STEC

Memory module manufacture – where PCB assembly and semiconductor packaging converge

Memory module manufacture is increasingly seeing PCB assembly requirements converging with those from semiconductor backend manufacture like bare die and flip chip packaging. Specifically, modules are integrating bare dies with ever smaller chip capacitors and resistors having tighter component interspacing. Bump sizes and bump pitches are meanwhile decreasing to fit more I/Os on smaller sized package. This brings tighter and tighter accuracy requirements which are making memory module production one of the most challenging of manufacturing environments. Intensely competitive, it has exceptionally tight quality and reliability requirements despite the high component densities. DRAM modules can measure less than 20 by 140 mm but have to combine up to 10 memory devices, a controller, and over 100 chip components.

California-based STEC makes solid state memories for OEMs worldwide, concentrating on the high-performance sector. Its product range includes the market’s widest array of DRAM modules, flash memories and solid state drives (SSDs). All are high value devices for servers, telecom, and embedded and other complex products that need ultra reliable assembly. In 2006, STEC contacted Assembléon to increase production levels from just below the 10,000 units/week/production line from their existing equipment.  The company bought an Assembléon AX-501, which was primarily used as a chip shooter.

Like other chip shooters, the AX-501 needed a separate line balancing machine for populating boards with high IC counts. Memory modules like DRAMs do have high IC count, though, and STEC wanted a machine that would combine chip and IC shooting. It also wanted to pick ICs directly from a Jedec Tray Stacker and, with memory devices sometimes delivered in tape and reel rather than in Jedec Trays, it needed a system with very flexible setup. Relocating the work area of the Standard AX-501 Placement robot so it could pick & place from the  Jedec Tray Stacker increased line capacity to 30,000 units/week/production line. But STEC wanted more, and in response Assembléon set up a new project to further increase output and accuracy of the AX-501 itself. 

Figure 1: Malaysian technicians prepare a previously installed AX-501 before fitting a TPR

The joint project resulted in Assembléon’s recently introduced Twin Placement Robot (TPR), which has revolutionized STEC’s flash memory and DRAM module production. The TPR combines two independent placement robots within one module to place ICs while the rest of the AX-501 modules continue to place chip components. Four of STEC’s seven production lines at its Penang facility in Malaysia now have two AX-501s (separated by a board flip unit) producing nearly 70,000 units/week – around eight times the previous levels. Another two lines, including one NPI (New Product Introduction) line, have a single AX-501, and a line with two Assembléon MG-1 machines handles low volume production. Seven of the ten AX-501s now have a TPR, with the remaining three to be upgraded soon (the TPR can be retrofitted, Figures 1 and 2).

Figure 2:  The TPR can be easily retrofitted on a previously installed AX-501

STEC is delighted with the new setup. “The TPR system has exceeded all our requirements and expectations” enthused Jose Del Valle, STEC’s Director of Offshore Engineering. “It has greatly increased the flexibility of the A-Series platform and picks at high volume from trays at the front of the unit. Because of the TPR, we are able to place Flash and DRAM at near 0402 speeds. We have increased throughput and accuracy while also decreasing changeover times.“

Multiple heads ensure smooth placement at high throughput

Although the original AX-501 did place ICs, the machine was focused on chip components. Assembléon’s parallel placement technique uses up to 20 Compact Placement Robots (CPRs) placing up to 20 components at a time. For the same throughput as the sequential placement machines that dominate the rest of the industry, the AX-501 has up to 20 times longer to pick and transport a component, calculate its trajectory, allow it to settle, place it with controlled force, and check the placement. That makes the placement process very stable. The lower accelerations, decelerations and rotations make for a smooth placement process which greatly reduces the number of components twisted or thrown off.  Automatic calibration further reduces variation in the pick & place process to maintain six-sigma accuracy even with changing environmental variables like temperature. The resulting single-figure defects per million (dpm) placements is the industry’s lowest.

With each head placing up to around 6,000 cph (to IPC 9850), chip-shooting rates for the AX-501 with 20 CPRs are 121,000 cph. The AX-501 is actually arranged into five base segments. Each segment takes four CPRs or two Standard Placement Robots (SPRs). The SPRs, like the CPRs, place chip components but have double the number of feeder positions.

