Laminate Moisture Control Before Lamination
Laminate absorbs water from the air, and the amount it holds depends on the resin system, the humidity and how the material has been stored. When a stack is laminated with that water still inside, the press turns it into steam, and the result is a void, a delamination or a measured thickness that does not match the stack-up.
Where the Moisture Comes From
Prepreg and cores arrive in a sealed bag, and once the bag is open they begin to equilibrate with the room. The rate depends on the resin, the thickness and the relative humidity, so a material that was dry on Friday can be wet on Monday without anything having gone wrong.
Inner layers that have been through an oxide or a plasma step can also carry water, and the water that is absorbed after imaging and before lamination is the least visible. Our laminate properties notes describe how the resin system affects the figure.
Why Baking Is Not Optional
Baking removes the absorbed water before the press closes, and the schedule is set by the material and the thickness rather than by convenience. Too short and the water is still there; too long or too hot and the resin advances, which changes the flow and can leave a lamination that will not fill.
The bake schedule belongs in the travel card with the material lot, because a schedule that is right for one resin is wrong for another. Substituting a material without re-checking the bake is a common source of a delamination that appears only after the assembly reflow.
Measuring Moisture Rather Than Assuming
Moisture content can be measured by weighing a sample before and after a controlled dry, by a moisture meter, or by a thermal method that drives the water off and measures it. Any of the three is better than an assumption based on the calendar.
The measurement is worth taking on incoming material as well as on work in progress, because the bag may have been opened before it reached the shop. A supplier who repacks material into smaller bags changes the exposure without changing the label.
Delamination and Its Appearance
Delamination shows as a separation between the resin and the glass, usually at a corner or around a large copper area, and it is found by microsection, by a thermal stress test or by an ultrasonic scan. A small one can pass the electrical test and fail later.
The relationship between moisture and delamination is well enough established that a delamination investigation normally starts with the bake records rather than with the press. Our warpage notes cover the related problem of a stack that moves during lamination.
Thickness and the Stack-Up
Water that turns to steam occupies volume, and the volume has to go somewhere. Where the press cannot close fully, the dielectric ends up thicker than the stack-up predicted, and that thickness changes the impedance of every controlled line on the board.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the shop price the board against the dielectric and copper weights that will actually be used. It also gives the shop the figure against which the finished thickness can be checked.
Storage Between Steps
Between lamination and drilling, and between drilling and plating, the panel is again exposed to the room. The exposure is short and the water is on the surface, but a long queue in a humid area still shows up as a smear or an adhesion problem later.
Dry storage, a nitrogen cabinet or simply a shorter queue are all acceptable answers. The point is that the humidity in the shop is a process variable, and treating it as a background condition is what makes the variation hard to explain.
Interaction With Assembly
Moisture matters again at assembly, where the finished board and the components are heated. A board that was dry when it was laminated can still have absorbed water during storage, and the absorbed water drives the same steam damage at reflow.
That is why the bake before assembly is based on the moisture sensitivity of the assembly and the storage history rather than on the board alone. Our layer assignment notes describe how the stack choice affects the thermal behaviour of the finished board.
Records That Make the Process Controllable
The record is short: material lot, bag open date, bake schedule, measured moisture before lamination and the measured thickness after. With those five items, a delamination or a thickness excursion can be traced to a cause instead of being attributed to the press.
The measurements that matter are the repeatable ones, and a shop that records them can hold a tight thickness tolerance. A shop that does not will hold the tolerance on the boards it inspects and miss it on the ones it does not.
Process Control and Verification
On a design of this kind, baking is the item that decides how the rest of the board is arranged. 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.
Process Control and Verification
On a design of this kind, baking is the item that decides how the rest of the board is arranged. 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.
Process Control and Verification
On a design of this kind, baking is the item that decides how the rest of the board is arranged. 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.
FAQ
Does baking damage the prepreg? It can if the schedule is too aggressive, because the resin advances. The schedule is set from the material data rather than from a general rule.
Is moisture a problem for a thin board? It is, and a thin board equilibrates faster, so the exposure window before lamination is shorter than it is for a thick one.
What does gopcb provide for moisture control? We provide bake schedules set from the material and thickness, incoming and in-process moisture measurement, exposure time control between steps, thickness verification against the stack-up, coupons from the same panel, and records for each batch.
