In this article we’ll take a broad look over solder paste printing effects, screening can requirements and overcoming any faults you might encounter. During reflow soldering the printed solder paste deposits melt and form solder joints to interconnect the component terminations to the substrate pads. There are a large number of solder paste manufacturers, each with a range of solder pastes that all have subtle nuances in their formulations which in turn require small adjustments to the reflow profile to achieve the desired results.
Surface-Mount Technology (SMT)
This is a method for creating electronic circuits where the components are mounted directly on the printed circuit boards (PCBs). Previously, the electronics industry had used wire leads going into holes to fit components but now it has largely been taken over by SMT. One of the reasons for this is that an SMT component is often smaller than its lead counterpart simply because it’ll either have no leads or shorter leads. Some features include: flat contacts, short pins, terminations on the body of the component (passives), a matrix of balls (BGAs) or short leads in a gull-wing formation (QFPs).
Surface mount stencils are so much more than sheets of metal with apertures copying the PCB layout, used to deposit solder paste. Stencils are absolutely the most important tool where its optimisation and design influences the very success of every surface mount assembly-line. Achieving success with lead-free printing with either pin-in-hole reflow or fine-pitch, is almost the same as employing the more traditional solder alloys. The stencils contain multi-level aspects that are unique thicknesses on the same stencil which delivers the individual paste volumes that are required by the sheer diversity of components found on the PCB. At Tecan we combine our deep understanding of the latest surface-mount industry requirements with extensive manufacturing experience in order to offer the best stencil solutions based on three technologies:
- Precision etching: this is for reducing the stencil thickness locally, creating recesses or leaving raised areas which can then be lasered into apertures or whatever else may be required
- Laser-cut stencils: these are used for their improved aperture precision as well as dimensional tolerances for fine-pitch apertures
- Laser-formed stencils: used when a tight deadline is in place on the finest pitch components. Generally, the stencil material is electroformed nickel with the apertures made using laser technology. During the laser cutting process the beam normally liberates trace elements to the cut surface of the aperture walls. As the material here is more than 99% pure hard nickel the resulting apertures are naturally polished and provide optimum paste release.
Advantages of SMT
- Components can be applied to each side of the circuit board
- It has lighter and smaller components
- A much simpler and automated assembly
- Fewer holes that need to be drilled through the abrasive boards
- It features an improved mechanical performance under shake and vibration conditions
- Lower inductance and lead resistance (this means better performance for high frequency circuits)
- Any errors in component placement are automatically corrected thanks to the surface tension of the molten solder which pulls the component into alignment with the solder pads
Reflow and Profiles
When surface mount began, the heat sources employed were infrared. Infrared ovens have made leaps and bounds in the last thirty years to achieve fantastic results but nowadays many manufacturers use forced air convection ovens to reflow the solder paste. Vapour phase technology has also enjoyed similar success in the development of reflow soldering and is popular with certain lead-free process requirements. The reason being is that the boiling point of the chemical used is constant and every part of the printed circuit board (PCB) assembly benefits from the same temperature.
Achieving a reflow profile that is suited to both the solder paste and the components used is absolutely critical to the success of the surface mount process. Guidance is always provided by the solder paste manufacturer and should be used as exactly that. The PCB you are processing may require small variations from the manufacturer’s suggested reflow profile to enable achievement of acceptable quality levels.
In order to adjust a profile you need to understand what’s happening on the PCB. For this reason, a profiler should be used with thermocouples mounted to monitor the temperature at various critical points throughout the process to ensure the temperatures achieved enable reflow of the solder paste.
Reflow profiles can be broken down into four distinct areas:
1 – Pre-heating: This increases the temperature of both the PCB and the components without any risk of thermal shock to either.
2 – Soaking: Allows for stabilisation of the temperature across the variation of each different component.
3 – Reflow: This occurs once the temperature is increased above that of liquidus. Typically 40 to 60 seconds is spent above this temperature (time above liquidus) to assure that all the solder fillets are totally reflowed.
4 – Cooling: Necessary to ensure the prevention of excessive intermetallic layers which could cause embrittlement of the resultant solder fillets.