Solder Paste Metal Load and Print Behaviour
Paste is a mixture of solder powder and flux, and the proportion of powder is the metal load. It is stated as a percentage by weight, and it decides how much solder a given deposit finally contains.
How It Is Specified
The figure is by weight, which means the volume fraction is different because the metal is denser than the flux. The two are related by the densities and the difference is significant.
The distinction matters when a deposit volume is converted into a joint. Our paste volume notes describe the calculation.
Effect on the Deposit
A higher metal load gives more solder for the same printed volume and it also raises the viscosity. A paste that is too stiff does not release cleanly from the aperture.
The release depends on the aperture shape as well. Our stencil notes describe the tool.
Slump and Its Measurement
Slump is the movement of the deposit between printing and reflow. A paste with a low metal load and a weak rheology slumps, and the slump produces bridging on a fine pitch part.
The slump is measured by comparing the deposit before and after a standing time. Our SPI notes describe the measurement.
Matching Paste to Aperture
A fine aperture needs a paste that releases easily and resists slump, which usually means a smaller powder size and a controlled rheology. Using one paste for every product is a compromise.
The powder size is specified as a type. Our component selection notes describe the specification practice.
Metal Load and the Final Joint
The final joint contains the metal from the paste plus the plating that dissolves into it. A change of metal load changes the joint volume and therefore the fillet.
The acceptance is judged on the joint. Our joint criteria notes describe the criteria.
Storage and Its Effect on the Figure
The flux can separate during storage, which changes the local metal load. Mixing restores the homogeneity and it has to be done in a controlled way.
Our paste conditioning notes describe the routine.
Verification
The verification is a metal load stated as weight with the volume fraction considered separately, a slump measurement after a standing time, a powder type matched to the finest aperture, and a print volume trend that shows the paste behaving consistently.
Our first pass yield notes describe how the data is used.
Process Control and Verification
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. 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.
Checks Before Release
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.
Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.
FAQ
Is a higher metal load always better? It gives more solder per unit volume and it makes the paste harder to print. The right figure depends on the aperture.
Does metal load by weight equal metal load by volume? No, because the densities differ. The conversion should be made explicitly where the joint volume matters.
What does gopcb provide for paste control? We provide a metal load stated by weight with the volume fraction considered, a slump measurement after a standing time, a powder type matched to the finest aperture on the board, and a print volume trend that shows the paste behaving consistently.
