SMT Quick-Tips 4: Selecting a Selective Soldering System
From: Author:Mark Hardy Publish time:2021-08-31 16:30 Clicks:0
SMT Quick-Tips 4: Selecting a Selective Soldering System
Flux materials
As covered in prior chapters on Wave soldering, there are a variety of flux types and chemistries available, including low pH, high-solids content, and water soluble, alcohol-based, and others. Selection of a particular type of flux is frequently not an option for the contract manufacturer, since it is usually determined by the end-user’s application, so they must be able to accommodate work using many types of flux.
No-clean fluxes are generally preferred because they require little to no post solder cleaning, except for a visible residue that should be removed. If the user opts not to use a no-clean flux, it is very important to control the amount of flux applied to the board. In most cases, controlling the solder head to cover the area previously sprayed will burn off the flux and eliminate the necessity of cleaning the residue in a subsequent step.
Remember that solder types used for selective must be compatible with solder used on the top of the board, and this will likely have a material effect on the flux type used.
Spray Fluxing
Thru-hole penetration is the ultimate goal of any fluxing system, but there are a number of factors that affect the performance of a spray fluxing system. Factors to consider for spray flux deposition include:
- Compatibility of materials of construction: Make sure the application technique is compatible with your flux chemistry.
- Motion and speed of the spray head: The faster or slower the spray head moves, the leaner or heavier the deposition will be.
- Flux flow rate delivered to the spray head: Will affect the quantity of flux deposited on the board.
- Pitch pattern between spray strokes: Determines whether consistent coverage will be achieved over a wide area.
- Spray pattern: Check for controls that will help minimize overspray and waste while targeting the critical areas to be covered.
In a spray fluxing system, flux can either be delivered by ultrasonic energy or by pumping from a canister in a housing below the machine into a spray head where it’s atomized with air. Nozzle-free spray heads are preferable because they are less likely to clog. Spray is delivered in a cone shape, in a variable band of material onto the board. The narrower the band, the more likely flux will be directed into the plated holes. While it covers a lot of real estate, a wider band doesn’t necessarily penetrate deeply into the through-hole to get good flux coating in the fillet.
Another important factor for effective spray fluxing is the speed and position of the spray head. Fluxers with reciprocating heads provide a greater opportunity to direct flux to plated holes from a variety of angles, and electronically driven heads are more apt to overcome issues with sticking due to flux residue and airborne contaminants than pneumatic systems which are more susceptible to this problem.
Finally, evaluate the fluxing system based on its construction attributes. A self-cleaning system will reduce downtime for maintenance. A robust machine will perform continuously without excessive maintenance attention.
Micro-drop Jet Fluxer
A micro-jet (or drop-jet) fluxing system sends an undiluted bead or bubble of flux directly through the fillet, the thru-hole and the solder ring of the circuit board in droplet form. A micro-jet system will generally use less flux than a spray system because it’s only applying flux precisely where it’s needed. Most systems allow for a variety of dot patterns which are pulsed through the nozzle, ensuring accuracy.
Micro-jet technology provides very fine control of bead size and positioning, thus reducing liquid media waste and virtually eliminating post solder cleaning. An advanced micro-jet flux system is capable of depositing precise amounts of liquid flux with extremely high accuracy, achieving pinpoint thru-hole penetration, with no wasted overspray.
In the early years of micro-jet fluxing, designs were adapted from currently available ink-jet printer technologies, and often were not as well suited to the PCB market as needed. However, as the technology matured, dedicated designs have been developed that were (and are) better suited as integrated systems in the selective soldering machine.
Micro-Jet technology, however, still comes with a couple drawbacks:
1. Programming is typically more complicated and time consuming; however, once programmed, repeatability is excellent and the recipe can be stored for producing the same boards again in the future.
2. Initial cost of a micro-jet system is higher than spray fluxing, but again, with its added efficiency, pinpoint accuracy and long-term reliability, this cost can often be made up very quickly in a production environment.
Biggest Advantage – Less Flux – Less Cleaning
One of the most important technical considerations is the actual control software of the manufacturer, as this is more important than the control of the fluxer itself.
Both the spray fluxer body and the micro-drop fluxer body can look identical – it’s only the methods of dispersement that differ.
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