Component Coplanarity for Fine Pitch Packages
Every package that is soldered with leads or bumps depends on those terminations reaching the pad at the same time and with the same pressure. Coplanarity is the measure of how well they do that: the deviation of the seating plane from a perfect surface. On a coarse pitch part a few tens of micrometres of deviation is absorbed by the solder; on a fine pitch device the same number means some joints never form while others bridge.
What Coplanarity Means
Coplanarity is expressed as the maximum distance between the lowest and highest termination when the package rests on a flat reference. It combines the flatness of the leads or balls, the flatness of the package body, and the effect of temperature, because a package that is flat at room temperature can warp when it reaches reflow temperature.
Most standards quote a limit at room temperature, which is convenient for measurement but incomplete for the process. The number that matters is the coplanarity at the moment the alloy is molten, and for plastic packages that value can differ substantially from the ambient figure because of moisture and thermal expansion mismatch.
Package Types and Their Failure Modes
Gull wing leads bend and recover, so a lead that is out of plane may still contact the pad and produce a marginal joint rather than a complete open. Ball grid arrays behave differently: a ball that is short leaves a gap, and the missing joint is invisible without X-ray. Land grid arrays and quad flat no-lead packages have no compliant termination at all, so the package body has to sit flat within a few tens of micrometres.
Each family therefore needs its own limit and its own inspection method. Treating a no-lead package with the tolerance intended for a gull wing part is a common way to accept components that will fail at assembly.
Measurement Methods
Optical methods are the fastest. A laser scanner or a shadow moire system maps the underside of the package and computes the seating plane, reporting a single coplanarity value plus a map of individual terminations. Automated optical inspection with a side view can screen for bent leads on gull wing parts as they pass through the line.
Contact methods use a flat glass plate or a granite surface and measure the gap at the extreme terminations with a micrometer or a dial indicator. They are slow and destructive to the sample, but they are also the reference against which optical systems are calibrated, which is why incoming inspection labs still keep them.
Solder Volume as Compensation
Solder can absorb some non-coplanarity, and that is the practical reason paste volume matters. A slightly thicker deposit bridges a gap that a thin one cannot, because the molten alloy wets both surfaces and pulls them together as it forms a fillet. The compensation has limits, and it depends on the alloy and the pad geometry rather than on good intentions.
The useful figure is the gap that the deposit can close given its area and thickness. If the deviation exceeds roughly a quarter of the deposit thickness after reflow, the joint becomes unreliable even when it looks connected. Where the design is marginal, the answer is a stencil change rather than a tighter incoming limit.
Placement Force and Self Alignment
Placement can either hide or expose non-coplanarity. A nozzle that presses the package down before release forces the extreme terminations into the paste, which improves contact on a warped part but can also squeeze paste away from the centre. A nozzle that releases high leaves the part resting on its highest terminations.
Self alignment then does what surface tension allows. A part whose leads are all near the pads will centre itself, while one that rests on a single corner cannot. Placement force settings are worth verifying whenever a fine pitch part appears in a new design, and the capability data described in our note on component tolerance and reliability shows how much variation the process can absorb.
Incoming Inspection Limits
An incoming inspection limit is a contract with the supplier, and it has to be achievable and measurable. The usual form is a maximum coplanarity for a given package family, sampled by lot, with a defined measurement method. Without a stated method the number is not enforceable, because contact and optical measurements disagree.
Sampling also needs a rule for what happens when a lot fails, and a rule for how the reels are handled afterwards. Baking, re-reeling and re-inspection all cost money, so the limit should be set from the process requirement rather than from what is easy to sort. Our discussion of component selection beyond the datasheet covers the wider set of parameters that deserve a limit.
Bridging and Short Risk
Non-coplanarity does not only cause opens. A package that rocks into place can push surplus alloy sideways, and on a fine pitch device the result is a bridge between adjacent leads. Bridging caused this way is often intermittent along a row, matching the pattern of the lead deviation rather than the printed deposit.
Inspection detects it in two ways. Optical inspection sees bridges on exposed leads, while X-ray finds them under a package and also shows the gap where a joint failed to form. The combination of techniques and the limits of each are set out in our comparison of X-ray and AOI for package inspection.
Working with the Supplier
Coplanarity problems are best solved with data. A measured distribution of values across several lots, together with the assembly defect rate, gives the supplier something to act on and gives the buyer a basis for negotiation. Sending back a single failed sample rarely changes anything.
Where the deviation is intrinsic to the package, the process can sometimes be adjusted instead: a thicker deposit, a longer soak that lets the whole assembly reach temperature evenly, or a placement force profile that seats the part. The gopcb engineering team evaluates these trade-offs during design review, when a change still costs nothing.
Additional Considerations for This Build
Practical attention to lead skew 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 lead skew explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Deliberate attention to solder bridging 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 solder bridging 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, lead skew 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.
FAQ
What coplanarity limit is typical? Around 100 micrometres is common for fine pitch gull wing parts and tighter for no-lead packages. The limit should follow the pitch and the deposit thickness rather than a single number applied to every family.
Does baking improve coplanarity? It removes absorbed moisture and therefore reduces warpage at reflow temperature. It does not change the room temperature figure, so a part that fails a flatness check still fails after baking.
Can a warped package be used at all? Sometimes, if the deposit is thick enough and the assembly reaches temperature evenly. It is a process compromise rather than a cure, and it should be confirmed by inspection before the design is released.
