March 1998
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Unique Molding System Design Offers Superior Device Encapsulation
The development of an advanced liquid molding machine and a complimentary encapsulant improve significantly on the decades-old process of transfer molding.By Dr. Daniel Wong, Kras Asia Ltd., Hong Kong, Lawrence L. Plummer, Kras Corp., Fairless Hills, Pennsylvania, and Masaki Kitaoka and Masa Yanagi, Nippon Pelnox, Kanagawa, Japan A new, fully-automated liquid molding machine, the KALMS (Kras Automatic Liquid Molding System), has
been developed which employs the concept of stationary modules with moving molds. In addition to offering
higher throughput than traditional transfer molding systems, KALMS (Figure 1 ) eliminates many of the problems connected with older molding equipment.
Figure 1 - Stationary modules with moving molds can also be used in a linear layout with even geater flexibility, ast shown.
Concurrent with the development of the KALMS, a project to develop a new encapsulant resulted in a material for use with the KALMS that does not require mixing before molding, offers high stability below 120 °C, low viscosity at room temperature, long pot life and no required post-cure.
This paper discusses the design and development of the machine and the encapsulant and traces the history of earlier molding processes.
History
The encapsulation of electrical and electronic components with a variety of plastics has been a common process for more than fifty years. The early electronics industry employed encapsulation for device protection from the elements, electrical insulation and to provide an identifying shape. Components such as resistors and capacitors were subjected to the high pressures of compression molding with materials such as phenolics and putty-like alkyds.Soon after the historic invention of the transistor by Bell Laboratories, the need to package an axial-lead diode, specifically a varistor, surfaced. Initial attempts to compression mold the device with alkyd resulted in severe damage to the delicate semiconductor.
Background
The first epoxy molding compound was developed almost simultaneously with the encapsulation requirements of Bell Labs. Prior to the introduction of B-staged epoxy, this material was applied as a liquid by casting and potting using a variety of small cases and shells. This was, at best, a slow and messy process, not suited to axial-leaded products. Soon after, Bell Labs, in partnership with two other companies-one a supplier of molds, the other a supplier of resins-invented a process for transfer molding semiconductors using epoxy resins.Today, almost 35 years later, a partnership between Kras Asia, an equipment and tooling manufacturer; Nippon Pelnox, a Japanese resin formulator; and General Semi (Formerly General Instrument) has pioneered a major breakthrough in the encapsulation process.
Epoxies, today possess outstanding properties. Automatic machines that cost over $1 million each are lined up in factories producing epoxy-encapsulated components. However, the process today remains almost identical to the first trials decades ago where an axial-leaded varistor was molded for Bell Labs by transfer molding.
Current epoxy resins are formulated like a cake mix with resin, hardener, filler pigments and release agents all blended together. The resulting formulation has built-in stability which enables the compound to be shipped and stored without significant degradation.
The resin, in a granular form or compacted into a more easily-handled pill or preform, awaits the heat and pressure of the transfer molding process to turn it into a liquid. When this takes place, the liquid epoxy flows into and around the device to be encapsulated via a series of gates and runners. Once heated, the polymerization (hardening) begins and is completed. Today's fast-cure materials, which use the new multiplunger or gangpot molding designs, are capable of curing in less than a minute.
In an age where almost all the costs to produce semiconductors are declining, epoxy resins continue on an unbroken upward price spiral.
Even with the advances in mold design to reduce waste left in the runner, cull and gates, waste is still a costly concern as are the environmental issues concerned with waste disposal.
Over the years, many attempts have been made to eliminate the preformulated cake mix concept of "one mix fits all."While resin systems have improved substantially from a quality and stability standpoint, shipping into the warm, humid manufacturing areas of southeast Asia continues to be a concern.
The process of metering liquid epoxy resin and hardener, then having to thoroughly mix these materials before dispensing or injecting into a mold was difficult and provided little success for component encapsulation. The problems of handling and cleaning the rapidly reacting and hardening epoxy was difficult and could be disastrous if the material froze in the transport mechanism.
Encapsulation-grade polyester, supplied as a liquid, was a process tried for awhile. The resin could be supplied
thoroughly mixed in a liquid form. It remained unreactive until it entered the molding area where a specific temperature triggered the reaction and cure. The liquid polyester was transferred from a storage vessel via a plunger or screw for each shot. The process worked, after a fashion, but unfortunately the polyester resin could not compete with the performance characteristics of epoxy.
