Dealing with blistering in connectors

Moisture absorbed by connectors before soldering becomes critical because of the high temperatures encountered in this process. The vapor pressure of moisture and other gasses trapped inside the walls of a plastic connector increase rapidly when the connector is exposed to the high temperatures of solder reflow. When this vapor pressure exceeds the strength of the walls of the connector, a delamination occurs as the vapor pressure creates an egress/exit. This delamination is commonly referred to as a “blister”.  Blistering can also be caused by rapid outgassing of  other volatiles in the insulator material (either coming from the base polymer, flame retardants or other additives) at temperatures near the melting point of the material because of the low material strength in relation to the vapor pressuer of the volatiles.

Examples of blistering on plastic parts

With the advent of lead-free soldering technology, solder oven temperatures are increasing, by up to 20°C. The resulting higher peak temperature are very close to the HDT and / or melting point of the various resins used as insulator materials. The result is that even small quantities of absorbed moisture are expelled very rapidly and will cause blistering where that may not have occured before in a particular resin.

All polyamidesabsorb moisture due to the presence of the amide linkage. The degree to which polyamides absorb moisture is related to the number of amide groups per given polymer chain length.  In the case of Stanyl the number of amide groups per chain is very high. This gives Stanyl its excellent mechanical strength, toughness, and thermal resistance, but it also makes it more susceptible to absorb moisture under humid conditions. Moisture absorption is less with most other polyamides, however even those polyamides with very low moisture absorption do absorb some moisture. Under the critical conditions of high temperature soldering, especially with lead-free solders, blistering will occur with any polyamide. Additives and volatiles in polymers may also cause blistering at peak temperatures.  

Blistering has also been found to occur with LCP's but the incidence is irregular and unpredictable as it has other causes. The very high flow LCP’s when used in very thin walled connectors (down to 0.1mm) are particularly prone to blistering. On some materials, the use of regrind can have a severe effect on the incidence of blistering. PA9T has been found to show blistering when regrind is used. More info..

Blistering on mPGA Socket Housing and commercial card connectors
@ 260°C peak PCB temperature

Blistering will result in almost all resins after exposure to the high temperature and high humidity environments encountered in Asian countries where the majority of the assembly takes place both captively and through contract manufacturers

Blistering in Stanyl parts is caused solely by moisture and if moisture can be eliminated, or expelled from the part, before the highest temperatures are encountered, blistering will not occur in Stanyl.

Historical problems with blistering of IC packages during surface mount assembly have resulted in JEDEC industry standards for moisture sensitivity and packaging requirements to eliminate this failure mode.

Stanyl accommodates the high temperatures of lead-free soldering without problems due to its intrinsic high HDT values coupled with high modulus at solder temperatures relative to other materials.

Stanyl parts with thickness of >0.5mm may exhibit blistering under specific combinations of design and environmental factors. Simple precautions to avoid blistering include
•   predrying connector parts prior to soldering
•   packaging connectors immediately after manufacture in moisture proof packaging

For other materials, such as PPA, PA6T and PA9T, which have a lower tendency to blister, the risk is still present. For complete security and quality assurance it is required that precautions be taken especially when lead-free soldering is being used. While moisture proof packaging solutions are the same, the required predrying may well be longer as water desorbs slower from these resins than from Stanyl.

Blistering of connector insulators has become more critical and now embraces most known material solutions. Leading connector manufacturers now accept that taking standardized precautions is the best way forward. This shifts the focus on insulator resin choice from the transient issue of blistering to the more fundamental suitability of a material over the life cycle of the connector. The overall performance of Stanyl in respect of all other connector requirements and system cost saving potential cannot be matched by other resins.

Part thickness has a strong influence on moisture absorption and on the incidence of blistering

Stanyl parts with a thickness of less than 0.5 mm safely “breathe out” excessive moisture during the soldering pre-heat phase. No blistering results when heated to the elevated temperatures encountered in all types of soldering processes.

For thicker wall sections (>0.5mm) blistering can occur with Stanyl. The likelihood of blistering is influenced by both the soldering conditions and design factors. DSM has developed technology which can help predict when blistering may occur with such thicker parts. Together with design assistance from our Connector Competence Center, the risk of blistering can either be eliminated or clearly identified. For more information contact your local sales office