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By Glenn Dody
Solder-bump-array technology, as on BGAs and CSPs, offers many advantages (and some disadvantages) that must be understood to prevent serious problems.
The ball grid array's (BGA) hidden solder joints are often falsely blamed for board test failures. In one study, 75 percent of all BGAs removed turned out not to be the cause of test failure but were accused because of the inability of the test technician to identify the real cause.1 The big, high-pin-count device with hidden solder joints was easy to blame when the test program could not pinpoint the problem.
Design for TestA test program should be designed specifically to pinpoint BGA problems to prevent wasted time and the loss of high-value devices. Boundary-scan technology should be used if possible. Powerful software tools are available to support the factory engineer in generating boundary-scan applications.2 If tester availability is a problem (and there is no time for a detailed boundary scan on each production board), a "streamlined" standard production test can be augmented for specific BGA testing either off-line or as time becomes available. This special test can be run when a BGA fault is in doubt. Another valuable tool is a small, real-time X-ray machine, which can positively identify solder shorts and confirm a decision to remove a BGA.
BGA Pad Types and SizesSoldermask-defined (SMD) vs. Nonsoldermask-defined (NSMD). NSMD pads are commonly used on most surface-mount devices. The SMD pad, invented by Motorola Inc. for mounting BGAs on very thin boards exposed to bending forces during installation, features larger pads with their edges covered with soldermask: The greater strength eliminates a common failure mode. There are advantages and disadvantages for each pad type.
The SMD pad's advantages include stronger mechanical strength and the greater copper surface provides higher heat dissipation to the board while achieving more heat resistance, i.e., multiple reflows, in rework. Among its disadvantages: features potential stress-concentration points that can affect long-term reliability, less space is available to route traces between pads, a bad HASL-coverage potential exists together with foreign-matter entrapment on pads, and dimensional tolerances on solderable openings are generally poor. Unless mounting on thin boards subject to bending, use NSMD pads.
BGA pad sizes. The Table lists BGA solder-bump pitches and corresponding recommended PCB pad sizes. On the size shown for 1.0 mm pitch, reliability and manufacturability are about equal, but board cost could be significantly higher for the larger pad sizes. This is because lines and spaces are reduced beyond the capabilities of some board vendors.
Concerning ceramic packages (CBGA), solder-bump size is 0.035" in diameter while PBGA is 0.030". The larger pad size is very important in obtaining maximum long-term solder-joint life because of the considerable thermal mismatch between the ceramic package and the organic PCB.
Design for Wave SolderingWave soldering is often required after surface-mount components have been joined on the board. BGA solder joints must not be allowed to reflow in this secondary operation; if they reach the melting point, some will open, requiring the removal and loss of an expensive package. The designer must provide a means to prevent full-array PBGAs from being subjected to wavesoldering operations. One strategy is to mount them in a location on the board that is protected from the full heat by a special wavesolder pallet. Another is the use of a periphery BGA that has no vias closer than 0.200" from its pads.
The BGA is made up of two laminated materials: a miniature PCB on the bottom with solder bumps attached, and a plastic cap molded in place across the top. These two materials match well in coefficient-of-thermal-expansion terms; when uniformly heated in the reflow process, both materials expand at the same rate and the package remains flat. The wavesoldering operation, on the other hand, causes the package to be heated from the bottom at a much greater rate.
The preheat section of the wavesolder machine is usually set to raise the board to about 100°C before entering the wave. (The temperature of the wave is 230°C or higher.) As the bottom of the board enters the wave, the higher temperature is transmitted through the substrate (the large number of vias under the BGA improves transmission). The bottom of the BGA heats rapidly, exerting more expansion forces than that of the top, which would make the outside edges of the package warp upward if the constraint of the solder joints around the outside of the package were absent. If vias are very close to solder joints, heat will be transmitted freely and the solder will melt, allowing the package to warp and open solder joints (Figure 1).
Figure 1. Blocking secondary reflow in wave soldering. With vias close to solder joints, heat will flow freely, endangering joint integrity and promoting warpage.