Costs and improved performance continue to drive developments in the electronic components industry. The industry wants servo-driven, fully-automatic equipment and shorter cycle times, increased yields, higher quality molding compounds and reduced molding compound consumption and waste.
For several years, Kras Asia has been involved in a partnership to develop a system to meet the expanding needs of this market.
General Semi, formerly the diode operation of General Instrument, is the world's largest producer of silicon rectifiers. General Semi wanted a process that offered fully automatic encapsulation, shorter cycle times, improved device performance, less material usage and a process where no molding compound came in contact with the axial leads. Keeping the molding compound away from the leads prevents a problem that has always plagued components-flash. Certain abilities of the system-KALMS and the encapsulant were determined to be necessary, as shown in Table 1.
Nippon Pelnox contribution to the partnership was the development of its patented epoxy formulation (U.S. patent # 5,610,443).
Even before the first designs of the machine and tooling were programmed on CAD, we had to determine if the liquid molding compound met and exceeded the quality levels of the state-of-the-art molding compounds then used by General Semi. After thousands of hours of tests, the material passed and exceeded performance expectations.
| A preformulated material that would not freeze in the transport or nozzle |
| The ability to control the exact amount of material for each shot |
| Exact control of the two-stage injection pressure |
| Stability of the mixed resin |
| A fail-safe method for cleaning nozzles and mold |
The next step was to design a test system to see if the material could be handled in a way that overcame the previous problems of liquid molding. In the patented formulation of Nippon Pelnox's epoxy, ELM- 1000, the hardener is a solid and is not desolved in the epoxy resin at ambient temperature, which enables the mixture to be formulated, transported and stored without reacting, except where and when required. The new resin offered a complete set of properties, before curing (Table 2) and after curing (Table 3).
Kras, as a major provider of encapsulation systems for over 38 years, understands the design and manufacture of machines, molds and component handling. New to this project, however, was the requirement to reduce material consumption substantially and to provide a means of dispensing the liquid resin without the molding compound touching the lead wires.
Concept Avoids Flash Deposits
To avoid flash, Kras developed a key concept called "The Capillary Gate Passage." (Figure 2). In this design, a series of small, needle-like nozzles enter the mold from the top and are positioned between the leads of two components. The nozzles clamp on the parting line of the mold. The capillary nozzles carry the exact amount of resin needed to encapsulate the two diodes. Unlike the runners and gates in conventional molds, the passage has a uniform profile and cross-sectional area which is the same size as the gate tip in a conventional mold. This unique design keeps the resin from contacting the leads while also using substantially less compound.This capillary concept provides increased heat and rapid curing, as the material with reduced viscosity enters and gently surrounds the device being encapsulated. Once the resin has cleared the capillary nozzle, the nozzles retract leaving a cull 1 mm thick by 2 mm in diameter. The filled mold leaves the injection station immediately, to be automatically replaced by another mold.
| Properties | Test Conditions | Unit | ELM-1000 Liquid Resin |
|---|---|---|---|
| Specific Gravity | 25°C | 1.95 | |
| Viscosity | 25°C, No. 7, 2 rpm | Poise | 1000 |
| 25°C, No. 7, 20 rpm | Poise | 544 | |
| Gelation Time | 150°C, 50 mg | Sec. | 40 |
| 150°C, 1.5 mg | Sec. | 65 | |
| 180°C, 50 mg | Sec. | 4 | |
| 180°C, 1.5 mg | Sec. | 18 |
| Properties after Curing |
Test Conditions | Unit | ELM-1000 Cured Resin | |
|---|---|---|---|---|
| 150°C 1 hour |
150°C 30 min |
|||
| Hardness | ASTM D-2240, | Shore D | 93 | 93 |
| HDT | ASTM D-648 | °C | >=200 | >=200 |
| Tg | TMA | °C | 170 | 180 |
| CTE | >=Tg, a2 | PPM/°C | 20 | 20 |
| Flexural Strength |
JIS K-6911, 25°C | kgf/mm2 | 10.2 | 10.5 |
| Flexural Modulus |
JIS K-6911, 25°C | kgf/mm2 | 1270 | 1220 |
| Lap Share Adhesion Strength |
JIS K-6850, Cu/Cu | kgf/mm2 | 220 | 233 |
| Volume Resistivity |
JIS K-6911, 25°C |  .cm |
2.8x1016 | 2.8x1016 |
| 24 hrs boiling | .cm |
1.1x1014 | 1.3x1014 | |
| 24 hrs PCT | .cm |
1.5x1013 | 1.7x10 13 | |
| Water Absorption |
JIS K-6911 24 hrs boiling |
wt.% | 0.67 | 0.60 |
| 24 hrs PCT | wt.% | 0.93 | 0.89 | |
| Fire Retardant |
UL-94 | V-0 | V-0 | |
| Impurities | K+, Cl- | PPM | <=10 | <=10 |
The New Encapsulation System
The KALMS is a servo-driven rotary machine which consists of eight individual diode molds. Each mold contains three chases of 40 cavities.The cycle begins at station one where an automatic loader accurately positions 120 diodes onto mold Number 1. This loading continues as each empty mold moves into position.
