Solder Paste Printing for Mixed Pitch Boards
A board that carries both a fine pitch part and a large connector cannot be printed with one set of parameters, and the compromise is usually made in favour of whichever part is inspected more.
Where the Conflict Comes From
The fine pitch part needs a thin deposit with a low volume and a small aperture, which needs a thin stencil, a fine powder and a gentle separation.
The large part needs a high volume through a large aperture, which needs a thicker stencil and a deposit that does not slump. The two requirements pull in opposite directions. Our area ratio notes describe the limit that decides which one governs.
Step Stencils
A step stencil gives a thin section for the fine pitch area and a thick section for the rest, which resolves the conflict at the cost of a special stencil.
The step has a transition region where the squeegee behaves differently, and no fine feature should be placed there. The step position should be designed with the layout. Our fabrication notes notes list the attributes that should be stated.
Aperture Modification
Where a step stencil is not justified, the aperture can be enlarged beyond the pad on the parts that need volume, accepting a bridging risk that is managed by the spacing.
The modification should be applied per part rather than globally, and the result measured rather than assumed. Our paste volume notes describe the measurement.
Paste Choice
A paste with a finer powder prints the small aperture and it dries faster on the stencil, which affects the large apertures later in the print.
The working life becomes the constraint on a mixed board, because the fine apertures set the powder size and the coarse ones set the volume. Our rheology notes describe the behaviour over the working life.
Print Sequence
Where the two areas cannot both be printed from one stencil at one thickness, the print can be made in two passes with different parameters, or with two stencils.
Two passes add handling and a registration step between them, which becomes a source of error that has to be controlled. The trade should be made on the volume.
Verification
The verification is the deposit measurement on both the smallest and the largest aperture, taken at the start of the run and at intervals.
A result that is acceptable on one and short on the other means the compromise has moved to the wrong side, and the stencil rather than the printer is the answer. Our paste inspection notes describe the measurement.
Design Inputs
The layout can help by grouping the fine pitch parts together so that the step covers a contiguous area, and by keeping the high volume parts away from the step.
That grouping is a design decision, and it is cheap at layout time and expensive afterwards. Our manufacturability review notes describe where the grouping is checked.
Process Control and Verification
On a design of this kind, aperture is the item that decides how the rest of the board is arranged. 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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
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.
Process Control and Verification
On a design of this kind, aperture is the item that decides how the rest of the board is arranged. 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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Process Control and Verification
On a design of this kind, aperture is the item that decides how the rest of the board is arranged. 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, aperture is the item that decides how the rest of the board is arranged. 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.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Is a step stencil always the answer? It resolves the thickness conflict and it costs more and it adds a transition. It is justified where the volume difference is large and the volume is high.
Which part should the compromise favour? The one with the smaller process window, which is usually the fine pitch part because its failure mode is an open.
What does gopcb provide for mixed pitch printing? We provide area ratio analysis to identify the governing feature, step stencil definition with the transition placed clear of fine features, per part aperture modification, paste selection with the working life stated, and deposit measurement on both the smallest and largest apertures.
