Samtec Connector Mini Classroom Series 2 | Normal Force, Circulation, Temperature and Other Points

【Abstract/Foreword】

Plating affects the life and quality of the connector system, including corrosion resistance, conductivity, solderability and, of course, cost.

[Small classroom background]  

This is part two of a series in which Samtec Quality Engineering Manager Phil Eckert and Principal Engineer Matt Brown discuss issues related to connector plating, titled "Normal Force, Cycles, Temperature, and Other Points."

The first part of the series, Samtec Connector Plating Small Classroom Series 1 | Connector Plating Knowledge Q&A, has received good response. Therefore, Samtec will gradually launch a series of follow-up popular science articles to answer connector plating questions often raised by Samtec quality and processing experts.

These questions primarily involve gold and tin plating, as these are the most common connector plating options.

Q1: Should the normal force of the contacts determine which plating to use?

A1:

The short answer is "Yes". For tin plating, a general guideline is to withstand 100 grams of normal force per mated contact to achieve a gas-tight connection. Oxides have the potential to form a resistive layer on the tin surface. To achieve a good connection, sufficient normal force is required to break through the oxide layer. When I say these connectors require 100 grams of normal force, we are not talking about miniature connectors. You don't typically see tin plating on these small connectors because it is difficult to generate proper normal force.

Gold plating is a precious metal that does not react with pollutants in the atmosphere, so a smaller normal force can be used, such as 30-40 grams.

You may be familiar with the phrase "30-40 grams, end of life", which refers to the contact normal force after considering thermal fatigue. This refers to the contact normal force after testing has been completed and exposure to higher temperatures has occurred. Typically the contact beam will be relaxed, but you still want to achieve 30-40 grams (gold plated). Note that this is just a general guideline; I have seen EOEMs using micro connectors that weighed less than 30 grams and worked fine.

We mentioned before that gold plating requires a contact normal force of 30-40 grams. We give a range because the required normal force will be affected by the contact design. That said, the more touchpoints the better.

Q2: What is the maximum continuous use temperature of gold plating and tin plating?

A2:

The maximum continuous use temperature for tin plating is 105°C and the maximum continuous use temperature for gold plating is 125°C. By the way, many customers use "maximum continuous use temperature" and "operating temperature range" as synonyms.

There is also peak temperature, which refers to a limited period of time when the device is exposed to temperatures above operating temperature. Reactions such as surface corrosion or oxidation can occur above operating temperatures, and all reactions are accelerated by heat. A general rule of thumb is that reaction rates double for every 10 degrees Celsius increase in temperature. Therefore, once a certain temperature is reached, reactions that occur slowly and do not affect the life of the connector for many years suddenly occur rapidly, causing the connector to rapidly fail. 

You should know the current carrying capacity (CCC) of the connector. Current flowing through a connector will increase the temperature in and around the connector, so 105°C and 125°C are a consideration at a given current level.

Q3: What are the typical thicknesses for these connector plating options?

A3:

For tin plating, Samtec recommends a tin layer of at least 100 µ" on the solder tail and in many cases greater than 100 µ" on the contact area. Some specifications do not specify an upper limit for tin plating. Due to the low price of tin, we recommend a minimum tin plating level of 100 µ".

For gold, 30 µ" is the standard for reliability. We have been exposed to many customers who require 30 µ" gold in their designs, but we also know of many more customers using 10 µ" gold, even as low as 3 µ" of gold (called "flash gold").

If the connector needs to pass Mixed Flow Gas (MFG) testing, where unmated contacts are exposed to chemicals such as chlorine, hydrogen sulfide, nitrogen dioxide, and sulfur dioxide, the OEM may specify the use of 30 µ" gold. Gold Content A 30 µ" connector will pass this test, but a 10 µ" connector will generally fail the test due to being porous.

Q4: What is the maximum number of mating and unmating times for each connector coating?

A4:

This is a difficult question that is often asked. The number of cycles a connector system can withstand can be affected by a variety of variables. Of course, the first is surface treatment. We know that gold plating takes longer. The base metal also has an impact, as some base metals produce larger normal forces. Connectors with very high normal forces will wear away the plating after many cycles. 

We perform 100 cycle tests on 30 µ" gold connectors. No doubt many customers experience connectors exceeding 100 cycles, but these test standard tests are repeatable and reflect real-world application conditions. Above is a typical Samtec test plan, and the tests performed after the cycle.

The simplest practical advice we can give is, if more cycles are needed, use gold, the more cycles, the more gold. The number of cycles is also affected by environmental conditions such as temperature, humidity, and atmospheric contaminants such as sulfur and chlorine.

Q5: What are the advantages and disadvantages of tin/lead plating?

A5:

Tin/lead plating has been used for the past 30 years and still is! In most respects, tin/lead plating produces better solder joints. Tin/lead also reduces whisker growth and is particularly useful in military and space applications. Dendritic whisker growth can form on pure tin plates and can cause short circuits. There are numerous examples of whiskers on pure tin causing catastrophic failures.

Although cases of tin whiskers leading to catastrophic failures are common, the consensus is that matte tin is generally acceptable as long as the plating quality is good, and some believe that a nickel base plate used with matte tin can further reduce the chance of tin whiskers.

Tin/lead plating also has a lower eutectic temperature, which helps protect sensitive components during processing. Tin/lead plating can lower oven temperature settings by 15°C to 30°C compared to pure tin plating. For example, lead-free ovens can be set to temperatures between 245 degrees Celsius and 260 degrees Celsius; leaded applications can be set to 230 degrees Celsius, and often lower.

The disadvantages of tin/lead are clearly related to environmental concerns in protocols such as RoHS and REACH.

【Summarize】        

As a company that has been in the connector field for decades, Samtec has solid technical accumulation and diverse practical experience in connector electroplating. We understand every challenge encountered in the connector design process.

We will continue to bring connector Q&A series in the future, so stay tuned!