Solder Ball Defect Analysis On Assembled Boards

A solder ball is a small sphere of solder that has separated from the joint and remains on the board surface, usually beside a pad or trapped under a component. Balls range from harmless specks to a real risk of a short between adjacent conductors, and the difference is often only a few tenths of a millimetre.

Analysing the defect means finding out where the solder came from. The source is almost always the paste itself, and the mechanism that freed it is what the analysis has to identify before a fix can be applied and verified.

Where The Solder Comes From

The solder in a ball comes from the paste, and it is separated from the deposit by one of several routes. Paste squeezed between the stencil and the board during printing remains on the surface as a smear. Paste printed onto the solder mask instead of onto the pad is never pulled back to the joint during reflow.

A third route is splashing, where a volatile burst during reflow throws small droplets of molten solder onto the surrounding surface. Splash balls are usually very small and distributed around the joint, which distinguishes them from the contamination type.

Stencil Printing And Paste Release

Printing parameters control the amount of paste left outside the aperture. Too much squeegee pressure forces paste under the stencil, and too little leaves paste on the stencil surface which later falls onto the board. A clean stencil and a correct separation speed are the two most effective controls.

Aperture design also matters. A large aperture with a small area ratio releases poorly and leaves paste clinging to the walls, which is then transferred to the next board as a smear. The area ratio is the aperture area divided by its wall area, and a value above about 0.66 is the usual requirement for clean release. The way apertures are dimensioned is described under design guidelines for manufacturability.

Reflow Behaviour

During reflow the paste first dries and then melts. If the ramp is too fast, solvent and flux volatiles escape violently and can eject small droplets of solder. A slower ramp with a proper soak lets the volatiles leave before the alloy becomes mobile. The alloy behaviour is described under lead-free versus leaded solder.

Solder mask that outgasses produces the same effect. A mask that was under-cured releases moisture and organics during reflow, and the escaping gas lifts solder away from the joint. Mask cure should be verified by a solvent resistance test rather than assumed from a recipe.

Solder Mask And Pad Geometry

Solder mask between pads defines the channels that solder can flow along. A mask that is too thin, or a pad that extends beyond the mask opening, gives molten solder a path away from the joint. The dam height and the opening size together control this.

Solder mask slivers between fine pitch pads are a common source of the problem. A deposit on a sliver has nowhere to go and forms a ball after reflow, so the mask opening should be generous enough to leave a clean dam rather than a thin ridge.

Inspection And Acceptance Limits

Inspection for solder balls is visual, usually with a defined ball size limit and a keep-out area around critical features. A ball below a certain diameter in a non-critical area is normally acceptable, while a ball of any size that could bridge two conductors is not.

The limit should be written with a measurement method. A rule such as no balls above 0.13 mm in diameter, and none within 0.5 mm of a conductor, is enforceable, while a general instruction to keep the board clean is not.

Corrective Actions

The corrective sequence starts with the printing process, because that is where most of the solder is released. Squeegee pressure, separation speed, stencil cleaning and aperture design are checked in that order, and each of them has a clear test associated with it.

If printing is clean, the profile is examined and then the mask. Mask cure and mask thickness are both worth measuring when the balls appear as a general contamination rather than as a few large spheres. How the defects are tracked through production is described under PCB design quality characteristics.

Process Control and Verification

On a design of this kind, solder ball is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, solder ball is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, solder ball is the item that decides how the rest of the board is arranged. 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.

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

Are solder balls always a defect? Not always. Small, isolated balls away from conductors are usually accepted, and the standards allow them within defined size limits. Balls that could bridge two conductors are always rejected.

Does a no-clean paste cause more balls? It can, because the residue is designed to stay on the board and can hold small spheres in place. The paste chemistry is a factor, but the printing process usually dominates.

Can balls be removed after assembly? Loose balls can be brushed or blown off, but balls trapped under a component cannot be reached. Prevention at the printing stage is far cheaper than removal after the fact.