Home made PCB tinning solution
Thiourea ($\ce{(H2N)2C=S}$) is a very important component of tinning solution because tinning is a redox process. That's because thiourea is a powerful reducing agent (Ref.1), chemically and biochemically used in redox reactions in vitro or in vivo:

$$\ce{(H2N)2C=S &lt;=&gt; (H2N)2C-S-S-C(NH2)2 + 2e-} \quad E^\circ = \pu{0.418 V} \tag1$$

The oxidizing agent in this process is $\ce{Sn^2+}$ ion, which reduces it self during the process:

$$\ce{Sn^2+ + 2e- &lt;=&gt; Sn} \quad E^\circ = \pu{-0.1375 V} \tag2$$

As a result, following redox reaction happens:

$$\ce{(H2N)2C=S + Sn^2+ &lt;=&gt; (H2N)2C-S-S-C(NH2)2 + Sn} \quad E_\mathrm{rxn}^\circ = \pu{0.2805 V} \tag3$$

Since $E_\mathrm{rxn}^\circ $ is positive, the reaction is spontanious in standard conditions and no electric current is necessary. When you dip your circuit in the solution (or touch surface to surface of the solution), metallic tin would deposit as a thin layer. 

As an alternative to $\ce{(H2N)2C=S}$, dimethylated or diethylated thiourea can be used in those reactions, which require electrode potential within the range of $\pu{0.418 V}$ or higher (Ref.2):

$$\ce{(CH3NH)2C=S &lt;=&gt; (CH3NH)2C-S-S-C(NHCH3)2 + 2e-} \quad E^\circ = \pu{0.460 V} \tag4$$

$$\ce{(C2H5NH)2C=S &lt;=&gt; (C2H5NH)2C-S-S-C(NHC2H5)2 + 2e-} \quad E^\circ = \pu{0.450 V} \tag5$$

In tanning process, theoretically, any component with oxidative potential bigger than  $\pu{-0.1375 V}$ can be used as the reluctant. Thus, best alternative come to my mind is ascorbic acid (Vitamin C). Recently, natural source of ascorbic acid source, extract of fruits of Averrhoa bilimbi (Sinhala name: Billin), has been used to prepare silver and gold nanoparticles from corresponding salts (Ref.3):



In any event ascorbic acid didn't work, there is a way to synthesize thiourea derivatives without using thiocyanate or isocyanate. The scheme is illustrated in the bottom box of above diagram (Ref.4). The $E^\circ$ of these derivatives should be closer to that depicted in equations $(4)$ and  $(4)$. However, any attemp to synthesize these compound should be done in certified chemical laboratory under the observation of an experienced chemist. Keep in mind that thiourea is a potent carcinogen.

Note: It is well known that silver polishing reagents contain thiourea ($50\%$ by weight; Ref.5). You may able to extract thiourea from such solutions.



References:


Paul W. Preisler, Louis Berger, “Oxidation-Reduction Potentials of Thiol-Dithio Systems: Thiourea-Formamidine Disulfide,” J. Am. Chem. Soc. 1947, 69(2), 322–325  (https://doi.org/10.1021/ja01194a048).
Paul W. Preisler, “Oxidation--Reduction Potentials of Thiol--Disulfide Systems. III. Substituted Thioureas and Compounds Containing Two Thioureido Groups,” J. Am. Chem. Soc. 1949, 71(8), 2849–2852  (https://doi.org/10.1021/ja01176a076).
R. S. Rimal Isaac, G. Sakthivel, Ch. Murthy, “Green Synthesis of Gold and Silver Nanoparticles Using Averrhoa bilimbi Fruit Extract,” Journal of Nanotechnology 2013, Article ID 906592, 6 pages (https://doi.org/10.1155/2013/906592).
Wei Tan, Jianpeng Wei, Xuefeng Jiang, “Thiocarbonyl Surrogate via Combination of Sulfur and Chloroform for Thiocarbamide and Oxazolidinethione Construction,” Org. Lett. 2017, 19(8), 2166–2169 (https://doi.org/10.1021/acs.orglett.7b00819).
Mark Laing, Mark Beeston, "Silver polish formulation containing thiourea," US Patent 5,958,854, 1999 (Application No.: 08/869,685 in 1997).