Minimizing Effects of Lead-free SMT Assembly on Connector Housing Resins

Estimated reading time: 6 minutes

Many connector suppliers are eliminating, or have already eliminated, lead from their contact plating process. The next phase will be lead-free solder compliance for board assembly.

By John J. McMullan

In this stage, the higher liquidus temperatures required for lead-free solders may have a profound effect on the connector insulator housings. Some materials currently used may deform (melt) or show signs of delamination on the side walls, referred to as "blistering." (Figures 1 and 2) The melting is caused by the temperature exposure being greater than the melt temperature or heat deflection temperature of the housing resin, and delamination occurs due to rapid moisture egress under higher SMT temperatures, namely 245° to 260°C at printed circuit board (PCB) level.

Key to elimination of blistering is moisture control for most of the temperature-capable engineering resins used in connector housings. Many Asian OEMs, who have been converting to the lead-free solder process for the last few years, require that their connectors be packaged to minimize moisture absorption for this reason. One company* has undertaken a program to evaluate current packaging techniques and determine their effectiveness in moisture control.

Figures 1 and 2. The vapor pressure of moisture and other gases 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."

To ensure applicability to this study, IPC/JEDEC J-STD-020B, "Moisture/ Reflow Sensitivity Classification for Non-Hermetic Solid State Surface Mount Devices" and Sony's SS-254, "Test Methods for Electronic Components, Lead-free Soldering Parts Design Standard" were used to define the conditioning parameters for the tested injection-molded parts.

These standards contain several exposure conditions, defined by relative humidity and time at temperature, and use a lead-free reflow profile that results in a 250°C temperature (0/-5°C). Peak oven temperatures of more than 300°C are typical to generate this surface temperature (Table 1).

IPC/JEDEC J-STD-033A, "Handling, Packing, Shipping and Use of Moisture/ Reflow-Sensitive Surface Mount Devices" packaging requirements were used as the guideline for the testing of moisture control effectiveness. Board assemblers have focused on handling more expensive ICs but have failed to take similar precautions with board connectors. But the problems listed previously for connectors in these hot oven environments can also result in board rework.

Moisture Absorption

Many currently used high-temperature resins will pass lead-free solder surface mount assembly if care is taken in preventing/retarding moisture absorption to keep the level below that where blister onset occurs. Blister onset temperature is dependent upon several factors, including wall thickness, percentage of moisture in the part, speed of connector heating, and peak temperature of the reflow process.

The higher the peak temperature, the lower the moisture content can be before blistering. Lead-free solders result in higher peak temperatures, meaning more blistering.

Moisture Prevention

Any means of protection from a high-temperature/high relative humidity environment for molded parts, including minimizing the time the parts are subjected to these environments, will extend shelf life and minimize blistering of these parts, as well as result in higher temperatures for blister onset.

Since the majority of non-LCP connectors fall into the IPC/JEDEC J-STD-020B MSL levels of 2 to 5a, the packaging required to conform to the IPC/JEDEC J-STD-033A standard includes moisture barrier bags (MBB), desiccant, moisture-sensitive identification label (MSIL) and a caution label. The key parameter in blistering of connector housings is the control of moisture absorption in the molded parts.

Moisture Barrier Bags

Moisture barrier bags are defined in the IPC/JEDEC J-STD-033A standard as "A bag designed to restrict the transmission of water vapor and used to pack moisture sensitive devices." Water vapor transmission rate (WVTR) is the measurement of the effectiveness of these bags in the restriction of moisture permeability.

The company has analyzed the types of bags that are available for connector packaging and come up with some recommendations based upon WVTR ratings (Table 2). The high-density polyethylene (HDPE) bags are better for preventing moisture ingress than low-density polyethylene (LDPE) bags.

An experiment was conducted where 10 kgs of SCSI connectors in a 30 percent glass-reinforced, flame-retardant resin** were packaged in the suggested plastic bags with a surface area of 1 m2. They were subjected to two different storage conditions: 26°C at 65 percent RH and 35°C at 90 percent RH.

At a 260°C peak temperature lead-free solder condition, the blister onset moisture level for many high-temperature resins is ≈1.5 wt%. Table 3 shows that, at the storage conditions of an air-conditioned warehouse in Southeast Asia (Condition 1), even the thinnest tested LDPE bag would provide enough moisture protection to keep the connectors below the blister threshold level for 90 days. At the higher-thickness LDPE or at either of the two HDPE thicknesses, storage time would be substantially longer.

If the packaged connectors were left exposed to the natural environment (Condition 2), only the HDPE bags would provide any real safety factor and then only the 100 µm thickness would allow for 90-day storage without blistering.

Desiccants

According to the IPC/JEDEC J-STD-033A standard, parts having MSLs higher than 1 require desiccants be used as an integral part of the packaging of these parts.

The effectiveness of desiccant in combination with MBBs was verified using silica gel with moisture-sensitive connectors (MSLs >1). MBBs in two different thicknesses (0.06 and 0.2 mm) were used in this test, along with 11 g (≈1/2 unit ) of silica gel. The connectors were exposed to an environment of 35°C at 90 percent RH. The results (Figure 3) show that the addition of desiccant to MBBs is effective as the desiccant absorbs/retards the initial moisture uptake of the connectors.

Figure 3. Effectiveness of desiccant on moisture absorption of packaged connectors.

According to the IPC/JEDEC J-STD-033A spec, desiccants must meet the MIL-D-3464, TYPE II standard. Approved options include clay and silica gel. For standard MBB sizes used for shipping connectors (loose or on reels) the amount of desiccant required is less than two units.

Moisture Analysis

If all moisture absorption precautions are taken and connectors are still exhibiting some delamination during SMT assembly, measuring the amount of moisture in the parts to determine the steps required to solder without connector blistering may be required.

The company compared three moisture measurement techniques: halogen moisture analyzer, titration*** and vaporimetry.

The test specimens used were smart media card connectors that had been immersed in water overnight and then conditioned at 40°C and 95 percent RH for 24 hours to absorb high water content.

All techniques provided similar moisture percentages in parts. For ease of use, the halogen moisture analyzer was considered the best.

Pre-solder Drying

The higher preheat temperatures typical in lead-free solder profiles may provide more safety from blistering for thinner-walled parts than current tin-lead solder profiles due to the direct relationship between moisture desorption rate and temperature.

For plastic resins capable of withstanding the heat of lead-free SMT processing, a suggested drying temperature of 120°C can be used to dry parts to eliminate blistering.

Conclusion

Higher temperatures required for lead-free SMT soldering will force a change from many currently used connector housing resins, which will melt, suffer pin retention failures or fail to meet dimensional stability requirements. In order to continue to use non-LCP resins for lead-free SMT assembly, the connector industry can use established and accepted IPC/JEDEC standards to develop new handling requirements.

*DSM Engineering Plastics.** Stanyl PA46.*** Karl Fisher.

John J. McMullan, global segment manager — connectors, may be contacted at DSM Engineering Plastics, P.O. Box 3333, Evansville, IN 47732, (630) 904-5234; Fax: (630) 904-5233; E-mail: John.McMullan@DSM.com