BGA X-Ray Inspection in PCB Assembly: Coverage and Void Criteria

Why "X-ray inspection required" leaves the scope open

BGA, QFN, LGA and other bottom-terminated components hide their solder joints under the package body. Optical inspection sees the outline and the fillet edge at best. X-ray is the usual way to look at the joints themselves.

The problem is that a single line in the PO, such as "X-ray inspection required", answers none of the questions that decide cost and acceptance. Which packages? Every board or a sample? Judged against which criteria? What do you get back as evidence? Two suppliers can quote the same line and deliver very different things. One may image the first board of the lot. Another may image every BGA on every board and store the images. Both can say they did X-ray inspection.

This article covers the four decisions that turn the line into a specification: coverage, frequency, criteria and evidence. It also covers what happens when a board fails.

Decide which components are in scope

Start from your BOM and placement file, not from a general rule. List the reference designators that need X-ray and the reason for each.

• Required. Packages whose joints cannot be seen at all: BGA, CSP, LGA, QFN and DFN thermal pads, bottom-terminated power modules. Fine-pitch BGAs and large-body BGAs are usually the first to name.
• Conditional. Parts where X-ray adds value only in some cases: press-fit or through-hole connectors where you want to check barrel fill, shielded parts after the shield is fitted, and packages placed on via-in-pad.
• Not required. Leaded parts whose joints optical inspection can judge.

Write the list as reference designators in a controlled document. "All BGAs" sounds clear but becomes unclear when a revision adds a small CSP or a module with an internal BGA. A designator list makes changes visible.

If you also want via-fill checks on the bare board, keep that separate. It belongs to the PCB fabrication order, not to the assembly X-ray scope.

Decide how often: first article, sampling or 100%

Frequency drives most of the cost. Three common levels:

• First article only. X-ray the first assembled boards of a new build or after a change. This confirms the process setup, not every board.
• Sampling per lot. A defined number of boards per lot or per shift, for example to watch for drift in paste printing or reflow. State the sample size and what happens if one sample fails.
• 100% of listed components. Every board, every listed designator. Typical for high-reliability products or for parts with a history of problems.

You can mix levels: 100% on two critical BGAs, sampling on the rest. Write it per designator so the quote reflects it.

Choose the criteria and state the source

X-ray finds several defect types, and voids are only one of them. Your criteria should cover at least:

• Bridging between balls or pads
• Missing balls or open joints
• Head-in-pillow or non-wet joints, where X-ray can suggest but not always prove the defect
• Ball shape and size consistency across the array
• Voids in balls and in thermal pads

For each, name the document that sets the limit. IPC-A-610 is the common acceptance reference for assemblies, and IPC-7095 discusses BGA design and process, including voids. Many buyers have their own customer specification with tighter limits for thermal pads or for safety parts. Name the document, the revision and the class. If you set your own void limit, write it as a number you chose, and say how it is measured: per ball, largest single void, or total void area as a percentage of the ball or pad.

Thermal pads under QFNs and power modules deserve a separate line. Large-area pads often void more than balls do, and the acceptable level depends on your thermal design, not on a general standard.

Agree how the inspection is done

The same criteria can give different results if the method changes. Write down what matters to you:

• 2D top-down only, or angled or 3D views where available for head-in-pillow and stacked parts
• Whether void percentage is measured by software or judged by an operator
• How double-sided boards are handled, where parts on the other side overlap in the image
• Who approves the inspection program for each designator

You do not need to specify the machine. You do need to know whether the method can detect the defect your criteria describe.

Decide what evidence you receive

"Inspected and passed" on a delivery note is not evidence. Choose the level you need:

• A pass/fail record per board serial number and designator
• Measured void values per designator for the sample or for every board
• Stored images, either delivered with the shipment or kept for a stated period and available on request
• A summary report for first article and after any process change

Images take storage and handling time, so ask for them where you will actually review them. Keeping the records traceable to board serial numbers matters more than the file format.

What happens when a board fails

Write the failure path before production starts:

• The failing board is marked and segregated, not reworked silently.
• If sampling finds a failure, the lot is held and the sample is widened or the lot is moved to 100% inspection.
• Rework of a BGA, if allowed at all, needs your approval and a repeat X-ray after rework.
• The supplier reports the likely cause, for example paste volume, stencil, reflow profile or component moisture, and the corrective action.

This links X-ray to your general defect disposition rules, so the two documents should use the same terms.

PO wording you can adapt

The text below is a starting point for your own terms. It is not legal advice and not PCB168 standard terms. Replace every [N] with values you have chosen.

> X-ray inspection. The supplier shall X-ray inspect the components listed in buyer document [document no., revision] at the frequency stated per designator: [first article / N boards per lot / 100%]. Acceptance per [IPC-A-610 class N / customer specification no., revision]. Void limit for listed thermal pads: [N] % of pad area; for BGA balls: [N] % per ball, measured [by software / by operator]. Bridging, missing balls and open joints are rejectable. Records per board serial number and designator shall be [delivered with the shipment / retained for N months]; images for first article shall be delivered. A sampling failure places the lot on hold pending 100% inspection of the listed designators and buyer disposition. BGA rework requires buyer approval and repeat X-ray inspection.

Keep the designator list as a separate controlled document so that a BOM revision triggers an update through change control.

How this fits with the rest of your PCBA RFQ

X-ray is one line in a larger inspection plan:

• The full list of inspection and test items for the RFQ is covered in defining test and inspection requirements for a PCBA RFQ.
• Acceptance classes and how defects are dispositioned are covered in PCBA solder joint acceptance class and defect disposition.
• Moisture control for BGAs before reflow, a common cause of voids and head-in-pillow, is covered in moisture-sensitive device handling in the PO.
• Release of the first assembled boards is covered in first article inspection report release terms.

PCB168 lists X-ray inspection for BGA and via-fill verification on the capabilities page. Send your designator list, criteria document and expected evidence with the Gerber, BOM and CPL through the quote request, so the scope is priced before the order.

Checklist before you send the RFQ

• Designator list for X-ray, with required and conditional parts
• Frequency per designator: first article, sampling or 100%
• Sample size and sampling failure rule
• Criteria document, revision and class named
• Void limits for balls and thermal pads, with measurement basis
• Method notes: 2D or angled views, double-sided boards
• Evidence level: records, values, images, retention period
• Failure, hold and rework approval rules
• Change trigger when the BOM or designator list changes

FAQ

Do all PCB assemblies need X-ray inspection?

No. It is needed where joints cannot be inspected optically, mainly BGA, CSP, LGA and bottom-terminated parts. A board with only leaded parts usually does not need it.

What void percentage is acceptable for a BGA?

It depends on the criteria you adopt. IPC-A-610 and IPC-7095 discuss voids, and many customer specifications set their own limits. Name the document and class in the PO, or write a number you chose with its measurement basis.

Can X-ray detect head-in-pillow defects?

Not always with a top-down 2D view. Angled or 3D views improve the chance. If this defect matters to you, ask what method the supplier will use and how it will be confirmed.

Is 100% X-ray inspection worth the cost?

For critical BGAs on high-reliability products it often is. For other parts, first article plus sampling may be enough. Decide per designator rather than for the whole board.

Should I ask for every X-ray image?

Only where you will review them. Records per serial number with stored images available on request are often a good balance.

Does X-ray inspection replace functional test?

No. X-ray checks joint structure. Functional or in-circuit test checks electrical behaviour. Most plans use both.