Introduction to PLCC Packages

PLCC, which stands for Plastic Leaded Chip Carrier, is a surface-mount integrated circuit package used for housing and protecting electronic components. PLCC packages were developed in the 1970s as an alternative to dual in-line packages (DIP) that allowed for higher pin counts in a more compact square package.

The key characteristics of PLCC packages include:

• A square or rectangular plastic housing
• J-shaped metal leads that extend from all four sides of the package
• Pin counts ranging from 20 to over 100 pins
• Surface mountable using either solder paste and reflow or Wave Soldering

Advantages of PLCC Packages

PLCC packages offer several advantages compared to other SMT package types:

1. Compact size – The square shape enables a high density of pins in a smaller footprint compared to equivalent DIP packages. This allows for miniaturization of PCB designs.

2. Improved electrical performance – The J-leads act as controlled impedance paths, reducing inductance compared to DIP. The leads are also spaced further apart than QFP or QFN, reducing crosstalk.

3. Easier handling – The J-leads are less fragile and prone to bending than gullwing leads on QFPs. The package is also thicker making it easier to pick up and place.

4. Lower cost – Being an older, mature package technology, tooling and component costs for PLCC tend to be lower than newer SMT packages. The plastic housing is inexpensive.

5. Thermal performance – The exposed pad underneath conducts heat well to the PCB. Having leads on all four sides also helps dissipate heat.

PLCC Package Variations

There are a few common variations of the PLCC package:

PLCC Socket Packages

PLCC sockets allow a PLCC component to be plugged in rather than soldered. This enables replacing or upgrading components without soldering. Sockets are commonly used for microcontrollers, EEPROMs, FPGAs and other programmable devices.

Sockets have pins that match the PLCC footprint and a spring-loaded mechanism to secure the device. The socket is soldered to the PCB while the PLCC plugs in.

Shrink PLCC (PLCC-S)

Shrink PLCC, or PLCC-S, is a smaller version of the standard PLCC package. It maintains the same J-lead and 4-sided configuration but in a more compact housing. Typical sizes range from 5x5mm to 12x12mm.

PLCC-S offers higher density than standard PLCC, though at the cost of some thermal performance and lead fragility. It is a good choice when board space is highly constrained.

Quad PLCC (PQCC)

Quad PLCC extends the PLCC to even higher pin counts by adding additional rows of pins. Standard PLCC has 2 or 3 rows per side (dual or triple row) while PQCC has 4 rows of pins per side.

This allows for pin counts over 200 in the same size package as a lower pin count PLCC. The tradeoff is higher cost and the need for fine pitch soldering capability.

Comparison Table

PLCC Applications

PLCC packages are used extensively in through-hole to surface mount conversions, programmable logic devices, embedded microcontrollers, and industrial/automotive electronics. Some common applications include:

Programmable Logic Devices

PLCCs are very popular for housing PLDs such as CPLDs and FPGAs. The high pin counts in a compact size are well suited to the I/O demands of programmable logic. PLCC sockets are often used to allow the PLD to be swapped or reprogrammed.

Example components:
– Xilinx XC9500 CPLD in 44-pin PLCC
– Altera MAX7000S CPLD in 84-pin PLCC
– Lattice ispLSI 1000 in 44-pin PLCC

Microcontrollers

Many 8-bit and 16-bit microcontrollers are available in PLCC packages. The quad-sided leads facilitate routing the many I/O pins. PLCC is an especially good fit for industrial and automotive applications that require the robustness of a J-lead.

Example components:
– Microchip PIC18F458 8-bit MCU in 44-pin PLCC
– Philips 80C51 8-bit MCU in 68-pin PLCC
– Infineon C167CR 16-bit MCU in 84-pin PLCC

Memory

Both volatile and non-volatile memory ICs are offered in PLCC packages. The small size helps keep the cost down while providing adequate pin counts. Common memory types are SRAM, EEPROM and flash.

Example components:
– Microchip 25LC256 256Kb SPI EEPROM in 8-pin SOIC
– Cypress CY62157EV30 8Mb SRAM in 68-pin PLCC
– STMicroelectronics M29F400 4Mb flash in 44-pin PLCC

Embedded Computing Modules

Many single board computer and computer-on-module form factors use PLCC sockets to interface the module to a baseboard. This allows easy upgrades and maintenance in the field.