This unique character of the BGA has caught many new users by surprise, especially with designs that cannot be changed quickly. One effective quick fix is manual application of Kapton tape under the BGA area to block the molten solder from the heat-transmitting vias.
CBGAs are not as sensitive to the problem. Because the body is composed of a single piece, it comprises a large thermal mass that heats slowly and will not warp. The wavesolder operation probably can be optimized to prevent reflow. In contrast, the PBGA comprises a low thermal mass, which heats rapidly and features open vias very close to the pads.
Assembly Process PitfallsMoisture sensitivity. One of the more frequent problems new BGA users encounter is caused by moisture sensitivity of the package. Usually more moisture and delamination sensitive than conventional quad flat pack (QFP) packages, the effects are generally more catastrophic. Because solder spheres cover the entire bottom surface of a full-array package, delamination causes this sensitive portion of the package to bulge downward, exerting excessive force on the spheres in the middle. Exerted on only a few spheres, the force frequently is sufficient to mash them together, causing shorting (Figure 2). X-ray can clearly show the shorting defect, and delaminated or "popcorned" packages show a distinctive pattern.
Figure 2. Delaminated PBGA vs. PQFP caused by moisture sensitivity. Delamination can cause a "mashing" of some of the solder spheres, leading to shorts.
In contrast, the QFP will usually delaminate below the lead-frame flag, and even though the bottom of the package may bulge downward, there is usually no shorting or opens because the leads hold the package above the board.
Also, because they are located at the interface most prone to delamination, wire bonds may be broken on the BGA. QFP wire bonds are not usually in the area of delamination and not as likely to be damaged.
At one time or the other, most in the profession have violated the strict requirements of "time out of the bag" on QFP packages without noticeable problems. The package shown in Figure 2 can be electrically good and reliable in the field even with a crack in the bottom simply because the die and wire bonds are not in the delaminated area. Because there is frequently no negative feedback on delaminated QFPs, complacency in requiring strict compliance of exposure guidelines has become a problem. However, because of possible catastrophic failure of more expensive BGAs due to excessive moisture absorption, strict compliance to time-out-of-bag rules must be observed.
PCB and component handling. Handling bare boards or the solderable surfaces of any component with ungloved hands must not be permitted. Finger oils can spoil solderability and cause random open solder joints. Root cause identification of this defect is difficult. To preempt their possibility, all personnel handling bare boards or components should be required to wear gloves or finger cots.
Solder-paste characterization. Beyond simply accepting the solder-paste vendor's recommendation, it is highly recommended that the material be characterized for the process at hand (per IPC J-STD-005, "Requirements for Soldering Pastes") using the specified viscosity, slump, tack and wetting test procedures.
Placement force. BGA solder spheres should not be forced into the solder. The relatively large size of the sphere will displace a significant amount of paste if pushed to the board pads and threaten a movement of material sufficient to cause shorts. For QFPs, it is desirable to apply enough force to push all leads into the solder paste where they will receive a cleaning action of the flux during the preheat stage of reflow and take the solder uniformly. Preheat cleaning action is not important for collapsing BGA bumps because the drop will expose pure nonoxidized solder to make a good joint. Many placement machines will apply the same force for BGAs and QFPs unless special programming is applied.
ReflowTemperature profile characterization. This is a most important operation that is often "blotched." Setting up a profile board correctly is tedious and requires careful work, a task that is even more critical when high-mass components are involved. Furnaces vary greatly in uniformity and stability. Hence, careful profiling is a must.
Most BGAs (like most QFPs) can take any standard reflow soldering temperature. The important factor in determining the heating profile is the solder-paste-flux characteristics. Some require a long preheat to activate, while others burn up before reflow in a long cycle. The solder paste vendor's reflow recommendations should be a starting point. Actual characterization should be performed in the process at hand by IPC-STD-005 to ensure the best solderability of all components and the board.
