Wednesday January 26th 2005
ITRI Innovation Curo Park, Frogmore, St.Albans
Area Array Technology is part of mainstream electronics but very little inspection criteria is available in the industry. The assembly process for BGAs is fairly well understood for tin/lead alloys but with the move to lead-free assembly there are critical issues to consider during inspection. The workshop was presented by Bob Willis and Keith Bryant with both theory and practical demonstrations during the day.
The workshop covered optical and x-ray inspection of solder joints. It includes an introduction to the lead-free assembly process with specific attention to BGA and area array devices. It provided a step by step guide to the procedure of inspection for optical and x-ray showing you how to do it. Inspection criteria are included for x-ray and visual criteria on different lead-free terminations and pad surfaces.
Optical inspection should follow a simple procedure like this:
BGA OPTICAL INSPECTION
Visual examination of Ball Grid Array (BGA) solder joints are best achieved using a endoscope system which are available from a number of suppliers world wide. These systems have been specifically designed to allow solder joints to be examined between the base of the device and the printed board. Often there are limited standoff heights and overhang restrictions due to the body of the package. The close proximity of components to the body of the device to be examined may also limit the opportunity for inspection particularly when scanning along the sides of devices.
Although not covered in this article X-ray criteria is available and is the popular alternative to optical inspection. X-ray was of course the first choice for BGA inspection when area array devices were first introduced and is commonly used in industry. The author conducts regular training courses on inspection criteria for both optical and X-ray and produces the first training videos and interactive CD-ROMs on the subjects.
It is possible to examine the outer row of ball terminations with traditional inspection equipment depending on standoff height and the overhang on the side of the BGA. However this is not feasible to use on inner ball terminations or where the overhang obscures the terminations when tilting the board assembly.
Both X-ray and these specialist inspection systems have limitations on the process issues and defects they can detect. If the budget is available the ideal situation is to have both methods of controlling and defining the manufacturing process or have samples from production examined by an external source.
Ideally some visual standards should be provided for operator reference during final inspection or during process monitoring. This is true for both X-ray and optical inspection as assessing joints can be daunting for staff members new to these techniques. Criteria should include conventional BGA, Chip Scale Package (CSP) and flip chip terminations with tin/lead or lead-free eutectic and high temperature ball terminations.
Examples of the soldering quality on high temperature balls often featured on Ceramic Ball Grid Arrays (CBGA) should be included due to the difference in surface appearance. With balls that become liquid with the paste there is little evidence of any demarcation between the two solder surfaces. With high temperature balls like 90%lead/10%tin there will be a demarcation line between the solder alloy and the ball. This is because the ball does not reflow but the paste does. There can be differences in the height the solder rises up the ball due to the paste volume and the wettability of the terminations.
In the case where lead-free termination BGAs are used with tin/lead paste the ball may not completely reflow at 210-225C. These temperatures are quite normal for a surface mount assembly with tin/lead. Tin/Silver/Copper Sn/Ag/Cu or SAC balls will have a reflow temperature of approximately 217C.
Example of conventional and high temperature ball terminations joints. The difference in the appearance is due to the ball alloy.
The simplest form of inspection for BGA alignment is again a feature on the PCB as the joints and the pads are not easily seen. It is good practice to use BGA alignment marks on the corner of the package.
If measurements are to be taken during inspection then the following can be used as a guide. It is good practice to taken some measurements on typical BGA devices during early production runs against specific devices and their locations. If problems do arise these measurements can be useful for reference and for comparing reworked devices. Remember that the measurements here are only a reference as the final dimensions will depend on the original ball size on the device, paste thickness and pad surface on the board. In the case of reworked devices if paste or flux only reflow was used for replacement it would affect the final ball size.
SIZE OF BALL TERMINATIONS
Ball diameter should be equal to or larger than the original ball diameter on the BGA prior to reflow due to the addition of paste. In the case of high temperature lead/tin ball termination's which do not reflow there should be no change in ball size.
STANDOFF HEIGHT OF BGA PACKAGE.
Measure the difference in height between the board and the base of the BGA laminate on a minimum of two of the four corners. Compare the variation in height of the BGA by scanning along the length of the BGA on two sides. The height will be equal to or less than the original ball height. The stand off height will reduce due to the size of the device and its weight.
Variations in height between the centre and the edges of the BGA may indicate warpage of the fibre glass device or PCB. It can also indicate voiding in the ball terminations. This is more commonly noted if all termination measurements are taken. This technique is much simpler with X-ray as it can be automated. Warpage can occur up or down in the centre area of the part. Although uncommon warpage can even be seen on ceramic packages.
The table above shows some typical measurements taken on Plastic Ball Grid Array PBGA devices.
CHECK FOR SOLDER SHORTS
Scan along two adjacent sides of a BGA using light from the opposite side of the devices. There should be no restrictions to viewing each of the termination outlines. If light is restricted short circuits may be visible under the device. Viewing two sides ensures that shorts can be seen; viewing one side only shorts can be missed.
Example of satisfactory joints, unfortunately there were multiple shorts on the device but only in one plane.
Check for complete reflow of solder paste and ball terminations on tin/lead or lead-free joints. In the case of high temperature balls on ceramic parts the ball will not become liquid. During soldering the paste will reflow and allow wetting to take place between the high temperature ball and the pad surface. There will be a distinct line between the ball and the solder at the interface. This is due to the different metal surfaces being joined.
CHECKING FOR CRACKING OR POPCORNING
BGA failures do occur for a variety of reasons, they may be component, printed board, process or design related. On some occasions both X-ray and optical inspection can easily provide the answer and that is when other techniques must be used. Microsections, dye penetrant and acoustic scans may assist is pinpointing the root cause. The essential aspect in all analysis is the experience of the engineer who is tasked with finding the root cause of the failures. There is, however, no substitute for practical experience.
The following is a good procedure for inspecting boards using x-ray:
X-RAY INSPECTION PROCEDURE
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 time above liquidus temperature of solder paste alloy. X-ray inspection should be conducted after all rework of area array devices.
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. Pads with micro vias included, lead-free pastes and some surface finishes are also know to contribute to void formation. Reference may be made to IPC 610 and IPC 7095/7095A for reference standards and criteria.
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 broadly circular in appearance and consistent in size. Measurement of a ball under the centre of the BGA and four outer positions could allow confirmation of complete reflow. These measurements could be compared with historical data. 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. Use of oblique angle X-ray views is ideal as it will often help the investigation of the joint interfaces by allowing a better observation of the interface, this might otherwise be obscured if the view is limited to top down.
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%. IPC does have less stringent criteria.
INSPECTION OF FINE PITCH & 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 possibly 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 visible 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%. IPC does have less stringent criteria.
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%. IPC does have less stringent criteria.
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%. IPC does have less stringent criteria.
Delegates had the opportunity to win a CD-ROM which includes inspection methods and procedures for BGA devices. The CD also covers many defect examples and causes as well as illustrating the reflow of different terminations with unique micro video clips.
Modern Lead-Free Assembly Processes
Lead-free Alloys - BGA - Area Array Reflow
BGA In Process Monitoring
Inspection Techniques
Inspection Procedures - Optical Inspection - X-Ray Inspection
Defect Guide
IPC Inspection Criteria
Further Information
For more information about BGS inspection and Soldertec Services please contact Zoe Sullivan on +44-1727-871 307 or zoe.sullivan@itri.co.uk
|