Example form factors:
– PC/104 and PC/104-Plus
– PCI-104
– EPIC (Embedded Platform for Industrial Computing)

Designing with PLCC

Integrating PLCC components into a PCB design requires consideration of the footprint, soldering, and circuit board layout. Following some best practices will ensure a manufacturable, reliable design.

PLCC PCB Footprint

The PCB footprint for a PLCC package consists of landing pads for each J-lead as well as a central Thermal Pad. The size and spacing of the pads is determined by the package drawing.

Some key dimensions to consider are:
– E and D, the body length and width
– B, the lead width
– e, the lead pitch
– L, the lead foot length
– A and A1, the seated height and body thickness

A typical 44-pin PLCC has a lead pitch of 1.27mm and an overall length of 17.15mm. The land pattern would have 11 pads at 1.27mm spacing along each edge with an 11.1mm square thermal pad in the center.

Always consult the manufacturer’s package drawing for specific dimensions. It’s also a good idea to check the land pattern against the generic PLCC footprints in IPC-7351.

PLCC Soldering Considerations

PLCCs can be soldered using solder paste and reflow, wave soldering, or manual soldering. Some tips for each method are:

Reflow Soldering

• Use a no-clean solder paste with a lead-free or tin-lead alloy
• Print paste using a stainless steel stencil between 0.004″ and 0.008″ thick
• Follow a reflow profile suitable for the solder alloy used and PCB thermal mass
• Ensure adequate ventilation to exhaust solder fumes

Wave Soldering

• Use a no-clean flux and lead-free compatible solder pot
• Pre-heat the PCB to within 50-75° C of the solder temperature
• Convey the PCB at a 5-8 degree angle to the wave to prevent bridging
• Adjust conveyor speed to control dwell time and barrel filling

Manual Soldering

• Use a chisel tip between 1.5-3.0mm wide
• Select a lead-free solder wire between 0.020″-0.031″ diameter with no-clean flux core
• Set the iron temperature between 300-350° C for lead-free solder
• Touch the tip to both the lead and pad and apply solder wire to the joint, not the iron
• Solder leads individually or use solder bridges and wicking to remove excess

Layout Guidelines

Proper component placement and routing are critical for PLCC packages due to their high pin density and quad-sided lead configuration. Some guidelines to follow are:

• Provide a ground plane under the component for a low-impedance return path
• Decouple power pins to ground near the PLCC, preferably under the package
• Avoid routing traces between pads as this creates an acid trap that is hard to clean
• Use 45 or 90 degree bends rather than routing traces under the package
• Keep traces as short as possible, especially for clock and other high speed signals
• Distribute power and ground pins evenly on all four sides of the package
• Route signal groups (buses) together to minimize loop area and crosstalk
• Consider the effects of thermal expansion on long traces attached to PLCC pins

Following these layout practices will minimize parasitics, avoid manufacturing issues, and ensure reliable solder joints.

Troubleshooting PLCC Soldering Issues

Like any surface mount package, PLCCs are susceptible to certain soldering defects. Knowing how to prevent and correct these issues is key to successful assembly.

Bridging

Solder bridges are a common issue due to the closely spaced leads. Bridges can form during soldering or from poor paste printing.

To correct bridging, use solder wick and flux to remove the excess solder. Then, clean the area thoroughly with isopropyl alcohol.

Proper stencil thickness, print parameters, and reflow profile can prevent solder bridging. Using a thicker stencil or increasing print pressure/speed causes more paste to be printed and increases the risk of bridging.

Poor Wetting

Poor wetting, where the solder fails to flow properly to the pad or lead, can occur due to insufficient flux, contamination, or improper heat. It will result in a dull or lumpy joint.

Applying additional flux and heat can correct minor poor wetting. In severe cases, the lead may need to be reflowed. Preventing poor wetting requires a clean PCB surface, fresh solder paste, and adequate reflow temperature.

Tombstoning

Tombstoning is when one end of a component lifts off the pad due to uneven heating or surface tension. It is uncommon in PLCCs due to the lead length but can occur on fine pitch packages.