Selection of profile board, components and thermocouples. The profile board must be a fully populated production board to test and characterize the actual thermal load. Trying to save costs by not using a fully populated board, or using a close substitute assembly or "clip-on" thermocouples, can cost much more in rework and scrap product. The idea is to set up a permanent profile board that can be passed through the furnace many times to ensure a continuously uniform operation. Selection of the components to monitor should include:
- The highest mass part near the center of the board.
- A low-mass part on one corner of the leading edge of the board.
- A low-mass part on the diagonal trailing corner.
- Any ceramic semiconductor package (BGA or QFP) regardless of the location.
- Any other critical part.
Use of new, fine-gauge (36-gauge wire preferable), calibrated, welded bead thermocouples are recommended. Never use broken wire that has the ends twisted together.
Actual temperature measuring point. The temperature to be monitored must be kept in mind to know and document the precise temperature of the solder joints (not the board or the package). There may be the rare occasion in which the body temperature of an especially temperature-sensitive device must be monitored. The purpose of profiling is to achieve the maximum solderability that a specific solder-paste flux can provide, i.e., to set the optimum temperatures and dwell times, both in preheat and peak times, as recommended by the solder-paste vendor (possibly modified by in-house paste characterization).
Thermocouple-attaching TechniquesQFPs. The profile board should be reflowed to keep everything in place. (One should "guess" at a profile for this.) The best way to attach a thermocouple on a QFP is to unsolder a lead, remove most of the solder and place the device bead between the lead foot and the PCB pad. A small amount of thermally conductive, electrically insulative epoxy should be used to fix the thermocouple in place. The volume of epoxy should not be more than the amount of solder connecting a normal joint. Use of so-called "high-temp" solder should be avoided. There are two good reasons for this: The thermocouple material will not take solder using normal solder flux. Thus, a large amount of solder must be used to encapsulate the thermocouple to hold it in place, which is likely to change the thermal mass of the area and affect the temperature reading. Second, high-temp solder is electrically conductive. If a connection between the two thermocouple leads above the welded bead is made, temperature will be sensed at that point and not between the lead and pad as intended. Another way inaccurate readings can occur is to allow the bare thermocouple wires to get twisted above the welded bead, giving an indication of the temperature at the twist point instead of at the solder joint. Kapton tape or epoxy should be used on the insulated thermocouple lead at various points on the board to hold it in place and to keep mechanical strain away from the welded bead measuring point.
PBGAs. The plastic BGA is a low-thermal-mass package and, because of the many large solder-bump attachments, the temperature will follow the board temperature fairly well. A good and easy way to place the thermocouple is simply to thread the small thermocouple bead between the bumps, between the package and the board, to get it near the center of a full-array package. On periphery BGA packages, the thermocouple should be placed in the center of the rows of solder joints on one side.
Figure 3. Thermocouple-attachment technique for CBGAs. Two thermocouples should be attached: one in the center and another in a corner.
Ceramic BGAs. CBGAs can be heavy thermal masses and thus require special attachment techniques. CBGA temperatures can vary from the corner of the package to the center, and the amount of variance can be significant in the case of a rapid temperature-ramp rate. To be on the safe side, two thermocouples should be attached to the packages one in the center and another in a corner. The following special attachment procedures should be used to get the thermocouples embedded into solder joints on this heavy, expensive package (Figure 3).
1. Before the CBGA is soldered on the board, a drill just big enough to allow the fine-gauge thermocouple bead to go through the hole should be used to drill two holes through the board: one through the center of a pad in one corner of the BGA site and the other in the center of the site.
2. The normal amount of solder paste is placed on the BGA site, followed by placement of the CBGA and reflow.
3. From the bottom of the board, the same drill bit placed in the previously drilled holes should be used to drill about 0.020" into the solder ball of the BGA above.
4. Place a very small amount of thermally conductive, electrically insulating epoxy in the hole and push the thermocouple through the hole up into the BGA solder bump. This is the exact point where the temperature is to be measured.
5. Kapton tape or epoxy is applied to fix the insulated thermocouple wire on the board to avoid mechanical stress on the thermocouple bead.
