaces indicating cold solder. Probe joints for continuity issues signalling poor bonding.
Q2. What is the ideal temperature for hand soldering with lead-tin alloy?
For Sn60Pb40 solder, ideal tip temperature is around 370℃ to 400℃. Higher temperatures above 450℃ should be avoided to prevent damage to components.
Q3. How does excess flux cause insufficient solder problems?
Too much flux can actually impede solder flow rather than helping it. It also leads to charring which deposits residues that hinder wetting. A thin uniform layer of flux should be applied.
Q4. Can inadequate solder volume be a reason for insufficient solder defects?
Yes, using too little solder wire compared to the thermal mass of the joint can lead to insufficient solder. Larger wire diameter or longer application time is required.
Q5. What is the effect of oxidation on solderability?
Metal oxide formation on surfaces interferes with solder wetting by creating a barrier layer. Flux helps remove oxides but preventing oxidation via protective coatings or oxidation inhibitors also improves solderability.