When choosing smaller parts, consider ImAg or ENIG instead.
The board surface names HAL and HASL (hot air leveling and hot air surface leveling) refer to the same thing. They are interchangeable terms. With that out of the way, I’ll get to my point, which is HASL is not the right surface for all applications.
Take a look at FIGURE 1. This is a 0.5mm pitch BGA land, using lead-free HASL. Don’t expect good results with this board. That's not a statement about quality; it is a good quality HASL board. Even the bumps on the pads are not out of line for a HASL PCB. It’s not a defect. It’s just the way HASL works.
Figure 1. Lead-free HASL leaves bumps that could affect placement and soldering of smaller parts.
The catch is that, while the PCB is perfectly good, it’s not the correct board surface to use for all parts. HASL is fine for larger parts, but for small components, it’s archaic and often not reliable.
BGAs require a flat surface (also called a planar surface). With the bumps common on HASL boards, the BGA won’t have a flat surface. The solder paste won’t adhere evenly to the pads. The BGA will probably slide off the pads before reflow. It may end up far enough off that it can’t self-center, as BGAs usually do.
The HASL pads won’t all have an even amount of solder left on the board. Some pads will have more, some less. When added to the solder paste, the pads with more solder may end up bridging.
The issues become even more severe as the parts get smaller. Wafer scale parts, 0.4mm pitch parts, 0201 passives, and other similarly or smaller-sized components are essentially incompatible with the HASL surface.
So, what do you do? Order boards with immersion silver or ENIG. Both give a nice flat surface that BGAs like.
Now look again at Figure 1. There is something else about this board we don’t recommend. Hint: The problem is in the pad layout.
These are solder mask defined (SMD) pads. The solder mask covers the outer part of the pad, so the solderable copper surface is determined by the size of the opening in solder mask, not by the area of the copper pad.
For BGAs 0.5mm pitch and larger, we (and pretty much everyone else) recommend non-solder mask defined (NSMD). With an NSMD pad, the solder mask opening is larger than the pad. This leaves more copper area to adhere to, including the sides of the copper pad. It tends to be much more reliable.
FIGURE 2 illustrates the difference. The left-most pad in the image illustrates an SMD pad, while on the right is an NSMD pad. The NSMD pad leaves a lot more surface area of the copper pad for the solder ball to grip on, including the sides.
Figure 2. The non-solder mask defined pad (right) has more surface area for the solder ball to grip on, including the sides.
BGAs with 0.4mm pitches might need either SMD or NSMD pads, depending on a number of circumstances. When in doubt, look in the back of the datasheet.
is marketing manager and chief technology champion at Screaming Circuits (screamingcircuits.com);