Solder joints are one of, if not the most crucial factor in whether or not your design and PCB functions properly. A large chunk of issues during assembly and testing can stem from solder joints and the methods used during this step of manufacturing. Soldering is the process in which two or more items are joined together by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. This allows your voltages and signals to pass through the different resistors, capacitors, ICs, and other components that you have designed on your PCB.
Common Ways to Make Solder Joints
Wave soldering, reflow soldering, and hand soldering are the most common forms of creating solder joints. During wave soldering, parts are inserted into plated through holes, sometimes trimmed and prepped beforehand. Then the stuffed PCB travels on a rail system over flowing solder in a wave solder machine. In this machine there is a wave of molten solder that the leads and bottom of board touches. This results in solder on the pins and pads, but not on the PCB itself.
Reflow soldering is mostly for SMT components. This is a process that utilizes solder paste which is a mixture of prealloyed solder powder and a flux-vehicle that has a toothpaste like viscosity. A stencil and squeegee system is used to apply the past only to the pads. Components are then placed onto the solder pads with a pick and place machine. The paste ensures that the components stick onto the board. After that the board is heated by running through a reflow oven in order to melt and create the solder joints.
Standards for Solder Joints
The industry started with the use of leaded solder but many companies and entities are making the shift to lead-free solder due to environmental concerns. Lead-free solder melts at a higher temperature, and has introduced a variety of challenges as compared to its leaded counterpart. Vinatronic has put together this article as a solder joint acceptability cheat sheet for managers, designers, engineers, and hobbyists to recognize common defects and allow them troubleshoot their PCB assembly’s solder joints.
IPC-610: Acceptability of Electronic Assemblies, is the gold standard for quality. If you don’t have a copy, we would highly recommend procuring one. This document gives criteria for soldering and often has examples of defects across their different classes of acceptability. The team over at IPC continually updates this standard and will only improve as time goes on.
Common Lead-Free Solder Joint Defects
- Cold Solder
- Bridging
- Solder skip
- Insufficient or excessive solder coverage
- Overheated solder joint
- Component Damage
- Tombstoning
- Solder balls
- Fillet lifting in through holes
- Popcorning/cracking
- Surface corrosion and copper dendrites
- Tin whiskers
- Pad lifting
- Solder Paste Not Reflowed
- PCB Delamination
- Outgassing and Blowholes
Cold Solder Joints
You can identify cold solder by its dull and ununiform appearance. Generally it is caused by insufficient heat being used on the solder joint to completely melt it. Either the soldering iron or the joint itself was not heated up sufficiently. Be sure to check the soldering iron’s temperature and make sure it is set high enough to melt that particular solder type (lead-free solder has a higher melting temperature). Larger boards may also need more time in the reflow oven in order to get to a uniform temperature for SMT parts. In the wave solder machine, this could be an indicator of improper heat, angle, wave height, or duration as well. Sometimes colder solder joints still result in a functional board but it definitely poses a reliability concern. After extended use, these joints may fail.
Bridging
When solder across two pads joins together it is called bridging. This creates a short in the circuit. This is usually caused by issues in the stencil printing process or component placement. If excess solder is applied, once the component is placed on the board it may squish out and connect the two areas.
Solder Joint Skips
When a part doesn’t get solder it is called a skip. This may be due to improper solder paste application or could be from the through hole joint not running through the wave properly (wrong wave height, angle, speed). Could also be due to insufficient flux.
Insufficient or Excessive Solder Coverage
Some new hobbyists/engineers apply too much solder to overcompensate and it results in a bulbous amount of solder on a pad or pin. This is a reliability concern and should be looked out for.
On the flip side, not enough solder may result in intermittent function or improper function of a board. Be sure a through hole pin has sufficient 360 degrees of solder and that SMT parts have enough solder to make contact with the pad.
Overheated Solder Joint
Applying to much heat may burn the component, the pad, or other elements. Look out for discoloration and make sure temperatures are set properly in order to prevent damage.
