Footprint Design for Reflow Soldering
A footprint is the interface between the component and the process, and it determines how much paste is needed, where the joint forms and how much force the stencil applies. Getting it right removes a whole class of defects before they appear.
The dimensions on a component datasheet are mechanical, and the footprint has to convert them into process requirements. That conversion is the reason two products with the same package can need different land patterns.
Pad Extension and the Toe Fillet
The pad usually extends beyond the component termination on the toe side so that a fillet can form where it can be seen and inspected. The extension is a process allowance rather than a mechanical one.
Too little extension and the fillet has nowhere to form, so the joint looks starved. Too much and the paste spreads away from the termination, leaving a thin joint with a large ball of solder at the tip.
Heel and Side Clearances
The heel side determines how much solder is available at the point where the fillet has to rise up the termination. The side clearances control the amount of solder that can wrap around and the risk of a bridge to the neighbouring pad.
On a fine pitch part the side clearance is the parameter that has to give, which is why mask defined pads are used. The trade is described in the rules for pad design standards.
Paste Aperture Against Pad
The stencil aperture is normally the same as the pad, and it can be reduced or shaped to control volume. Reducing the aperture in the direction of the pitch decreases the bridging risk while keeping the fillet requirement satisfied.
The aperture should be specified as a percentage reduction with a reason, rather than adjusted on the printer without a record. An aperture change is a design change because it changes the joint.
Standoff and Collapse
For a leaded package the standoff is created by the lead geometry, while for a ball array it depends on the ball and the paste volume. The footprint influences the collapse only through the paste volume it allows.
Where a specific standoff is required, the paste aperture and the pad size have to be chosen together and the result verified by a cross section. A standoff that is too small can stress the package, and one that is too large can leave an open joint.
Thermal Pads
A thermal pad footprint must balance two requirements: enough paste to form a void free joint, and enough thermal connection to dissipate heat. Both are affected by the aperture pattern.
Segmenting the aperture into a grid gives the gas a route out during reflow, which reduces voiding. The same segmentation reduces the contact area slightly, so the pattern should be evaluated on the measured result rather than assumed.
Connector and Heavy Part Footprints
A connector needs a longer pad than a chip part, because the joint has to resist mechanical load as well as carry current. The extra length should be on the toe side where the fillet can be inspected.
Where a part is heavy, the pad may be widened to increase the joint area, provided that the widened pad does not create a bridging risk. This is a case where the mechanical requirement overrides a general rule.
Via in Pad Footprints
A footprint that places a via inside a pad requires the via to be filled and capped, and the aperture to be sized on the assumption of a solid surface. An open via drains the paste and produces a thin joint.
Where a filled via is used, the planarity of the cap is part of the footprint requirement, because a cap that is proud lifts the stencil. The pad and the via specification therefore travel together.
Footprint Libraries and Revision Control
A footprint library is only useful if the parts in it are validated, so each footprint should be checked against the component drawing and against a produced assembly. A library with an unverified footprint propagates the error to every product that uses it.
Changes to a footprint should be versioned, since a silent change alters the paste volume of every product that references it. The change should be treated with the same discipline as a schematic change.
Verification
The verification is a first article with a paste volume measurement and a cross section of the joint, compared with the drawing. Where the result differs, the footprint is the first item to review.
Recording the aperture reduction and the measured volume against the footprint creates a library that improves over time. This is a practical way to accumulate process knowledge, in the same spirit as the records kept for manufacturing tolerances.
Additional Considerations for This Build
Practical attention to footprint design pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating footprint design explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, toe fillet 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.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
Process Control and Verification
On a design of this kind, toe fillet 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.
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
Should the pad always be larger than the component land? In the toe direction it usually should be, so that a fillet can form and be inspected.
How much should the aperture be reduced? Only as much as the bridging or volume requirement demands, and the reduction should be recorded with its reason.
Is a datasheet land pattern always correct? It is a starting point derived from one process assumption, and it may need adjustment for the paste and stencil in use.
What causes a starved joint on a chip part? Usually a pad with insufficient heel area or an aperture reduced more than the volume requirement allowed.
