BGA Warpage and Solder Joint Open During Reflow

A ball grid array package and the board it sits on both bend when they are heated, and they bend by different amounts. At the moment the alloy is molten the package and the board can be far enough apart to hold a solder ball open, and the joint closes again as the assembly cools.

Why the Two Move Differently

The package is a small laminate or substrate with a large silicon die bonded to it, and the die has a much lower expansion coefficient than the substrate. The board is a larger laminate with its own stack and its own copper distribution, so its shape at temperature follows its own pattern.

The result is a relative displacement that is largest at the corners and at the edges of the array, which is where the opens appear. Our mixed thermal mass notes describe how a board with different masses responds to the same profile.

Coplanarity and Its Measurement

Coplanarity is the deviation of the ball plane from a reference plane, and it is normally measured on the package alone at room temperature and at the peak reflow temperature. The hot measurement is the one that matters, because that is the state the joint is formed in.

A package that measures flat when cold can be badly warped when hot, and the two figures can move in opposite directions. Our BGA inspection notes describe how the result is verified afterwards.

The Shadow Effect and Insufficient Paste

Where the package is warped away from the board, the paste deposit can remain on the pad while the ball lifts, and the joint that forms is thin or open. The region is often described as a shadow because it follows the shape of the warped package.

The first correction is to raise the paste volume at the corners of the array rather than across the whole footprint. A uniform increase pushes paste into the neighbouring joint and creates bridges where the package is flat.

Profile Changes That Help

A slower ramp and a longer time above liquidus let the package and the board equalise, so the relative displacement at the moment of solidification is smaller. The cost is more heat input to the rest of the assembly, which is the trade that has to be made deliberately.

A soak that brings the whole assembly close to the melting point before the final rise is the usual technique, and it is verified with a profile that includes the package underside. A profile measured only on the board surface does not describe what the joint experienced. Our <a href="https://www.gopcba.com/reflow-oven-profile-verification/” title=”profile verification”>profile verification notes describe how that measurement is arranged.

Support and Stiffening

A board that is supported at the edges only will sag in the middle, and a sagging board changes the gap under the package. Support placed directly under the package, or a pallet that holds the board flat, removes that contribution.

Where the board is thin and the package is large, stiffening or a thicker board may be the design answer. That change is usually more expensive than a profile change, so it is considered after the process options have been tested.

Package Side Choices

Package suppliers offer different substrate constructions and different die attach methods, and the warpage behaviour differs between them. Choosing a package on its cold coplanarity figure alone leaves the hot figure to chance.

Where a package is known to warp, the assembly design can accommodate it by using a thicker paste deposit at the perimeter or by adding a stiffener ring on the board. Both are decisions that have to be made before the stencil is cut.

Verification After Assembly

Verification is by X-ray for the joint at the corners of the array, and by a dye and pry or a cross section on a sample where the question is the shape of the joint rather than its presence. X-ray catches the open and the section explains it.

A coupon or a sample board that carries the same package and the same profile is worth keeping, because it can be sectioned to show what the corners look like without destroying a product. Our board quality notes describe how the standard is applied to the result.

Process Control and Verification

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. 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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

FAQ

Can an open joint under a BGA be reworked? It can be reflowed with local heat or replaced with a reballed package, and the board sees another thermal excursion at that site.

Does more paste always fix a shadow? No. More paste creates bridges where the package is flat, so the increase is normally applied to the perimeter of the array only.

What does gopcb provide for BGA assembly? We provide profile development with measurement under the package, paste volume adjustment at the array perimeter, support placed under the package, X-ray verification at the corners, sample sections, and records that tie the result to the profile and the package lot.