Component Damage
Too much heat may melt elements of a component and make them not function or even dangerous. Passing through the reflow oven can be a taxing on Moisture Sensitive Devices (MSD), unless they are packaged properly to prevent them from mechanical damage from handling or cracking. MSDs that are exposed to high levels of humidity may lead to cracking of the component body or pop corning during reflow.
Tombstoning
This is when parts get lifted up at an angle. Could be an issue with part placement in SMT boards or wave height in through hole.
Solder Balls
Generally this is when excessive solder turns into a ball during reflow. Solder balls reduces the insulation separation distance below the required minimum. They can be removed with a high-pressured airgun but may require a follow-up visual inspection with optical or x-ray.
Fillet Lifting in Through Holes
Lead-free soldering in particular, sometimes results in fillets lifting in THT leads. Sometimes, along with the fillet, the pad may pull away from the laminate as well. Although in most cases, this failure does not lead to a solder joint defect or an electrical failure. The reason for this type of concern is related to the expansion and contraction the PCB undergoes during soldering and the solidification of lead-free materials. IPC 610D inspection criteria covers this process issue.
Popcorning/Cracking of Components
Incorrect specification or use of the component can result in popcorning or cracking during solder reflow. This is usually when a leaded components is used for lead free processing. This could also be from not following maximum soldering temperature and duration suggested by the manufacturer of the component.
Surface Corrosion and Copper Dendrites
Excess flux and surface cleanliness are important in getting good solder joints. Contaminants on the PCB’s surface can lead to corrosion.
Formation of copper dendrites can result in intermittent failures in PCBAs. In lead-free soldering, VOC free fluxes may be used which result in more problems. The root cause is the flux residue remaining on the board surface, which forms a conductive path through the moisture layer when the board is subject to high temperature and humidity.
Tin Whiskers
Tin whiskers are a complex issue that could have an entire page dedicated to it but for simplicity’s sake, the tin coating on the surface of a circuit board can often lead to growth of tin whiskers that short neighboring tracks and pads. The whiskers mainly occur due to stress formed during the plating process.
Pad Lifting
High temperatures during the soldering process can cause PCB materials to expand and deform. Could even place strain on the pad. This could cause the pad lifting off the surface of the board or the fillet lifting from the pad. The fillet may also tear in the process. This contraction and expansion in the laminate and the solder are the main cause for pad lifting.
Solder Paste Not Reflowed
Low solder profile temperatures can result in non-reflow or incomplete reflow of solder paste. Insufficient temperatures can cause non-wetting of the pad or component leads. This can happen when a component termination point fails to reach peak reflow temperature for an adequate duration time. The way boards are populated and their sizes may result in different areas heating up differently. Although it takes only a few seconds to form a reliable solder joint, the components forming the joint do have to remain at temperatures above melting point for reliable assembly.
PCB Delamination
Delamination of PCBs is visualized as a blister or air gap in the board. This may be formed when water vapor from the circuit board material expands and escapes when it is heated up in the solder process. Lead-free soldering requires higher temps and more energy to be exerted, and delamination of the PCB materials may occur when moisture in the board expands during the reflow process. This could be fixed by baking boards before assembly.
Outgassing and Blowholes
Similar to delamination, outgassing and blow holes may be a process indicator that your bare boards have moisture in them. This small amounts of liquids absorbed into a board expand during soldering and outgas from the plated through holes. At these high temperatures, the liquid state goes into the gas state and releases. Since solder on the outside hardens first, holes/voids may be seen on the base of the termination. That escaped gas causes pinholes, blowholes, or voids, with size correlating to the volume of the released gas.
Conclusion
Ensuring quality solder joints will allow your design and your product to be successful. Although this list doesn’t cover everything, it is a good place to start in diagnosing what is wrong with your board or as general knowledge when receiving completed assemblies. The team at Vinatronic has over 25 years of experience soldering and inspecting solder joints. If you have any further questions or a project you’d like to get built up our team would love to help!
