INTRODUCTION TO X-RAY INSPECTION

All text Copyright Bob Willis, EPS

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INTRODUCTION TO X-RAY INSPECTION

The use of X-ray inspection is becoming very popular due to the increased use of Ball Grid Array (BGA) technology. With BGA it is impossible to inspect the termination points after component placement and soldering without the use of X-ray. This inspection technique can also be used for inspection of other surface mount solder joints and fine pitch leads which are difficult to fully examine.

Real time X-ray is achieved by placing a board assembly between an X-ray tube and a camera. As the X-rays are transmitted some are absorbed and some pass through the sample and are picked up by the X-ray sensitive camera. The video signal is then passed through an image enhancer to allow inspection and interpretation of results. The resulting output will display a grey scale image that represents differences in either density or thickness of the material.

If there is an increase in the density the image will appear darker. If there is an increase in thickness the image will again appear darker.  X-ray inspection may be used to confirm the correct solder joint formation after reflow soldering on all visible and invisible joints; it may also detect voiding. The techniques will also allow alignment of component terminations to be assessed. 

As with conventional assessment the use of different size pads on the same part can affect solder joint assessment and should be avoided.  Inspection on all components other than BGA should be conducted using normal inspection techniques prior to X-ray evaluation. 

This reference standard is designed to accompany an EPS training video on X-ray inspection. The video contains most of the X-ray images provided in this guide. The test images in this guide are contained on the video and are used to assess staff understanding of the images obtained during X-ray evaluation. The assessment section is at the end of the video and may be used independently of the main content of the tape.

Visual Inspection Criteria

All solder joints should meet the minimum visual standards covered in international or internal company standards. The specific joint measurements contained in the relevant soldering document may be used for X-ray inspection if the system is capable of either manual or automatic measurement.

As a guide the following visual inspection criteria may be used as a reference during X-ray inspection:

Chip component

A solder joint should be visible around the complete end termination of the component. In the case of capacitors the solder joint will also be visible on the sides of the part.

Gull wing leads

A solder joint should be visible around the complete lead in contact with the pad surface. A solder fillet should ideally be visible at the toe of the lead and the heel.

J leads

A solder joint should be visible around the complete lead and pad on the surface of the board. Ideally there should be a solder fillet visible at the front of the J lead and at the back of the lead.

Castellation terminations

A solder joint should be visible across the complete component metallisation. The solder joint should extend across the complete pad surface.

A set of A3 colour wall charts covering visual inspection standards for surface mount components are available from EPS.

X-Ray Inspection Criteria
     
A sample of two boards should be examined from each batch being produced during normal in process inspection. This should also be done when changing temperature profiles or when setting up new product profiles. Voiding is the most common fault detected using X-ray inspection. Voiding is normally a fault of the profile peak temperature or the time above liquidus temperature of the solder paste alloy. 

Some solder paste formulations are more likely to void than others and may require specific profile conditions. Double sided reflow products often exhibit voiding on second side reflow if the same profile is used for both sides.

A satisfactory example of the following four terminations of solder joint viewed using X-ray inspection is illustrated here for reference.

Chip component terminations with satisfactory solder joints on each part. There is no evidence of voiding in the fillets or beneath the end metallisation.

Gull wing terminations with satisfactory evidence of toe fillets, side lead fillets and heel fillets which are not normally visible during manual optical inspection. No evidence of voiding.

J lead terminations with evidence of side fillets, heel fillets and no evidence of voiding in the fillets or at the base of the lead.

BGA solder ball terminations with consistent round shape, an inner and outer ring illustrating wetting of the pad and no evidence of voiding in the fillets.

Inspection of Ball Grid Array

Inspection of the solder joints should start at the centre of the BGA. This area is the most likely to be the last point to reflow during soldering. It is the most likely area to exhibit voids, non reflow or component delamination. If X-ray is being used after rework the whole area beneath the part should be scanned.

All BGA termination points should be circular in appearance and consistent in size. Measurement of a centre ball location and four outer row positions will allow confirmation of complete reflow. BGA termination pads may include a wetting indicator.  If this is the case it will make solder joint inspection easier to assess. A wetting indicator is a minor change to all pad shapes or a track from the mounting pad which is left exposed. In each case the solder paste can wet away from the main pad in a controlled manner, wetting may then be confirmed by X-ray. 

The maximum void size in any one termination will be less than 10% of the minimum joint dimension. In the case of multiple voids the maximum area will be less than 10%. 

Inspection of Fine Pitch QFP (Quad Flat Pack) 
and PLCC (Plastic Leaded Chip Carrier)

Inspection should commence from one corner of the device and scan around all four sides. Attention should be paid to the presence of heel fillets, side fillets and possible toe fillets on gull wing leads.

Toe fillets will not always be visible during inspection due to the lack of wettable area on the lead tip. The heel fillets should be consistent in size. The heel fillet is the area which will be subjected to stress during any mechanical or thermal cycling. Voiding may also be present under the lead. Toe and heel fillets should be viable on all J leaded devices.

The maximum void size in any one termination will be less than 10% of the minimum joint dimension. In the case of multiple voids the maximum area will be less than 10%. 

Inspection of Passive Components

X-ray inspection of passive components should be left until last as they will normally be satisfactory if all other parts are confirmed as completely reflowed. Due to their small mass they are likely to reflow before any other component and less likely to exhibit voids. They may exhibit voiding on second side reflow operations.

When a chip component has successfully soldered it will have evidence of a fillet on the end terminations and possibly on the side terminations. The solder joint area under the chip termination should also be assessed.

The maximum void size in any one termination will be less than 10% of the minimum solder joint dimension. In the case of multiple voids the maximum area will be less than 10%

Inspection of Small Active Components

Small active components like SOT23, SOT89 and SOIC, (Small Outline) devices are again less likely to exhibit poor reflow. Their low mass makes complete reflow of these devices relatively easy. It is possible to see voiding on SOT89 components on the centre paddle termination.

The maximum void size in any one termination will be less than 10% of the minimum joint dimension. In the case of multiple voids the maximum area will be less than 10%. 

X-Ray Inspection Defects

As well as providing a standard for X-ray inspection of solder joints this guide provides examples of typical defects visible using X-ray. These include insufficient solder, solder balls, solder splash, non reflow and bent leads.

The following process defects illustrate the possible problems which may be easily detected by X-ray inspection:

Satisfactory Gull wing solder joints which exhibit minor voiding on the edge of the joints and under the lead visible as white dots. 

Unacceptable gull wing joints showing misplaced leads, lack of solder on one pin with limited side and toe fillets.

Unacceptable chip termination showing lack of one end fillet, solder ball under device and minor voiding under chip metallisation.

Unacceptable chip open circuit due to "tombstoning" of the component during reflow.

Unacceptable BGA with inconsistent shape of the ball terminations. Solder balls are also visible under the component.

Satisfactory heel, side and toe fillets on SOT23 with minor voiding in the side of the joints.

Two lifted components, one missing component on the top right of the image; all other terminations are satisfactory.

Assessment of X-ray solder joints is relatively easy but requires either experience or good visual inspection criteria. Examples are provided on the accompanying video tape and also included as evaluation sheets at the end of this document for operator assessment.  Examine the examples in conjunction with the last section on the video tape and indicate if they are considered to be satisfactory or unacceptable. 

Further copies of this inspection guide are available from EPS to assist operator training courses. A set of A3 wall charts covering BGA inspection criteria using X-ray equipment are also available.

If you are facing production problems then EPS can provide your company with a defect investigation service. Contact us on Tel: 01245 351502 Fax: 01245 496123 or Email: eps@bobwillis.co.uk