Removing moisture is still a hot topic

Moisture in boards can cause big problems, so manufacturers need to know how to find out if moisture levels are too high

September 8, 2010

by Terry Costlow, IPC online editor

Moisture and printed boards are like oil and water — little good comes when they’re mixed. Board fabricators and assemblers usually take care to ensure that boards don’t absorb too much moisture, since major problems can occur if humidity levels get too high.

“When you’re soldering, moisture can sometimes cause delamination or other problems. You want to keep boards dry so they don’t blow up during soldering,” said Joseph Kane, senior principal process engineer for BAE Systems Platform Solutions.

Most of the boards made in BAE’s facilities are complex designs that will go into high reliability systems like those used by the military, so the company doesn’t want to lose pricey modules for easily preventable reasons.

Moisture absorption is fairly easy to prevent. Fabricators can protect printed boards soon after manufacturing rather than leaving them exposed to humid air, and assemblers can keep them in sealed packages or dry cabinets until shortly before production.

Even when those steps are taken, companies will occasionally want to check boards to ensure that nothing’s gone wrong. Kane has done some extensive studies on the amount of moisture that can be present in boards. “Depending on the laminate and other details of the construction, if moisture exceeds some critical level, users need to bake them,” he said.

Much of Kane’s work is being done in conjunction with his membership on the IPC D-35 Printed Board Storage and Handling Subcommittee, which recently released IPC-1601, Printed Board Handling and Storage Guidelines.

Kane will describe the moisture testing processes at Electronics Midwest in Rosemont, Ill., during the IPC conference that runs September 28–30. His Tuesday, September 28 presentation is entitled: “Weigh/Bake/Weigh Testing for Moisture Content in Printed Boards.”

That testing technique is based on the new IPC-TM-650, Method 2.6.28, Moisture Content and/or Moisture Absorption Rate, (Bulk) Printed Boards, which has been developed in conjunction with IPC-1601 and provides techniques for determining moisture content by weighing boards, baking them until they’re dry and then weighing them again. 

IPC-TM-650, Method 2.6.28, suggests baking test samples for 24 hours following initial weighing. Kane wanted to see if that did indeed remove sufficient moisture to provide valid results, so he baked some boards for much longer, two weeks or more. Those tests proved that the 24 hour bake called out in this new IPC test method does indeed provide a useful measurement of moisture content.

Most companies don’t want to lose boards to a baking process for a full day, nor do they want to pay the utility bills to heat the oven. “One intention of the IPC-1601 guideline is to keep costs in check; we don’t want compliance to cost suppliers a lot of money,” Kane said. “It’s not just the cost; baking causes oxidation and makes the boards harder to solder.”  The guideline will highlight the effects of baking on a number of final finishes, including tin-lead, immersion tin, immersion silver, ENIG, ENEPIG and electrolytic gold.

Thankfully, it’s usually pretty easy to avoid the expense of baking. One  important step is for board fabricators to store boards properly as soon as possible. Boards need to be put into dry storage or sealed packages promptly before they sit around absorbing moisture, Kane explained.  The new IPC-1601 provides guidance on the types of packaging material available, and the procedures for their handling and storage, as well as recommended profiles for baking of printed boards, in cases where moisture absorption has not been prevented.