Solder Pot: Preparation, Placement and Process Control
A solder pot holds a volume of alloy that is permanently molten, and everything that touches it dissolves slowly into it. Copper, iron and nickel from the fixtures and the boards accumulate in the bath, and the alloy that produced good joints at the start of the quarter is not the same alloy at the end of it.
What Contaminates the Bath
The largest contributor is copper, which dissolves from the boards and from the barrels being soldered. Iron and nickel come from the pot itself and from the fixtures, and each has a solubility limit above which it starts to form intermetallic particles in the bath.
The particles change the flow and the wetting, and the joints start to look grainy before any measurement shows a problem. Our joint criteria notes define the appearance that the process is held to.
Analysing the Alloy
The alloy is analysed by taking a sample from the bath and measuring the composition against the specification. The sample point matters, because the heavy intermetallics settle and a sample from the surface does not represent the bulk.
The frequency is set from the throughput rather than from the calendar. A pot that runs continuously and processes a large copper area per hour will reach a limit far sooner than one that runs a few hours a week. Our board quality notes describe how the result is recorded.
Dross and Its Management
Dross is oxide that forms on the surface of the bath, and it is produced faster at higher temperature and with more agitation. It is removed rather than stirred back in, because pushing it below the surface puts oxide into the joint.
The removal is a routine operation with a defined interval, and the dross that is produced is a measure of how the pot is being run. A sudden increase in dross production usually follows a temperature increase or a change in the flux.
Temperature Control and Its Effect on Contamination
Higher temperature increases the dissolution rate of everything in contact with the bath, so running the pot hotter than the process needs accelerates contamination. A pot that is left hot overnight with no production loses alloy to the pot walls and to the fixtures.
The practical control is to run the pot at the lowest temperature that produces an acceptable joint and to shut it down when it is idle. The saving in alloy is larger than the saving in the ramp time. Our reflow defects notes describe the equivalent discipline in a reflow oven.
Fixtures and Their Life
Every fixture that enters the bath loses material to it, and every loss raises the contamination level. Coating the parts that touch the alloy, and replacing them on a schedule rather than when they fail, limits the contribution.
The fixture that is left in the bath between operations is the worst case, because it stays in contact with molten alloy continuously. A fixture that is removed and cooled lasts far longer.
Correcting a Contaminated Bath
A bath that is out of specification is diluted by discarding part of the volume and adding fresh alloy, or it is replaced entirely. The choice depends on how far out of specification the analysis is and how much copper has to come out.
Adding fresh alloy without removing any is a partial fix, because it reduces the concentration and does not remove the particles that have already formed. The operation is therefore a partial drain and refill rather than a top up. Our cleanliness notes describe the related problem of what the boards carry into the bath.
Records and Trend
The record that makes the process controllable is the analysis result, the throughput since the last analysis, the refill history and the dross production. Those four together show whether contamination is being produced at the expected rate.
A trend is more useful than a value, because it shows how long a bath has before it needs attention. A single result inside specification at the moment it was taken does not say that.
Process Control and Verification
On a design of this kind, alloy is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
Process Control and Verification
On a design of this kind, alloy is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Process Control and Verification
On a design of this kind, alloy is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
FAQ
Can dross be put back into the pot? No. It is oxide and it contaminates the joints as a non-wetting inclusion.
Does a lead free bath contaminate faster? It runs hotter and dissolves copper faster, and its limits are lower, so the interval between analyses is usually shorter.
What does gopcb provide for solder pot control? We provide alloy analysis at a throughput based interval, sampling that represents the bulk, temperature set at the lowest figure that meets the joint requirement, dross removal and fixture life control, partial drain and refill rather than top up, and trend records per pot.
