POLY-FUSE

and PolySwitch

SMD PPTC Application

Application Note

PURPOSE: This document leverages Littelfuse manufacturing

experience and technical expertise in the correct use of

Resettable Surface Mount Polymeric Positive Temperature

Coefﬁcient (PPTC) for optimal performance, longevity, and

safety. Use this application quick reference guide to properly

integrate PPTCs in your Printed Circuit Board (PCB) designs.

PPTC Theory

There are two main types of PPTC applications:

1. Safety regulation (USB port, power supply, battery,

motor control, etc.) and

2. Risk prevention (e.g. I/O port).

For other types of applications, please contact Littelfuse Sales

Technical Support for consultation or joint development.

During certain abnormal conditions such as overcurrent, overload,

and overtemperature, PPTC resistance will increase. This limits

the power supply in order to protect circuit components.

Customers need to eliminate these conditions immediately after

PPTC activation because the PPTC cannot act as a protective

device if these conditions happen more than once or occur for

an extended period of time.

SMD PTC Soldering Methods

Littelfuse recommends an industrial one-time lead-free reﬂow

soldering process when using PPTC. For reference, R1max refers

to the maximum resistance of a device at 20˚ C, measured one

hour after tripping or a one-time reﬂow soldering per our

datasheet recommended proﬁle.

Multiple Soldering Risks

Since PPTC is very sensitive to high temperatures, you run the

risk of damage when soldering the same unit multiple times.

When customers do the Printed Circuit Board Assembly (PCBA)

process, multiple Dual In-line Package (DIP)/Surface Mount

Device (SMD) components are on the same PCB. When using

two reﬂow soldering and one wave soldering, it is recommended

to place PPTC components and DIP components on the same

side of the PCB board, and only apply the PPTC on the second

reﬂow soldering process. Avoid exposing PPTCs to multiple

high-temperature soldering processes.

In special applications, customers will use PPTC in a variety of

different high-temperature processes. Design of Experiments

(DOEs) are required to ensure that the characteristics of the

PPTC components will still be able to meet the customer’s

needs after undergoing these processes.

Solder Pad Layout Design

Solder pad design should include the through-hole (semicircle)

area, which can help to ensure the electrical connection and

keep the PTC properly latched during soldering (after solder

climbing). The Automated Optical Inspection (AOI)/Charge-

Coupled Device (CCD) of the Surface-Mount Technology

(SMT) machine can determine the solderability quality through

the hole solder climbing status.

Coating Material or Clean Solvent

Risk to PPTC

For PPTC operations, it is not recommended to use any coating

material that contains silicon-based oils, solvents, electrolytes,

acids, or any other materials with similar permeability. Customers

should assess the application risk by themselves or contact the

Littelfuse sales technical team before applying a coating material.

PPTC Circuit Operating Parameters

• Normal operation current (Ihold)

• Maximum circuit voltage (Vmax)

• Maximum interrupt current (Imax)

• Normal operating temperature surrounding device

(Thermal derating effect, min ˚C/max ˚C)

Quick Reference Guide

Other Items Critical to PPTC Type Selection

When selecting a type of PPTC, the maximum voltage of the

circuit cannot exceed the PPTC Vmax speciﬁcation.

On the Ihold circuit, for the maximum normal operating current,

it is best to leave a 10–20% margin of error.

Selecting a PPTC type should be based on the thermal derating

compensation of the customer’s ambient temperature.

For additional information, please reference:

• POLY-FUSE

PTC Selection Guide

• PolySwitch

Device Selection Guide

• Product Catalog and Design Guide (PTC)

If you experience any application issues not addressed in these

documents, you can contact Littelfuse Global Support directly

for help and technical guidance.

Littelfuse Far East Quality

Electronics Business Unit

https://www.littelfuse.com/contactus.aspx