Figure 3: The Twin Placement Robot can increase line output of the AX-501 for boards with IC-to-chip ratios above 0.1

One of the base segments can now instead take a single TPR placing ICs at rates up to 16,000 cph (Figure 3). A TPR can significantly increase the line output for IC-to-chip ratios above 0.1. With 16 CPRs and a TPR, the combined IPC 9850 chip and IC output is up to 111,000 cph (also the real world rate). That gives Assembléon’s AX-501 the market’s highest output per square meter at nearly 9,000 components/m2. Placing chips and ICs with the same machine also saves STEC the labor, energy, maintenance and other operational costs of a separate line balancing machine (Figure 4).

Figure 4: The AX-501 with TPR combines chip and IC shooting on one machine

The TPR actually has two (Twin) robots under a single hood. These new robots have an extended work area for processing wider boards, and improved X and Y-axis designs. The Y spindle has been replaced by a linear axis guide, and the linear X slide has been improved by increasing the frame stiffness. Encoders and associated circuits also have higher accuracy.

Placement accuracy for the CPR and SPR is 35 microns at three sigma levels with camera alignment. The controlled speed of the TPR’s linear robot motion profile gives extra settling time and reduces the effects of vibration from other heads. That improves placement accuracy to 25 microns with 17-micron repeatability (both at 3 sigma, CpK>1) using camera alignment. That makes it the industry’s most accurate machine for high speed memory placements and package-on-package assembly. The improved accuracy is needed to reliably place the BGAs (Ball Grid Arrays) and SSOP (Shrink Small-Outline Package) memory ICs used by STEC. BGAs, particularly, can have very small bumps down to 80 microns.

Whatever mix of robot types, the output of AX-501 is adjustable by simply adding or subtracting heads. For example, one SPR can be temporarily replaced by two CPRs, doubling production capacity on the spot during peak periods. That means that Assembléon can offer STEC its ‘True Capacity on Demand’, which allows customers to order a machine with low capacity and increase output if a new product is successful. It also allows customers with seasonal sales patterns to rent extra capacity from Assembléon as needed. In both cases, there is no change to line layout.

The TPR places components up to 45mm by 45mm and 10.5 mm in height. The present design only allows a single TPR robot per AX-501, but Assembléon is working on increasing this.

Convenient feed from tray stacker

The TPR can pick ICs from tape and reels.  And larger components like QFPs and BGAs can be picked from one of two Jedec Tray Stackers, to allow two types of components. The tray stacker can hold up to 20 thick (12.19mm) trays or 30 thin (7.62mm) trays.

STEC’s products feature a wide array of interfaces and form factors, including custom OEM designs with embedded controllers and flash management circuits that need short design cycles and fast time to market. That in turn demands fast and versatile changeover of manufacturing setups. The stacker takes no setup time, and is open for easy visibility for the operator (Figure 5). The complete stack is transferred from the shipping box and placed into the feeder in one action. Using stacks of trays eliminates pallet set-up and teaching for each individual tray, with a complete stack easily being changed within a minute. As unused components remain stacked in their original trays, they are easily and safely stored for later use.

Figure 5: The Jedec Tray Stacker gives a convenient IC feed for the AX-501, and is open to give easy visibility for the operator  

Will meet future technology requirements

STEC now places 0402 and 0201 chip capacitors and resistors along with resistor networks, ICs and connectors. The AX-501 will also place 01005 (0.4 by 0.2 mm) chips with the same single-digit dpm should STEC need to, and is ready for new component sizes such as half-height (thin) components and 0050025 chips (25%of the size of the 01005). The AX-501 also takes the new 4 mm W4P1 all-plastic static-free tape for reliable 01005 component feeding. All this helps to make the machines future proof, adding new technologies without having to buy new pick and place equipment. 

STEC’s Del Valle concludes: “There are too many other advantages of the new setup to list here. We would like to thank Assembléon for developing the TPR for STEC. It has given us a true competitive manufacturing advantage, and it will continue to be part of our future standard configurations.”