Manufacturing challenges with plastic overmolded packages
07/01/2009
Anwar A. Mohammed, Infineon Technologies, Morgan Hill, CA USA
Plastic packages are getting very sophisticated. Significant improvement has been achieved in the last few years. Plastic packages are now being designed to handle over 100 watts of power. Notable improvements in material options and equipment availability for die attach, wirebonding and the molding have enhanced the quality and reliability to very acceptable levels. Advances in process monitoring equipment employing SAM technology and powerful X-rays, have minimized the occurrence of delamination and die attach voids. Several key manufacturing challenges associated with the assembly of plastic, overmolded packages are presented along with an exploration of material choices and bonding technologies and their impact on package reliability.
The assembly process for plastic over-molded packages typically contains wafer mounting, die attach, wire bonding, molding, laser marking, and DTFS. Each step is discussed below.
Wafer mounting. Wafer mounting is the process of converting the finished semiconductor wafer into a platform that could be used for introducing the die to the die attach machine, also known as the die bonder. The die attach machine can accept the die in a wafer ring frame or as a set of known good dies contained within a waffle pack. Wafer ring frames are more efficient because the dies can be electrically and visually inspected and mapped electronically in the form of a wafer map. Wafer maps are electronically portable and can be employed by a die bonder to select the appropriate dies or die sequence for manufacturing. Wafer mapping is a recommended best practice used by most savvy wafer manufacturers. A quality control gate at the end of the wafer mounting process, consisting of a high magnification, 10???30??, visual inspection is very prudent. This helps to filter out defects such as contaminated dies, chipped dies, cracked dies, and dies with sawing errors such as shallow cuts, wide cuts, and uncut dies.
From the reliability perspective, the above QA gate is very important because any micro crack initiation caused by improper handling or aberrant sawing needs to be filtered out here. The sawing process may employ mechanical sawing or laser sawing or a combination of the two but if it is not cleanly done, micro cracks are formed that are very hard and insidious to detect and become manifest only after exposure to temperature excursions making it a very costly discovery, usually at the customer site.
Die attach. The die attach process is the heart of the package assembly process???it has significant ramifications in the performance, reliability and the cost of the package. Some of the common die attach equipment available in the market today are supplied by ASM, Datacon, ESEC, K&S and NEC. There are various options available such as epoxy, solder or a eutectic materials (e.g., AuSn or AuSi) for use as the attach material. Because of the criticality of this process, many different process parameters such as the bond line thickness (BLT), die shear, and the die attach material coverage, need to be closely monitored.
Epoxies are commonly used as die attach materials for plastic packages especially for low power packages. This is an established process with years of reliability data easily available. The common mode for epoxy application is a screen printing process or a syringe dispensing process; however there are many different approaches available in the industry. The advantages of an epoxy attach would include a proven process with an acceptable epoxy coverage beneath the die. Many epoxies have been qualified to pass the typical military standard tests. However, for an electronic application where the heat dissipation from the die is critical, epoxies are a very poor choice. Some epoxies also tend to delaminate after temperature cycling. Most epoxies tend to have a degradation point that can withstand 260??C for at least 5 minutes to be compatible with the lead-free soldering process. One can also find epoxies that can handle high temperature exposure >300??C to accommodate die attach temperatures for materials such as AuSn eutectic.
Solders are also frequently used as die attach materials. The high lead, soft solder process using alloys such as 95Pb5Sn solder, has been around for decades. The advantages of solder attach include a proven and reliable process that can dissipate heat from the die much more efficiently. The high-lead 5Sn solder has a thermal conductivity of ~35W/mK. Most epoxies have a thermal conductivity <5W/mK. There are epoxies that can work consistently and predictably with a thermal conductivity of ~20W/mK.
The microelectronic industry has embraced epoxies that have thermal conductivities >50W/mK, but these materials need more development to exhibit process repeatability. Solders add to the processing cost by necessitating the metallization of the die bottoms and the plating or metallization of the leadframes and substrates where the die is placed. The soldering process has to be followed by a post-cleaning process. Sometimes, because of poor solder wettability there might even be a need for pre-cleaning. Reliability issues linked with solders include solder fatigue, tin whiskers, and silver migration.
To keep up with demanding power requirements, some savvy companies rely on eutectic attach materials such as AuSn and AuSi for die attaching to obtain superior thermal performance. The die attach temperatures are much higher relatively: ~320??C for AuSn, and ~420??C for the AuSi; but the thermal conductivity is significantly better with AuSn performing at ~70W/mK and AuSi performing at >150W/mK. The disadvantage of this approach is that it is a very tricky and sensitive process with a narrow processing window that needs to be monitored carefully.
Die attach is central to the performance of the package and as such needs to be monitored very carefully. The bond line thickness, also known as BLT, is important to ensure that the die attach is consistent. If the die attach material were too thin, it would cause delamination, and a thick bondline would lead to poor thermal performance.
Die coverage is another important parameter to monitor. If the die attach material exhibits too many voids, it would compromise the thermal behavior. For most temperature sensitive applications, the coverage should be better than 90%. The die shear test would ensure that the die adhesion to the substrate or the leadframe is at an acceptable level. For packages used for wireless applications, the die placement would be especially critical because any significant variations would change the wire length, hence the impedance characteristic of the package, thereby influencing the RF performance. Improper die handling during die attach can cause a serious and insidious problem called micro-cracking. These incipient cracks can have many causes such as improper die pick up, or excessive die placement pressure, which cause the initiation of small microcracks that might pass electrical testing but manifest themselves as opens in the field, which is the worst place to detect a failure.
