We buy a selective wave

The article contains a wide set of parameters with proposed standard levels that everyone should consider before buying a selective wave.

10.12.2021
 

Requirements from the point of view of the company's needs

The first step in selecting a selective soldering system is to review your organization's needs now and for the foreseeable future. This includes the number of chips planned for production, anticipated future growth and customer requirements, including the maximum PCB size and product range. The complexity of future product design should also be taken into account, and whether the operating mode will require a standalone serial or low-volume production system or an in-line system for large-scale production.

A key requirement to consider when choosing a selective soldering system is the combination of THT components and SMT components that will contain future products. This defines the solder joints that the selective soldering system will have to produce which in turn determines the types of solder nozzles required. If the complexity of future products cannot be determined with a mix of SMT and through-hole components, it is reasonable for the flexibility of a selective soldering system to be a major deciding factor in the decision-making process.

Overview of specifications and functions

All hardware specifications and system features should be taken into account when making comparisons of candidate selective soldering systems. These include the maximum board size, flux compatibility, required board edge clearance, and component spacing above and below the PCB. Other factors to consider are the capacity of the solder pot, solder wave height detection, available solder nozzle sizes, required nozzle spacing, fiducial alignment control, preheating capability, and the accuracy and repeatability of the X, Y and Z axes.

Equally important are factors such as ease of programming, consumption of consumables (i.e. nitrogen and solder), providing guarantees, as well as the availability of software updates at no additional cost.

Key process parameters

Among the process parameters related to selective soldering, the most important are X-Y positioning accuracy, flux dosing method, solder wave height control and preheating efficiency. The accuracy and reproducibility of both flux and solder dosing is critical to ensure accurate flux application and create an optimal solder joint. The repeatability and consistency of the solder wave height is required to ensure that contact is maintained between the solder wave and the underside of the PCB throughout the soldering cycle. The preheating temperature must be reached before and during the entire brazing cycle in order to properly activate the solids in the liquid flux and evaporate its base (alcohol or water).

Issues of special importance

Flux application

Selective soldering fluxes are available in many types: alcohol, water or rosin based fluxes, no-clean fluxes, fluxes with low pH and fluxes with high solids content. The choice of a specific type of flux for the selective soldering process is determined by the end use of the product and is critical to achieving a high integrity braze joint.

The criteria for selecting the flux composition should be based on the degree of solderability of the metal surfaces to be joined. Metal surfaces that can be easily soldered, including platinum gold, copper, tin-silver or palladium-silver, can usually be brazed using no-clean flux or an unactivated or mildly activated rosin flux. Metals that are more difficult to braze, such as nickel-plated brass, lead-cadmium bronze or beryllium copper, typically require either a fully activated rosin-based flux or a water-based organic / inorganic flux. In the latter case, after soldering, cleaning of the PCB is required in most cases.

Droplet dispensers are ideal for no-clean fluxes and low solids fluxes, but are not particularly suitable for use with other types of flux. Most drop dispensers on the market have a maximum solids limit of 8% above which they can potentially clog and not function properly. Some suppliers of selective soldering equipment even indicate that their equipment warranty will be void if their droplet dispenser is used with a flux other than the low solids flux, i.e. less than 8%. In addition, it should be noted that some of these suppliers do not offer other flux applicators.

Quality and throughput

The first step in measuring the quality of a selective soldering system is to review PCBs currently produced selectively or in wave with different types of through-hole components such as connectors, fine pitch components or capacitors, and compare the results based on trial production. It is suggested to select PCBs containing copper stray fields and multilayer wafers and document the quality results with X-ray imaging.

To fully assess the operational suitability of a selective soldering system, an additional trial should be performed on a larger production batch to determine cycle time and unit throughput per hour (UPH).

Operation costs

The consumption of consumables such as nitrogen and solder to compensate for the slag formed is the main ongoing operating cost of a potential selective soldering system. A nitrogen consumption of about 1.1 cubic meters per hour is preferred. The amount of slag generated is also an important factor: 42 grams in 8 hours can be taken as a benchmark. You should also consider the system in terms of maintenance time, minimizing downtime / costs. As a benchmark, 30 minutes can be taken to clean the soldering pot every 80 hours.

This article is based on the document 'Practical Methods for Evaluating and Qualifying Selective Soldering Systems' by Henry Reid of Hentec Industries / RPS Automation

Source: https://tek.info.pl & © Kurtz Ersa

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