Foam is a common issue in the cleaning world. If left unchecked, foam can cause the following symptoms:
• Leaks.
• Excessive rinsing.
• Pump cavitation.
• Pump seal failure.
Fortunately, it is among the easiest problems to correct.
There are several causes for foam. They include equipment parameters, flux selection, chemical selection, and process control. Each has its own unique solution.
Cause A:
• Liquid flux out of specification (solids content).
• Inadequate preheat.
Solution A:
Most wave soldering machines are equipped with a foam or spray fluxer. A foam fluxer utilises an air-stone placed at the bottom of a liquid flux reservoir. A small volume of air is injected into the air-stone, causing the flux to develop a foam ‘head’. The flux develops the head of foam due to a foaming agent that is added to the flux during manufacturing.
This foaming agent is designed to be burned off the board during the preheat and soldering process. If the foaming agents are not completely burned off, they will be carried into the cleaning system on the board’s surface. Foaming agents will foam when presented with high-pressure water sprays like those in a spray-in-air cleaning system.
It needs to be ensured that the boards are preheated properly, and that the dwell time with the solder is sufficient. The specific gravity of the fluxes may also be checked. All fluxes have a solids content. The only way to maintain that solids content is to monitor and adjust the flux’s specific gravity at least twice per day. If the flux’s specific gravity is not maintained, the solids content will rise, increasing the likelihood of foam.
If foam occurs in a water-soluble flux removal application using water only as a cleaning agent, the assemblies should be cleaned with a defluxing chemical additive. Most defluxing chemicals contain defoaming agents which will reduce or eliminate foam.
Cause B:
• Liquid flux added to the top side of an assembly (on wave solder applications).
Solution B:
If assemblies are being hand soldered (including hand rework), a squeeze bottle containing foaming flux must not be used. Application of foaming flux will cause the board to produce foam during the wash or rinse cycles. Only a non-foaming flux should be used in all hand soldering/rework applications.
Cause C:
• Inadequate volume of defluxing chemical.
Solution C:
If a defluxing chemical is being utilised, one must be sure that there is an adequate percentage of chemical being added to the wash solution. Most defluxing chemicals contain a de-foaming agent. If the percentage of chemical is too low, there may not be enough de-foaming agent to prevent foam. Frequently, the solution is to increase the percentage of defluxing chemical being used in the wash cycle.
Cause D:
• Liquid temporary solder masks.
Solution D:
Liquid ‘spot masks’, that are soluble in water, are notorious for causing foam. A better approach is to use a latex spot mask, and peel it off after reflow and prior to cleaning.
Cause E:
• Inadequate segregation of fluids between wash and rinse.
Solution E:
Excessive foam frequently appears in the rinse section of a machine (inline) or in the rinse cycle (batch). If there is insufficient segregation of the wash solution from the rinse water, low percentages of wash solution can be dragged out into the rinse water. The solution is to increase fluid segregation between the wash and rinse cycle.
Cause F:
• Wash solution loaded.
Solution F:
If the wash solution in your cleaner is being reused cycle after cycle, there is a possibility that the solution is loaded. A highly loaded wash solution may lead to inadequate cleaning and foaming.
If the suggestions above do not help, one may control the foam by using a de-foaming chemical. Most defluxing chemical suppliers sell a liquid de-foaming agent, which may be added to the wash solution to prevent or reduce foam.
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