Mold Number 1 is clamped in two stages: pre-clamp and power clamp. At the pre-clamp stage, four tons of clamping force is generated by actuators which move with the mold, keeping it closed. At the injection or molding station, a total force of 17 tons of clamping force is applied by a servo-actuator. After molding, the mold continues through the other stations with the pre-clamp tonnage.
The KALMS uses a multiple nozzle system. Each nozzle has a core (inner) plunger running through the center of the sleeve (outer) plunger. The sleeve plunger meters each shot via a reciprocal motion inside the nozzle case. Molding compound is injected using two controlled pressures. One advances the material into the cavity; the second produces a final, higher pressure pack.
The entire injection module is powered by two independent servo motors. These motors control movement for nozzle downward engagement, compound metering, injection, sealing and nozzle retraction.
To insure a reliable injection operation, a dedicated automatic nozzle cleaner is incorporated into the system. With self-cleaning features, the auto nozzle cleaner removes any residual compound remaining on the core plunger and the nozzle tip. As soon as the nozzles disengage from the mold, the nozzle cleaner engages the injection module.
After the first mold has been injected with compound, it moves through curing stations 3, 4 and 5. At station 6, the mold is opened and all 120 encapsulated diodes are automatically removed. A special ejector mechanism ensures that all culls have also been removed.
The empty mold then moves to station 7, where it is thoroughly cleaned with air, vacuum and rotary brushes. The cleaned empty mold moves to station 8 where the reloading of 120 diodes is completed automatically. The entire molding
Figure 2-The KALMS employs a concept called "The Capillary Gate Passage." This passage connects mold cavities to a point where compound is supplied from outside the mold.
| Test Conditions | Duration | Failure Rate | |
|---|---|---|---|
| O.P.Life | 100V/1A/75% | ~1000 hrs | 0/20 |
| HTRB | 1000V/150°C | ~1000 hrs | 0/50 |
| Storage | 175 °C | ~1000 hrs | 0/20 |
| Humidity Bias | 600V/85°C/ 85%RH | ~1000 hrs | 0/85 |
| Humidity | 65°C/90~95%RH | ~1000 hrs | 0/20 |
| Moisture | 65°C/-10°C/ 90-98%RH | ~960 hrs | 0/20 |
| PCT | 15P/121°C | ~288 hrs | 0/50 |
| Temp. Cycle | 150°C/-55°C/30 min | ~1000 cyc | 0/50 |
| Thermal Shock | 0/100°C | ~100 cyc | 0/50 |
| Power Cycle | 1A/4 min | ~1000 cyc | 0/50 |
| Forward Surge | 1000V/1A | 30A | 0/20 |
Conclusion
Future generations of the machine can be configured as inline systems, if desired, and the types of components, especially high-leadcount semiconductors, can materially benefit by the advances available from this process and the advanced molding material. Additionally, the developers are looking into the possibility of applying the liquid molding technology to reduce the wire sweep of fine pitch IC packages.Dr. Wong, Kras Asia Ltd., Hong Kong, may be reached at 852.2344.4141, fax 852.2343.4819. Mr. Plummer, Chairman/ CEO of Kras Corp., Fairless Hill, PA, he may be contacted at 215.736.0981,fax215.736.8953. Contact Mr. Yanagaio Mr. Kitaoka at Nippon Peluox, Kanagawa, Japan, at 81.0465.74.1211, fax 81.0465.71.7944.
(This article was originally presented in a different form at APCON '97 and is used by permission.)