Wirebonding. Wirebonding is the assembly process involved with inter-connecting the die to the electronic package. There are many different wirebonding machines available in the market offered by suppliers such as ASM, Delvotec, H&K, Kaijo and K&S. There are a variety of proven bonding techniques, e.g., ball bonding and wedge bonding, and each has its own pros and cons. Gold is a popular wire material and is commonly used at wire diameters of .7 mil, 1.0 mil and 2.0 mils. With the price of gold at stratospheric levels, a lot of companies are switching to copper wirebonding. Aluminum wirebonding has been around for decades and is especially popular for high-power applications.
Wirebonding is a critical process, which, if not carried out properly, has reliability and performance implications. Some of the common wirebond failures include bond lifting, shorts caused by wire sweeping during molding, and cratering caused by excessive bond force. Another well known wirebond failure is the ???purple plague’ which is caused when gold wirebonds are attached to aluminum pads. Some sensitive applications such as the wireless packages necessitate that the wire looping shape, the wire heights and the wire lengths of the wirebonds should be highly repeatable. Any variations of more than two mils would lead to a compromised performance. Contaminations on the die could lead to very poor bond pulls; however, sometimes cleaning up the dirt or foreign material by exposing the die to plasma cleaning can make a significant difference. Special bonding techniques such as stitching and reverse stitch on bond (RSOB) can improve the bond reliability.
Molding. Molding is the process of enveloping and protecting the die and the wirebond by a mold compound. The selection of the proper mold compound is critical for the performance and reliability of the package. For wireless packages, the dielectric constant of the mold is an important consideration whereas for power packages, the thermal conductivity of the mold compound is a key attribute. Typical qualification tests such as temperature cycling (TC) and high-temperature storage (HTS) are very dependent on the molding compound used. Typical bonding failures include wire sweeping, delamination, incomplete filling, cracking, blistering and flashing. Mold delamination may be caused by contamination which may be removed by plasma cleaning just prior to the molding process.
Lasermarking. Laser marking is the process of engraving identification and traceability marks on the semiconductor packages. Marking can include the product name, the date code, the manufacturing lot code, and the company’s name and logo. The old process of ink marking is slowly being replaced by laser marking. Some common marking errors are missing characters, illegible characters, and mis-oriented characters. The date code and/or the manufacturing lot code are critical for maintaining material lot traceability of the wafer run, the leadframe material, the die attach material, and the mold compound.
DTFS. The DTFS process typically represents four distinct backend assembly processes:
- The deflash process where the extra mold compound, also known as flashing, is removed from the package.
- The trimming process, which consists of removing the dambars that connect the leads.
- The forming process where the lead is formed into a particular shape (e.g., gull wing).
- The singulation process where the tiebars are cut off leading to the final separation of the individual package from the leadframe.
Be careful about die and package cracking because of the excessive level of mechanical stresses created during this process
Monitoring equipment
The relentless improvement of equipment technology deserves a lot of credit in ensuring the improved reliability and yields of the assembly process. Measurement of the dried bond line thickness for the die attach process is very important and can be effectively carried out by equipment supplied by companies such as Olympus and Hisomet. It is strongly recommended that equipment with SPC monitoring capabilities be used.
Equipment such as the SONIX, which employs scanning acoustic measurement (SAM) technology, are valuable in monitoring the solder or the epoxy coverage of the die attach material. Powerful X-ray machines like the ones offered by XTEK also do a commendable job in meeting the same objective.
Equipment from suppliers like the Dage group can be used to monitor the wire bond pull, the die shear and ball shear results. Once again, it is strongly preferred to have equipment with SPC monitoring capabilities.
Material selection
Proper material selection is a cardinal requirement to ensure package performance and reliability. It all begins with the selection of the proper package design to guarantee the expected product performance, cost and reliability. The package will play a critical role in influencing the mechanical, thermal and electrical performance of the device. The next important decision is to determine the die attach material, which will play an important role in determining the thermal performance of the package and in ensuring that the package can successfully pass qualification requirements, e.g., temperature cycling (TC) and thermal shock (TS). If the package is used for a wireless application, the selected wirebonding approach will determine its thermal and RF performance. The selection of the mold compound will play a significant role in determining whether the product will successfully pass the high temperature storage (HTS) and temperature humidity bias (THB) tests. Most of the above decisions will determine if you have a package that is lead-free, ROHS compliant, green, or none of the above.
Conclusion
Plastic packages with enhanced thermal properties and significantly improved reliability and performance are now available. High-volume manufacturing techniques have considerably reduced the assembly and test cost. The proper choice of package designs, materials and manufacturing processes are critical in developing customized power packages. It would not be surprising to see more and more high power (>10 W) packages of the future assembled in plastic packages.
Anwar A. Mohammed is a packaging expert serving the industry for over 15 years. He is the author of more than ten packaging patents and is a senior staff scientist at Infineon Technologies, 18275 Serene Drive, Morgan Hill, CA 95037 USA; [email protected].