Voided Solder JointsVoids occur normally in most solder joints made with solder paste. All QFP solder joints, for example, have voids but are usually very tiny. They occur from the flux being mixed with metal and volatile gases given off during reflow, and getting trapped inside the molten metal. A BGA solder bump contains much more volume than that of a QFP joint, which permits multiple small voids to combine into larger ones. Factors that affect void formation in BGAs were studied and reported in a paper.3 And long-term solder-joint testing in thermal cycling at Motorola Inc. has shown that, generally, a large amount of voids provide as good or better solder-joint life than joints free of these "defects." One study showed that a voided area up to 24 percent of the pad caused no negative effect and PBGA solder joints with voids had 16 percent better reliability than those without voids.4
Nevertheless, new BGA assembly processes should be evaluated for void content via X-ray. If more than 50 percent of the solder joint, as viewed from the top, consists of voided area, some action might be warranted. Certain solder-paste fluxes create more volatile gases and larger voids. Water-soluble fluxes in solder pastes usually cause more voids than no-clean fluxes. The solder paste vendors are well aware of the void issue with BGAs and can recommend materials that will reduce voids.
Defect Identification and PreventionEven if all of the above has been accounted for in design and process optimization, there will inevitably be defects that must be reworked. The key to reduction and elimination of defects is to identify each cause and to provide feedback to the point of initiation. To facilitate early identification, production workers should be encouraged to identify problems (or potential problems) as early in the process as possible.
Microscope InspectionsThe naked eye is incapable of seeing all the possible defects that cause a solder defect, especially in the area of solder-paste printing and rework-site cleaning. A good microscope, capable of 20 to 60X enhancements, should be available for inspecting the first board off the printer, setting up a new board or the beginning of a shift. Likewise, a microscope should be located in the rework area and every solder-paste print should be inspected. These are manual operations that can have minor defects that can spoil the work. The microscope inspection applies to BGAs but is doubly important for fine-pitch leaded parts. The finer the dimensions, the more important the magnification step.
SummaryThis article concentrates on the problem areas of BGA assembly but does not address the device's positive attributes. The BGA has proven to be a highly effective technology that will eventually dominate the packaging device industry. All negatives and disadvantages can be overcome with understanding. BGAs provide ease of assembly and extremely high yields. It behooves all to master this technology to get ready for steadily decreasing solder-bump pitches that will soon match flip-chip densities.
These are the key points to avoid problems using BGA:
- Maintain absolute control of moisture-sensitive BGAs' "time out of the bag."
- Initial design for manufacturability includes:
- Best possible test routines to avoid BGA false alarms
- Use of wavesolder strategies to avoid secondary reflow
- Ensure that correct pad type and sizes are used
- If using CBGAs, ensure minimum solder-paste volumes are provided.
- Personnel must not handle solderable surfaces with bare hands.
- Characterize solder paste per IPC J-STD-005.
- Excessive placement force on BGAs can cause solder shorts.
- Reflow temperature profile characterization must be done carefully and accurately, especially with heavy-mass CBGAs.
- Identify root cause of all defects and work to prevent them.
- Use microscope with good lighting to inspect the first solder-paste print in production, and, in rework, inspect every part-removal site, after printing solder and before placement of new part.
REFERENCES1 Susan Fauser et. al., "High-Pin-Count PBGA Assembly: Solder-Defect Failure Modes and Root Cause Analysis," Compaq Computer Corp., Proceedings of SMI '94.
2 Ray Dellecker, "Boundary-Scan Programming and Test," Data I/O Corp., Proceedings of NEPCON Texas '98.
3 Anthony A. Primavera et. al., "Factors that Affect Void Formation in BGA Assembly," Universal Instruments Corp., Proceedings of IPC/SMTA Electronics Assembly Expo '98, Providence, R.I.
4 Donald R. Banks et. al., "The Effects of Solder-joint Voiding on PBGA Reliability," Motorola Inc., Proceedings of SMI '96.
GLENN DODY may be contacted at Dody Consulting, 9201 Honeycomb Drive, Austin, TX 78737; (512) 288-2747; Fax: (512) 301-7131; E-mail: firstname.lastname@example.org.