BGA Reballing Practice, Stencil Choice and Inspection
Reballing a ball grid array package is one of the more demanding rework tasks in electronics assembly, because every ball is a joint and there may be several hundred of them on a package only a few millimetres square. It is done to rescue an expensive part, to change the alloy for a process reason, or to recover a package that has been removed from a board. Doing it well depends on a few details: the moisture state of the package, the quality of the site preparation, the stencil, and the inspection that proves the result.
When Reballing Is the Right Answer
Reballing is worth considering when the package is expensive or scarce, when the required alloy differs from the one already attached, or when a package removed from a prototype board has to be reused. It is rarely worth considering for a low cost part in high volume, where a new part is cheaper than the labour and the risk.
The decision also depends on the thermal history of the package. A part that has already been through several reflow cycles has consumed part of its moisture and thermal budget, and each additional cycle carries more risk than the last. The record of previous cycles should be part of the decision rather than an afterthought.
Package Preparation and Moisture Bake
A BGA package is a moisture sensitive device, and the heat of ball removal and reattachment is enough to cause internal damage if it has absorbed water. The package should be baked to the supplier specification before any thermal work, and the bake should be recorded, because a second reball of the same part needs to know what it has already been through.
The bake also has to be compatible with the carrier the part sits in. A tray that cannot take the temperature will deform, and a deformed tray can tilt the package and produce a ball array that is not coplanar. Deformation of the package itself from a previous reflow also has to be assessed before starting.
Ball Removal and Site Cleaning
The existing solder balls are removed with a hot plate, a hot air tool or a dedicated removal station, and the objective is to remove the alloy without damaging the pads or the solder mask. A low melting alloy or a wick can help, but every additional step removes some of the pad’s own solder and thins the copper.
Site cleaning is where the quality of the result is decided. Residual solder left on the pads produces a ball that is offset or a joint with a void, and residual flux or underfill produces a non wetting pad. The cleaned site should be inspected at magnification before the stencil is placed, and the pads should all look identical, because a pad that looks different will behave differently.
Stencil Alignment and Fixturing
The stencil is the tool that places the paste or the balls in the right pattern, and its alignment determines whether the finished array is concentric with the pads. A stencil designed for the package is far better than a generic one, because the aperture size and the ball pitch have to match the land pattern exactly.
The fixture has to hold the package flat and the stencil in contact with the top surface. Any gap between the stencil and the package lets paste squeeze sideways and produces a ball that is too large; any tilt produces a gradient across the array. A magnetic or vacuum fixture is usually worth the cost for anything beyond occasional work.
Paste Versus Preformed Balls
The paste method prints solder paste into the stencil apertures, removes the stencil and reflows the package so that the paste forms balls. It is quick and the equipment is simple, but the ball volume depends on the print quality and the reflow, and small variations across the array are difficult to avoid.
The preformed ball method places solid spheres into fluxed apertures, and the volume of each ball is fixed by the sphere rather than by the printing. That makes the array far more uniform and gives a better coplanarity result, at the cost of handling small parts and of a placement step that has to be done accurately. For fine pitch packages the preformed route is usually the more reliable.
Reflow and Attachment
The reflow profile should be the one the package and the alloy require, with enough soak to bring the whole array to temperature evenly and a peak that fully forms each joint. The package is thin and light, so it heats quickly, but the ball array is dense and the thermal mass per unit area is higher than it looks.
Cooling should be controlled, because a fast quench on a package with a large difference between the die and the substrate can warp it. Warpage after reflow is one of the reasons a reballed package that looked perfect on the bench fails at board assembly, when it is heated again.
Coplanarity and Ball Integrity Checks
Coplanarity is the flatness of the ball array, and it is the property that decides whether the package will sit on its pads and reflow correctly. It is measured by placing the package on a flat reference surface or with an optical system that maps the height of each ball. A single low ball can leave an open joint that only appears after thermal cycling.
Ball integrity covers the shape, the diameter and the attachment to the pad. A ball that is offset, a ball with a visible void at its base and a ball that has merged with its neighbour are all rejects, and each points at a different fault in the process.
Inspection: X-Ray, Visual and Cross Section
Optical inspection sees the outer rows and the package surface, which is enough to catch gross placement errors but not the interior of the array. X-ray sees the whole array and is the standard method for checking ball presence, position and gross voiding.
Cross sectioning is destructive but it is the only way to see the interface between the ball and the pad and the intermetallic layer that formed during reflow. Our BGA inspection notes explain how the two methods divide the work, and our solder defect guide groups the failures that appear in a ball array.
Process Discipline and Records
Every reball should be recorded with the package identification, the bake history, the stencil used, the material and the inspection result. That record is what allows a failed part to be understood, and it is what stops a package that has been reballed three times from being treated as new.
At gopcb the same discipline applies to the land pattern the package will eventually be placed on, which is covered in our land pattern notes. Our component tolerance guide explains how the diameter and pitch of the balls interact with the pad design, and our quality guide describes how a rejected array is classified.
FAQ
How many times can a package be reballed? There is no fixed number, but each cycle adds thermal history and consumes pad solder. The practical limit comes from the condition of the pads, and it should be assessed by inspection rather than by counting cycles alone.
Is a moisture bake needed before reballing? Yes, for any package that has been stored or has been through a previous thermal cycle. The heat of ball removal is enough to cause internal damage in a package that has absorbed water.
Can a reballed package be used for production? It can be used for rework and for recovery of a finished assembly. For volume production, the economics and the risk usually favour a new part unless the package is genuinely unobtainable.
