Some months back I asked the question on Technet was there an x-ray inspection
standard, No Answer. So  I produced one, this is the updated document. Any one
have anything to add please send separate comments, not the whole document and
suggest where any changes may go.

Bob Willis
Electronic Presentation Services
2 Fourth Avenue, Chelmsford, Essex CM1 4HA. England.
Tel: (44) 01245 351502
Fax: (44) 01245 496123
Home Page: http://ourworld.compuserve.com/homepages/bwillis
Email: [log in to unmask]


INTRODUCTION TO X-RAY INSPECTION

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.