What damage does the assembly process do to a pcb? (part 3)
In part 2 of this post I explained how the T260 and T288 material data sheet values could be used as an indicator of how durable a laminate system (FR-4) shall be when exposed to heat. The higher the temperature applied the less time it takes to delaminate the FR-4. Traditional dicey cured epoxy systems do not stand up to lead-free assembly temperatures as well as one would think. The newer phenolic cured epoxy systems are much better suited and able to withstand the higher temperatures applied with lead-free assembly techniques.
GREAT! PROBLEM SOLVED!
There are some potential challenges to the newer phenolic epoxy systems. Understanding some basic facts is the first step in preventing non-conformances, aka scrap. Over the next few posts I shall discuss the potential challenges of the phenolic laminate systems and how they relate to the assembly process. To begin with phenolic cured FR-4 systems are mechanically weaker than their traditional dicey cured FR-4 counterparts. Below is a comparison between two high end grades of laminate from the same manufacturer. Values are taken from the vendor’s material data sheet of the material and not of the finished printed circuit board. These values are as good as it gets. The values you shall see on a finished board shall be lower after you add variables such as the bare board manufacturing process and the actual board design.
Epoxy System: Dicey cured Tg 140°C (Traditional FR-4)
- Flexural Strength axis 1 = 600 MPa
- Flexural Strength axis 1 = 500 MPa
- Copper peel strength (1 Oz as is) = 10 Lb / in
- Copper peel strength (1 Oz heated) = 9 Lb / in
Epoxy System: Phenolic cured Tg 180°C (Lead-Free Assembly Compliant FR-4)
- Flexural Strength axis 1 = 500 MPa
- Flexural Strength axis 1 = 420 MPa
- Copper peel strength (1 Oz as is) = 7 Lb / in
- Copper peel strength (1 Oz heated) = 7 Lb / in
Flexural Strength is defined as a materials ability to resist deformation or rupture under load. It is also referred to as the modulus of rupture, bend strength or fracture strength. See Flexural Strength
Peel Strength is defined as the average load per unit width required to separate a flexible member from a rigid member or another flexible member. See Peel Strength
The values above are indicators of the materials durability and toughness when exposed to mechanical force or trauma. The values listed for the phenolic cured system are lower and hence weaker than the dicey cured system. The phenolic materials have a tendency to be more brittle and fracture more readily than their dicey counterparts. Hence, they now have an increased potential for mechanical fracturing and in extreme cases, delamination.
What does this have to do with the assembly process?
Quite a lot actually.
Understanding that the phenolic materials are weaker and easier to fracture is important. What operations are affected?
Standoff of Fastener Insertion. There are many different types of standoffs and fasteners that are pressed or swagged into a plated or non-plated hole. The forces applied in the installation process may result in an increased occurrence of Haloing about the hole. Haloing is mechanically induced fracturing or delamination on or below the laminate surface. In extreme cases the hole may even rupture or delaminate entirely. Potential solutions may include increasing the hole size, using a different tool or applying gradual amounts of force.
De-panelization. Small printed circuit boards are typically provided to assemblers in the form of a connected pallet or array. Rails may be attached along the edges of the boards with drilled perforations, a V-Score or nothing at all (punched). The process of de-panelizing the board out of the pallet may result in an increased occurrence of non-metallic burrs or haloing along the break edge. In extreme cases the edge may rupture or delaminate entirely. Potential solutions may include applying a deeper V-score or changing the spacing of the drilled perforations at the break point.
Pad Cratering. This is a relatively new non-conformance. After a ball grid array collapses in the assembly operation the solder cools down and solidifies. When materials cool down they contract. The bga device and board expand in size at different rates when heated. When the bga device and board cool down they contract at different rates. This places stress on the connection points. The stress may result in a laminate fracture that may fail outright or later in the field when the device is cycled on and off. Pad cratering is a challenge to be overcome. See Pad Cratering for more detailed information and a potential solution to this non-conformance.