What damage does the assembly process do to a pcb? (part 4)
In part 3 of this series of posts I discussed how phenolic cured laminates are mechanically weaker than their dicey cured laminate counterparts. I pointed out some of the material properties listed on the material data sheets that explain and support this point. Whereas the phenolic systems are better at thermal management, the dicey systems are better under mechanical stress. There is no right or wrong here. The systems just perform differently under different circumstances. Understanding the differences and how they relate to the applied assembly process are important to ensure success.
On this post I would like to discuss the effect moisture has on the printed circuit board. What most people don’t realize is that printed circuit boards are hygroscopic. Boards shall absorb available moisture from the surrounding environment to the point of equilibrium. The various FR-4 laminate data sheets list a Moisture Absorption value. The values are calculated in accordance with the IPC-TM-650 126.96.36.199A specification.
The 188.8.131.52A specification basically tests a solid rectangular piece of FR-4 free of copper, no holes and the edges sanded smooth. First the sample is preconditioned, aka dry baked. Immediately after dry baking, the sample is weighed. This is the dry weight. The sample is then submerged for 24 hours in Distilled Water. The sample is removed, towel dried and then weighed. This is the wet weight. The wet and dry weights are then plugged into a formula listed in the 184.108.40.206A specification to produce the Moisture Absorption value.
The value listed on the FR-4 data sheet is for a pristine piece of FR-4. It does not reflect the value you would see on the actual printed circuit board. The tested sample is a solid smooth rectangular block. There are only 6 planes of contact sanded down to remove edge fractures. A printed circuit board has many more openings where moisture can be absorbed through. Through holes, scored and routed edges. Drilling, scoring and routed edges result in some material fracturing. The amount of fracturing is dependent upon the fabrication process and the FR-4 used to make the board. Fracturing promotes moisture absorption. The moisture values listed on the data sheets indicate the FR-4 material’s minimum potential to absorb moisture. How much moisture is not that important. How to manage the moisture is important.
What does this have to do with the post topic, “What damage does the assembly process do to a pcb?”
If moisture is not managed properly, a lot.
The greatest danger faced by an assembler is Electro-Static Discharge, aka ESD. ESD can easily destroy electronic devices outright or damage them enough so that they fail later on in the field. Controlled environments and humidification are employed to minimize a static charge build up in the work area. I’ve been through some contract manufacturing facilities and have seen them employ elaborate misting systems that would pump moisture into the assembly area. The point is that assemblers work in a moisture rich environment. Once the board is unpackaged and enters the assembly area it starts to absorb moisture. How much moisture is a function of how much time the board spend in that environment.
How much moisture a printed circuit board absorbs is a function of several parameters.
- The area relative humidity.
- Time in the area.
- The bare board fabrication process.
- The type of FR-4 laminate used.
- Moisture content of the printed circuit board as received.
Dry baking a printed circuit board prior to shipping is considered an option by some printed circuit board fabricators. They may not dry bake the printed circuit boards unless requested to do so. When in doubt ask the fabricator what their policy is or specify dry baking as a requirement.
Moisture in the printed circuit board shall become a problem when the board is exposed to assembly soldering temperatures. Hygroscopic materials containing moisture are placed under stress when exposed to heat. The moisture wants to expand when heated. The expansion stress is referred to as the Coefficient of Hygroscopic Expansion, aka CHE. It may also be referred to as the Coefficient of Moisture Expansion, aka CME. Select hygroscopy for more information. The material data sheets provided by FR-4 laminate manufacturers do not list values on CHE/CME data. The import thing to understand is that materials bearing moisture expand when heated. The moisture expands at a different rate. This places stress on the laminate.
What’s the worst thing that can happen?
- Delamination. If enough moisture is present the FR-4 shall delaminate. Delamination caused by moisture expansion is considered a mechanical failure. The stress exceeded the laminate mechanical bond or flexural strength. The delamination shall be at the weakest point containing the most moisture. Zones for failure may include pre-preg to copper, pre-preg to core or between plys of pre-preg. The delamination may propagate from a drilled hole, a common point of entry for moisture. Phenolic cured epoxy systems are mechanically weaker than their dicey cured cousins. Phenolic laminates may be more susceptible to moisture delaminations due to this property.
- Warp and Twist. If moisture is present and the design is unbalanced enough, the board shall expand unevenly with a warp and twist. How is this possible? It is important to understand that the assembly temperatures are above the FR-4 laminate Tg value. Below Tg the epoxy is hard and solid. Above Tg the epoxy becomes soft and pliable. Copper expands at a low and steady rate as it is heated and is not a hygroscopic material. When the epoxy is heated above the Tg the epoxy becomes soft and pliable. The moisture absorbed by the FR-4 expands at a higher rate than the epoxy and copper. An area of the design with low amounts of copper (Zone 1) expands at a greater rate than an area of the board with a high amount of copper (Zone 2). The epoxy, being soft and pliable bends easily resulting in the warp and twist between the two different zones. When the board cools down below the Tg the epoxy becomes hard and solid again. The warp and twist between the two zones is locked into place.
How do you minimize the effect of moisture at assembly?
Dry bake the assembly after populating it with components but prior to any thermal exposure to soldering temperatures. Dry baking at low temperatures for long periods of time works best. Forcing the moisture out prior to assembly temperatures eliminates the effect of moisture on the assembly process. More information on dry baking parameters and procedures may be found on this blog at the following link, click here for dry baking posts.