September, 2017
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Continued from previous page solder ball to open.
Although we keep improving our algorithms to find this kind of defect, it is still a manual process in the industry. In order to obtain a good image, an X-ray inspection machine with high magnification, resolution and geometric maneuver- ability is needed. A wide range of geometric maneuverability will allow side views, as well as a tilted view of the BGA assembly. This is called 2.5D inspection.
Open Circuit. An open circuit, or “open,” occurs when the solder ball fails to collapse and doesn’t touch the pad on the board. Unlike the head-in- pillow, an open is always an open and can sometimes be discovered during electrical tests.
Opens are either caused by insufficient solder on the pads, or by BGA-to-board misalignment and coplanarity problems. It is also possi- ble that the solder on the pad ran to the trace, thus leaving the pad with- out enough solder height to allow the collapse of the solder ball. To identify opens, use an X-ray machine with enough magnification and resolution to properly identify the separation between the solder ball and board pad. The machine also needs 2.5D inspection capabilities.
Solder Splatter. It is not uncom- mon to find little solder balls around an assembled board. These solder splatters are especially dangerous if found in between the solder balls of the BGA. This splatter can easily move around during the normal use of the board and cause a short in the BGA. Humidity is usually the culprit here. Moisture on the solder paste can cause it to explode during reflow. Failure to bring the solder paste to room temperature prior to reflow can cause moisture to condense on the paste. Solder paste also has solvents that need to outgas gradually, so make sure the ramp-up of the reflow profile is not too fast. Another source of splatter is solder residue under the stencil, so only use properly-cleaned stencils.
Solder Mask and Metallization. The solder mask’s job is to keep the solder paste constrained to a specific area. However, when the solder mask is not properly deposited onto the bare board, it can cause the molten solder paste to run onto the trace.
The proper diagnosis of this problem is a function, in part, of the board layout. When designing the board, there are a couple of options when it comes to BGAs and solder masks. The mask can either be designed on the pad or not.
If the solder pad is defined, the edges of the pad are covered with sol- der mask and proper solder mask adhesion and deposition must be ver- ified. If the solder mask is not defined on the pad, the pad edges are exposed and alignment becomes crit- ical. If the solder mask is not cen- tered on the pad the trace can be overexposed, thus causing solder run and potentially an open or short cir- cuit.
Voids. The percentage of voiding inside each BGA solder ball is the key performance indicator in the BGA assembly process. The IPC-A- 610D Standard1 (Acceptability of
See at SMTAI, Booth 1215
Electronic Assemblies) categorizes greater than 25 percent BGA joint voiding as a defect.
Voids are caused when the entrapped flux from the solder paste vaporizes and rises in the molten sol- der paste. Due to buoyancy, voids are usually located near the package interface.
This problem has both a process and a chemistry solution, and both solutions must be investigated. One of Creative Electron’s customers recently solved a serious voiding problem by applying a longer dwell time below liquidus, rather than jumping to an expensive solder paste.
To measure the void inside each
solder ball, first define where the voids are. Then the ratio between total ball area and void area can be calculated.
This percentage will determine if the assembly passes or fails based on a specific criterion.
All these calculations can be done using software. Creative Electron’s TruView software auto- matically identifies solder balls and measures, on a per-ball basis, the total void, the largest single void, the eccentricity (how round the shape is), the diameter, and the area. The software also includes tools that go beyond pass/fail criteria and visualize individual balls using 3D rendering.
Page 73 X-ray Uncovers the Best and Worst of BGA Assembly
Solder Ball Shape. The ideal shape of a solder ball is round. When look- ing at the X-ray image of a BGA assembly we hope to see a grid of dark circles. Unfortunately, that’s not always the case.
The main reason behind mis- shapen balls is the alignment between the BGA and board. If the solder ball is not right on top of the pad on the board, it will elongate (due to surface tension) to make the metallurgical connection, thus creat- ing an oval-shaped profile. Another issue is the excessive deposition of solder on the pad. This excess solder has nowhere to go, so it balls up in different shapes and forms.
Continued on page 75
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