The use of High-lead, High Melting Point solder ( HMP and Gold) has significant challenges that require a specialized skill-set, and an equally important sense of discipline to properly utilize in high-temperature circuit board applications.
The very metallurgy of HMP solder, typically 85-93% Lead, creates a condition whereupon the solder resists flowing uniformly and completely, especially when applied to Plated-Through-Hole (PTH) connections which create a greater heat sink effect than SMT components.
On top of these difficulties let’s also add the most common mistake that other CM’s make on PTH components – feeding HMP solder from both sides of the board which creates a cavity of unburned flux and air in the center of the via. At high temperatures this via’s connection will absolutely fail. HMP solder must only be fed in from the Secondary Side.
The skill becomes imperative in knowing the correct tip temperature, contact duration, and rate of solder-wire feed to create a one-touch, complete, solder joint without reheating the solder joint a second time which creates a brittle, unreliable solder connection. This brittle connection will have a low mechanical adhesion strength and will most likely fail when the PCB is exposed to the shake and vibration of the downhole environment.
Think about this – a $3 dollar part in a $5 million dollar downhole tool, on a $250,000 a-day rig can fail due to a single, unreliable, solder joint.
Despite the requirement for these critical soldering techniques and disciplines, our third-party inspection service routinely inspects assemblies from other CM’s that show the telltale evidence of reheated solder joints.
At MIL SPEC we wrote the book on High Temperature soldering.
HMP is its own world. It requires very specific knowledge, techniques, tools, - and patience.
Example of a High Melting Point (HMP) Printed Circuit Board (PCB)
Example of excellent HMP solder joints.