The lifted lead must be reflowed with extra flux. Reducing the temperature ramp rate during reflow helps prevent tombstoning. Printing less solder paste on the side opposite the leaded end of the package also balances out the surface tension.

Inspection

Visual inspection should always be performed to verify solder joint quality. X-ray inspection can be used to check for voids in Ball Grid Array packages but is not necessary for PLCCs.

Proper inspection looks for a concave fillet with good wetting to the lead and pad. The fillet should extend 50-75% of the lead length. Bridges, poor wetting, and lifted leads are defects.

PLCC vs Other SMT Packages

PLCC is one of many surface mount package options available. It is helpful to compare its features and capabilities to some other common packages.

PLCC vs QFP

Quad flat pack (QFP) is another 4-sided SMT package. It has gullwing leads instead of J-leads. QFP is available in a wider range of sizes and pin counts compared to PLCC.

QFP generally has a smaller footprint than PLCC for the same pin count. A 44-pin QFP measures about 10x10mm while a 44-pin PLCC is 17.5×17.5mm. The smaller size of QFP allows for higher density designs but makes the leads more fragile.

PLCC has better thermal performance and lower lead inductance than QFP. The J-leads are mechanically and electrically superior to gullwings. PLCC is also easier to handle and socket.

In general, QFP is better for very high pin counts and fine pitch applications while PLCC excels in small to medium pin counts where thermal or mechanical performance are priorities.

PLCC vs BGA

Ball grid array (BGA) packages have a grid of Solder Balls on the bottom rather than peripheral leads. BGA can achieve much higher densities than PLCC – over 1000 pins in some cases.

The main advantage of BGA is the ability to escape a large number of signals from a small package. BGA is preferred for complex, high-speed devices like processors and ASICs. PLCC pin counts top out around 84 pins.

However, BGAs require more precise assembly equipment and cannot be easily socketed or inspected after reflow. They also are not well suited to high-vibration environments. PLCC is much more forgiving and serviceable for small-scale production.

Cost is another consideration. BGA packaging and assembly are more expensive than PLCC, though this is justified for very high pin counts. For low to moderate pin counts, PLCC offers better economy.

PLCC vs DIP

Dual inline package (DIP) is a through-hole package with two rows of pins. It was the dominant package before the advent of surface mount technology.

PLCC was originally developed as a drop-in SMT replacement for DIP to facilitate the transition to surface mount. A PLCC takes up about 1/4 the board space of an equivalent DIP. It also has better electrical and thermal characteristics.

The main advantage of DIP is that it is very easy to handle, socket, and hand-solder. It is still used for some through-hole designs and in hobby electronics. However, for any remotely space-constrained design, PLCC is far superior.

In summary, PLCC hits a sweet spot between the density of QFP/BGA and the robustness of DIP. It remains an excellent choice for low to medium pin count devices in cost-sensitive industrial and consumer applications.

FAQ

Q: Can PLCC packages be socketed?

A: Yes, PLCC sockets are widely available and commonly used for components like microcontrollers and PLDs that may need to be swapped or upgraded. The socket is soldered to the PCB and the PLCC component plugs into the socket.

Q: What is the maximum number of pins for a PLCC package?

A: Standard PLCC packages top out at 84 pins (square package with 21 pins per side). For higher pin counts up to about 200, quad PLCC (PQCC) packages are available which add additional rows of pins. Beyond that, QFP or BGA Packages are typically used.

Q: Are PLCC packages suitable for high-speed signals?

A: PLCC is acceptable for moderate speed signals up to a few hundred MHz. The J-leads have lower inductance than gullwing leads which helps with signal integrity. However, for very high speed signals, leadless packages like QFN are preferred to minimize parasitics.

Q: How do you remove a soldered PLCC component?

A: Removing a PLCC requires heating all the leads simultaneously to melt the solder joints. This can be done with hot air or a specialized desoldering tool. Once the joints are molten, the component can be lifted off the pads. Solder wick is used to clean up any residual solder.

Q: What are the dimensions of a 44-pin PLCC?

A: A 44-pin PLCC has a nominal body size of 17.15 x 17.15mm (0.675 x 0.675 in). The actual dimensions may vary slightly by manufacturer. The lead pitch is 1.27mm (0.05 in) and the